CSIRO LAND and WATER. Sediment supply and transport in the Murrumbidgee and Namoi Rivers since European settlement

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1 Sediment supply and transport in the Murrumbidgee and Namoi Rivers since European settlement By Jon Olley and Anthony Scott CSIRO Land and Water, Canberra Technical Report 9/02, December 2002 CSIRO LAND and WATER

2 Sediment supply and transport in the Murrumbidgee and Namoi Rivers since European settlement By Jon Olley and Anthony Scott CSIRO Land and Water, Canberra Technical Report 9/02, December 2002

3 Acknowledgements This report was funded by the Murray-Darling Basin Commission and the Cooperative Research Centre for Catchment Hydrology. The authors would also like to thank the following people and organizations for kindly providing information, reviewing the draft report or contributing photos; Ian Prosser, Chris Moran, Janelle Stevenson, Andrew Hughes, Peter Wallbrink, Gary Caitcheon, Gary Hancock, Sue Vink (CSIRO Land & Water), Carolyn Young, Greg Bowman, Lee Bowling, Monika Muschal (Department of Land & Water Conservation, NSW), Pat Feehan (Goulburn-Murray Water, Victoria), Geoff Titmarsh, Bernie Powell (Department of Natural Resources, QLD), Lisa Robins (Robins Consulting), Tony Jakeman, Lachlan Newham, Barry Croke (CRES, Australian National University), Klaus Koop (Environment Protection Agency, NSW). Copyright: 2002 CSIRO Land and Water. To the extent permitted by law, all rights are reserved and no part of this publication covered by copyright may be reproduced or copied in any form or by any means except with the written permission of CSIRO Land and Water. CSIRO Disclaimer: To the extent permitted by law, CSIRO Land and Water (including its employees and consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using this publication (in part or in whole) and any information or material contained in it. MDBC disclaimer: The contents of this publication do not purport to represent the position of the Murray- Darling Basin Commission. They are presented solely to stimulate discussion for improved management of the Basin's natural resources. Front cover photo; The Murrumbidgee River at Tharwa, south of Canberra. (photo; Anthony Scott) ISSN

4 Table of contents 1. INTRODUCTION...5 WHAT ARE THE ISSUES? THE MURRUMBIDGEE...8 THE RIVER TODAY...8 THE RIVER AT THE TIME OF SETTLEMENT...19 CHANGES SINCE SETTLEMENT THE NAMOI...36 THE RIVER TODAY...36 THE RIVER AT THE TIME OF SETTLEMENT...44 CHANGES SINCE SETTLEMENT SYNTHESIS REFERENCES

5 Acknowledgements This report was funded by the Murray-Darling Basin Commission and the Cooperative Research Centre for Catchment Hydrology. The authors would also like to thank the following people and organizations for kindly providing information, reviewing the draft report or contributing photos; Ian Prosser, Chris Moran, Janelle Stevenson, Andrew Hughes, Peter Wallbrink, Gary Caitcheon, Gary Hancock, Sue Vink (CSIRO Land & Water), Carolyn Young, Greg Bowman, Lee Bowling, Monika Muschal (Department of Land & Water Conservation, NSW), Pat Feehan (Goulburn-Murray Water, Victoria), Geoff Titmarsh, Bernie Powell (Department of Natural Resources, QLD), Lisa Robins (Robins Consulting), Tony Jakeman, Lachlan Newham, Barry Croke (CRES, Australian National University), Klaus Koop (Environment Protection Agency, NSW). Copyright: 2002 CSIRO Land and Water. To the extent permitted by law, all rights are reserved and no part of this publication covered by copyright may be reproduced or copied in any form or by any means except with the written permission of CSIRO Land and Water. CSIRO Disclaimer: To the extent permitted by law, CSIRO Land and Water (including its employees and consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using this publication (in part or in whole) and any information or material contained in it. MDBC disclaimer: The contents of this publication do not purport to represent the position of the Murray- Darling Basin Commission. They are presented solely to stimulate discussion for improved management of the Basin's natural resources. Front cover photo; The Murrumbidgee River at Tharwa, south of Canberra. (photo; Anthony Scott) 4

6 1. Introduction Sediment is a natural part of a river system. The amount and characteristics of the sediment supplied to the river influences the size and shape of the channel, the form of the bed sediments, the quality of the water, and the supply of nutrients, all of which affect the health and structure of a river s ecosystem. European settlement 180 years ago brought massive changes in land-use across the Murray-Darling Basin. The clearing of woodlands, the introduction of grazing stock, the drainage of valley bottoms, and the clearing of riparian vegetation caused a large increase in erosion. In less than one hundred years nearly every valley in south-eastern Australia was affected. Massive volumes of sediment were delivered to the rivers, and the form of the rivers, and the surrounding landscape, were changed dramatically. Vegetated valley floors were incised; clear flowing waters became turbid; deep pools were filled with sand; and rivers which abounded with fish and other life became dominated by algal growth. This report examines sediment supply and transport in two of the major rivers in the Murray-Darling Basin, the Murrumbidgee and Namoi Rivers (Figure 1). It describes the state of the rivers today and at the time of settlement. It assesses the relative effects of land-use change, rainfall variation, and flow regulation on the supply and transport of sediment in these systems. Finally, lessons learned and options for improving the health of these and similar rivers are discussed. What are the issues? The form and health of riverine ecosystems is influenced by diversity and extent of physical habitat. In this context physical habitat includes the size and shape of the channel, the form of the bed sediments, the quality of the water and the supply of nutrients, all of which Figure 1: Location map showing the Murray-Darling Basin, and the Murrumbidgee and Namoi Catchments are influenced by the supply and characteristics of the sediments. The size and shape of the channel is largely determined by the sediment supply, flow regime and channel gradient. Significantly changing the sediment supply or type of sediment delivered to a channel changes the channel and bed morphology. For example increasing the volume of sand delivered to the river alters the gross morphology of the channel, decreasing the amount of geomorphic complexity by filling pools, and smothering large woody debris. As there is a strong correlation in streams between habitat complexity and species diversity (Giller et al. 1994; Huston 1994; Palmer 1997), decreasing the amount of geomorphic complexity decreases the biotic diversity. 5

7 Previously deep water holes are now covered by sheets of sand The concentration of fine sediment suspended in the water column alters the light regime and this affects the phytoplankton habitat and benthic biofilm production. The decrease in light transmission associated with increases in the concentration of suspended sediments, reduces the rate of photosynthesis, and thus the production of new algal material. Davies- Colley et al. (1992), working in New Zealand, found that an increase in suspended sediment of ~30 mg/l in a previously clear stream resulted in a 50% reduction in plant production, and that increases as low as 9 mg/l could still have a significant effect. In muddier waters, when suspended solids reach 150 mg/l, almost no light penetrates beyond 8 cm depth. The depth of light penetration controls the depth at which algae, a primary producer, can grow. In effect, high turbidity limits the volume of stream habitat that is actually available to much of the stream biota. Increases in suspended sediment concentrations also adversely affect macroinvertebrates (Newcombe 1991; Metzling 1995; Quinn 1992) and fish populations (Koehn and O Connor 1990). A significant fraction of phosphorus (P), carbon (C) and nitrogen (N) are transported in particulate form. Most of the phosphorus is associated with the surface coating on the mineral grains, and a large proportion of this is ultimately bioavailable (Cullen 1995). Nitrogen and carbon are associated primarily with particulate organic matter that is transported in association with the mineral sediment. The supply of C, N and P to downstream reaches is intimately linked to the transport and transformations of the organic and inorganic sediments. In-channel chemical transformations and bacterial processes change the bioavailability of these nutrients in the river. Suspended and bed sediments can act both as sinks and sources of nutrients depending on the prevailing environmental conditions. In addition to their influence on the physical habitat, many pesticides and heavy metals are transported into river systems in association with sediments. By their very nature these are toxic to a variety of life-forms. Clear flowing streams are now turbid 6

8 Upper catchment wetlands which were once extensive, are now rare. Land-use changes associated with European settlement significantly increased the supply of both fine and coarse sediment to rivers in the Murray-Darling Basin. This has completely altered the form and diversity of the physical habitat along these rivers. Many reaches are now drowned in sand, and waters that were previously clear are now highly turbid. The balance of nutrients delivered to the rivers has also been altered. Wetland habitats in the upper catchment have been lost. Erosion which was triggered more than 100 years ago continues to supply sediments at rates greater than those prior to settlement. Most valleys now contain extensive gully networks 7

9 2. The Murrumbidgee The river today The Murrumbidgee catchment can be divided into three distinct geomorphic regions (Figure 2); the upper, mid, and lower Murrumbidgee (Page 1994). The highest point in the catchment is about 1900 m above sea level, falling to about 55 m at its junction with the Murray River. The upper region is mountainous and hilly. Grazing is the main land use in this region. The steeper areas are still covered by native vegetation. Most of the valley bottoms in the cleared areas of the upper catchment are deeply gullied (Figure 3). Tantangara Dam (Figure 2), which was built in 1960, now traps all of the sediments derived from the highest reaches of the catchment; and 99% of the flow is diverted. The channel bottom in the reach immediately below this dam, down to the confluence with the Numeralla River, consists of coarse gravels which are heavily coated with fine organic material. The sediment supply in this reach is low. At the Numeralla confluence the volume of sediment delivered to the main channel increases markedly. Large sand deposits are present along the lower reaches of most of the major tributaries, and much of the main channel. Above Burrinjuck Dam, sediment supply to the main channel of the river is also affected by dams on the Cotter River, Molonglo River and Ginninderra Creek. About 25% of the total catchment area above Burrinjuck Dam is separated from the main channel by water storage dams. The floodplain along the main channel of the Murrumbidgee River in the upper catchment tends to be narrow (<500 m) with much of the river confined between steep hillslopes. Redbank Weir BALRANALD MURRUMBIDGEE RIVER GUNDAGAI Jugiong Creek CANBERRA Legend Subcatchments Lower Middle Upper Reservoirs Weirs NARRANDERA WAGGA WAGGA Blowering Reservoir Tumut River Kilometers Burrijuck Reservoir COOMA Numeralla River Tantangara Reservoir Tugg. Ck. Figure 2: Murrumbidgee Catchment showing the lower, middle and upper subcatchments. 8

10 The two large storage reservoirs, Burrinjuck and Blowering (Figure 2) separate the upper region from the mid-region. These dams now trap most of the sediment delivered from the upper basin (Wasson et al. 1987). The midbasin area is characterised by undulating terrain dissected by numerous gully networks (Figure 3). Most of the major tributaries of the mid to lower Murrumbidgee join the river in this region. The major land uses within the tributary catchments are cultivation for wheat and cereals (~22 % of the area), and grazing (59 %). The remaining land is forested, some of which is harvested. The floodplain between Gundagai and Wagga Wagga is ~1 to 2 km wide (Page 1994). Sand deposits are present along much of the channel. The turbidity of the water in the Murrumbidgee in this area is highly variable, with the highest turbidities tending to be coincident with discharge events in the tributary catchments below Burrinjuck and Blowering Dams (Figure 4). The peak turbidities tend to occur on the rising limb of the hydrographs. In the lower catchment, downstream of Wagga Wagga, the floodplain widens to about 20 km and the river becomes highly sinuous as it crosses the Riverine Plains. The gradient in the lower catchment is low (<0.01%), and large sand deposits are present in the channel downstream to Hay. These sand deposits are deepest in the reaches just below Narrandera, where aggradation of the river bed is resulting in significant channel widening. The floodplain below Wagga Wagga is marked by a large number of relict channels and billabongs (Schumm 1968; Page Legend Elevation (m a.s.l.) Murrumbidgee River Gullies Reservoirs Kilometers Figure 3: Map of the mid and upper Murrumbidgee regions showing the extent of the gully networks. Note that gullies in the ACT were not mapped. 9

11 et al. 1991). There is considerable retention of water on the floodplain and much of the water which leaves the channel during floods is not returned to the river when the flood waters recede (Olive et al. 1996). The only major tributary in the lower basin is the Lachlan River, which joins the Murrumbidgee between Hay and Balranald. The Lachlan passes through an extensive area of low gradient marshes before it enters the Murrumbidgee River. Flow is dissipated in this region, and consequently the Lachlan only contributes flow to the Murrumbidgee during infrequent large flood events. The water in the lower catchment is generally more turbid, and the turbidity is less variable than in the mid-catchment (Figure 4). In the lower river the peaks in turbidity are generally not as strongly associated with peaks in discharge. During flood events peak suspended sediment concentrations, and turbidity, generally increase along the river to Wagga Wagga, and then gradually decrease further downstream (Figure 5). In non-flood periods both the suspended sediment concentrations (Figure 5) and turbidity Figure 4: Turbidity along the Murrumbidgee River NTU Tumut NTU Jugiong Turbidity NTU NTU Arbitrary Gundagai Wagga Wagga Hay Discharge Ml (x10 4 ) Turbidity (brown lines) and daily discharge (blue line) at stations along the Murrumbidgee River for the year The turbidity data is from the local government water treatment plants and the discharge data from the adjacent DLWC gauging stations. NTU Balranald Days since

12 Figure 5: Suspended sediment concentrations along the Murrumbidgee River Suspended sediment mg/l Distance downstream of Burrinjuck (km) In non-flood periods the suspended sediment concentrations along the river increase downstream. These samples were collected during both irrigation and non-irrigation periods between 1990 and During floods peak suspended sediment concentrations generally increase to a maximum around Wagga Wagga then decrease downstream. The data shown is from the flood in August increase downstream. A strong rainfall gradient exists across the basin; average annual totals vary from 1600 mm in the Snowy Mountains to 300 mm at Balranald. Prior to regulation the river experienced extreme variations in annual flow, with most of the flow concentrated in winter and spring (Ebsary 1994). The river is now a major source of irrigation water and flow is heavily regulated, both by the two major dams, Burrinjuck and Blowering, and by a series of weirs below Wagga Wagga (Figure 2). The combination of regulation and extraction for irrigation has resulted in flow along the mid reaches of the river now being spring and summer dominated when water demand by irrigators is highest. Before regulation this period was dominated by low flows. Regulation of the flow in the river involves daily assessment of the irrigation needs, tributary inflows, transmission losses, and travel times. The aim is to release sufficient water from the upper catchment dams to satisfy all of the downstream requirements, and also provide a minimum flow of 125 ML/day at Balranald (Ebsary 1994). Most of the water extraction for irrigation occurs around Narrandera. Floods most commonly occur in winter/autumn but they can happen throughout the year. The impact of regulation on flood events varies with flood magnitude; with the occurrence of small to moderate floods now reduced (Page 1988). Downstream trend in discharge and sediment transport A suspended sediment and discharge budget for the Murrumbidgee River downstream of 11

13 Burrinjuck and Blowering Dams was developed by Olive et al. (1994) for the three years 1990 to 1992, using turbidity and discharge data from six locations along the river. A relationship between turbidity and suspended sediment concentration was determined for each site (Figure 6) and used to convert the daily turbidity records into daily suspended sediment concentrations. Sediment concentration was then multiplied by the daily flow to give daily suspended sediment load at each location. The annual suspended sediment load was calculated from the sum of the daily loads. Downstream trends in the discharge for the period 1990 to 1992 are shown in Figure 7. A significant proportion of the annual flow over this period was derived from the two storage reservoirs, Burrinjuck and Blowering. Discharge from these reservoirs accounted for 70-75% of the flow at Wagga Wagga, where discharge was a maximum (5.74 x 10 6 ML/y). Below Wagga Wagga discharge decreases rapidly due to extraction of irrigation water and transmission losses. Flow at Hay was approximately 50% of that at Wagga Wagga, and less than 35% of the water that passes Wagga Wagga is delivered via the Murrumbidgee to the Murray River. Downstream trends in the suspended sediment load are also shown in Figure 7. Suspended sediment loads cannot be calculated for Jugiong as there is no discharge data available at this site. However, high turbidity readings at the Jugiong water treatment plant are typically associated with peaks in discharge from Jugiong Creek. This suggests that most of the load at Jugiong has been derived from Jugiong Creek. The load carried by the Tumut River at Tumut was approximately 15,500 t/y; a small fraction of the annual load at Gundagai (371,000 t/y). From Gundagai the load increased downstream to 552,000 t/y at Wagga Wagga. This increase is due to suspended sediment Figure 6: From turbidity to sediment concentration: Turbidity is a perceived lack of clarity in water (Gippel 1989). It is related to the optical properties of the water, particularly to the light scattering by solid particles. It is commonly measured using either an attenuation turbidimeter which measures light loss along a known path-length of the water, or a nephelometric turbidimeter which measures scattered light at some angle (usually 90 o ) to the beam. The most commonly used measurement units are nephelometric turbidity units (NTU) based on formazine standards. Turbidity is easy to measure and is often used as a surrogate for the concentration of suspended solids. Suspended solids (mg/l) Y = 1.79 x r 2 = 0.79 n = Turbidity (NTU) This figure shows the relationship between turbidity and suspended sediment at Balranald on the Murrumbidgee River. Solid and dashed lines are the linear regression and the 95% confidence limits respectively. Olive et al., (1994) determined the relationship between turbidity and suspended sediment concentrations at each of the water treatment plants on the Murrumbidgee River by sampling a wide range of turbidity/suspended sediment conditions over a four year period. The turbidity and suspended sediment concentrations were well correlated at all of the sites. Turbidity records from each site were converted to suspended sediment concentrations using these relationships. 12

14 from the tributaries between Burrinjuck Reservoir and Wagga Wagga. At Gundagai approximately 60% of the flow was generated from Burrinjuck Reservoir with the remainder from the tributaries. Most of the tributary flow occurred early in the event. The peak flow passing Gundagai was dominated by discharge from Burrinjuck Reservoir. There was virtually no contribution from the Tumut catchment and Blowering Reservoir. Below Burrinjuck Dam there was no regulation of flow as the weir gates were opened at all locations, and there was no abstraction for irrigation except in the lower sections of the river around Redbank Weir, where there was some flood irrigation of grazing areas and Figure 7 Figure 7: Downstream trends in discharge and the suspended sediment load in the Murrumbidgee River (after Olive el al., 1994) 13

15 wetlands. The river overtopped its banks and extended over its floodplain for the entire section from Burrinjuck Reservoir to at least Redbank weir. Hydrographs for the flood are shown in Figure 8. The response in the Tumut River was very small compared to the large releases from Burrinjuck Reservoir, the peak of which occurred on 12 th July. No flow record was available from Jugiong. At Gundagai, the hydrograph was narrow with a maximum daily flow of 2500 m 3 /s occurring on the 13 th. The peak on the 15 th at Wagga Wagga was similar to that at Gundagai. Downstream, where the river gradient is reduced and the floodplain more extensive, travel times were longer. Discharge peaked at Narrandera on 20 th and at Hay on 2 nd August. The travel time of the discharge pulse from Burrinjuck to Balranald (a total distance of 1052 km) was of the order of 4-5 weeks. Downstream of Wagga Wagga there was a steady decline in peak flow and the hydrograph becomes broader. This is associated with a considerable loss of water on the floodplain, and dispersion of the flow as it moves through the system. While the flow pulse only lasted for 3 to 4 days in the upper river, at Hay, 760 km downstream, it occurred over 10 to 12 days. The reduction of the peak and broadening of the hydrograph is exacerbated in the lower river by the effects of regulation through the weirs. Knowing that a flood is coming, the operators release water from the weir pools to minimise any local flooding. This was done at Hay, Maude, Redbank and Balranald weirs. So the flood pulse arrived on a falling river as the weirs emptied. All these factors, combined with the extraction of water for flood irrigation in the Redbank area, resulted in the flood pulse dissipating upstream of Balranald. Sedigraphs for this flood are shown with the hydrographs in Figure 8. At Jugiong, the sedigraph shows an initial peak early in the flood, associated with discharge from Jugiong Creek just upstream (peak concentration = 102 mg/l). During the dam release section of the hydrograph, the sedigraph is much less peaked, with concentrations less than 100 mg/l. The response of sediment concentration at Tumut is small and is extremely spiked with peak concentration occurring on the same day as the peak discharge. Downstream at Gundagai, the shape of the sedigraph is similar to that at Jugiong, but concentrations are higher with a peak of 234 mg/l, and the sediment peak leads the discharge peak by one day. The Wagga Wagga sedigraph shows a highly peaked response with a maximum concentration of 489 mg/l and the sediment peak occurred 2 days before the discharge peak. The recession stage of the sedigraph at this site had concentrations generally less than those of locations upstream. At Narrandera the sediment peak, now with a maximum concentration of 378 mg/l, arrived four days before the discharge peak. The sediment peak was also slightly broader, with the spike extending over 4-5 days compared to the 3 days at Wagga Wagga. The downstream trend of decreasing peak concentration and increasing attenuation continues at Hay with a peak of 220 mg/l, and the pulse now extending over 9-10 days. The sediment has increased its lead over the water pulse with the peak now leading by 6 days. While there was no identifiable water pulse at Balranald, the sediment pulse is clearly identifiable. However, it is broader, extending over days, and the peak concentration is further reduced to 161 mg/l. The suspended sediment load at Gundagai was 152,000 t, with the Tumut River contributing only 1,200 t. The load reached a maximum at Wagga Wagga of 222,500 t, with the increase in this section of the river associated with the increase in suspended solids concentration, probably due to tributary contribution. Downstream of Wagga Wagga there was a progressive decrease in the load. At Narrandera the load was approximately 37% of that at Wagga Wagga, while at Hay it was 19% and Balranald 9%. This downstream decrease in the load below 14

16 Text Box 1: Fallout radionuclides Knowing the erosion processes that generate sediments is an essential part of targeting catchment conservation works. Erosion processes include sheet erosion from paddocks, rill erosion from cultivated fields, sidewall and bottom erosion from gullies, and stream bank erosion from river channels; often all of these processes supply material but in different amounts. In a large catchment it is not possible to determine which of these erosion processes dominate by simply looking. Two radioactive elements, Cs-137 and fallout Pb- 210, have been widely used to determine the dominant erosion processes supplying sediment. Fallout 210 Pb is a naturally occurring radionuclide, formed through the radioactive decay of 222 Rn gas. The parent of 222 Rn is 226 Ra, part of the 238 U decay series; these radionuclides are present in all soils. Some 222 Rn gas escapes from the soil into the atmosphere where it decays to 210 Pb. This 210 Pb is then deposited on the soil surface, primarily by rain (Wise 1980). The maximum concentrations of fallout 210 Pb in soils are found at the surface. Concentrations then decrease to detection limits at about 100 mm depth. Cs-137 is a product of atmospheric nuclear weapons testing that occurred during the s. Initially the distribution of this nuclide in the soil decreased exponentially with depth, with the maximum concentration at the surface. However, due to processes of diffusion the maximum concentration is now generally found just below the surface in undisturbed soils. The bulk of the activity of this nuclide is retained within the top 100 mm of the soil profile. These fallout radionuclides are both concentrated in the surface soil. Sediments derived from sheet and rill erosion will be expected to have high concentrations of both these nuclides, while sediment eroded from gullies or channels have little or no fallout nuclides present. By measuring the concentration in suspended sediments moving down the river and comparing them with concentrations in sediments produced by the different erosion processes, the relative contributions of each process can be determined using a mixing model (see Text Box 2) Concentration (Bq kg -1 ) Pbex 137 Cs 15

17 Figure 8: Hydrographs and sedigraphs along the Murrumbidgee River for the large flood (1 in 12 years) in July-August and Balranald. Only a small component of the flood sediment load which passed Wagga Wagga reached Balranald where there was no overbank flow. The contribution of sediment to the Murray River was small. Dominance of sediments derived from gully erosion Wagga Wagga was a function of the decrease in both the flow and the sediment concentration. Below Wagga Wagga there was considerable deposition, especially between Wagga Wagga and Narrandera where 63% of the load was lost. In this section of the river there is a decrease in the river gradient dropping from (between Gundagai and Wagga Wagga) to (from Wagga Wagga to Narrandera). There is also a considerable enlargement of the floodplain with widths of up to 20 km, compared with the area above Wagga Wagga where the floodplain is 1 to 2 km wide. A further 18% of the load was lost between Narrandera and Hay. Finally, an additional 10% was deposited between Hay The tributary catchments of the mid- Murrumbidgee were identified by Olive et al. (1994) as the major contributors of fine grained sediment in transport in the lower Murrumbidgee River. The large dams, Burrinjuck and Blowering, trap most of the sediment derived from the upper catchments. The three major potential sources of sediment within the mid-murrumbidgee tributaries are cultivated lands (~22% of the surface area), uncultivated pastoral lands (~59%), and the numerous channels and gullies found throughout this region. Sediment yields from the forested areas are considered to be low. By comparing concentrations of the fallout radionuclides caesium-137 and lead-210 (Text Box 1) in river sediments collected downstream of Burrinjuck Dam, with sediment collected from each of the major sites of erosion, Wallbrink et al. (1997) demonstrated that most (>80%) of the sediment in transport in the lower river was derived from the erosion of channel banks and gully walls (Text Box 2). Only a small fraction of the river sediments were derived from surface erosion. The dominance of sub-soil erosion was also demonstrated by Olley and Wasson (in press). They summarized fallout radionuclide data from sediment samples collected from the catchment upstream of Burrinjuck Dam. Values for the <63 µm fraction of the river samples ranged from to Bq/kg with a mean of Bq/kg, (n=41: Table 1). The highest observed 137 Cs in the <63 µm fraction from the river samples corresponded to a maximum surface soil (to 1 cm) contribution of ~25%. On average the samples indicate that surface soil contributes only 5 to 10% of sediment to the river. 16

18 Table 1: Activity concentrations of 137 Cs in <63µm size fraction from stream samples collected from the Upper Murrumbidgee River Location n 137 Cs Bq/kg Source Range Mean Molonglo River to Wallbrink & Fogarty, 1998 Queanbeyan River to Murray et al., 1991 Murrumbidgee near Canberra to Variations in rainfall, discharge and sediment load Olive et al. (1995) examined variations in rainfall, discharge and sediment load for the period The summary statistics are given in Figure 9. The period ( ) was relatively wet with mean annual rainfall of 591 mm compared to the long term mean of 558 mm. There is considerable variation in both river flow and sediment load from year to year, with sediment load varying more than flow which in turn varies more than rainfall. The dominance of high flow years in transporting sediment is clearly shown (Figure 9) with the four years 1950, 52, 56 and 1974 contributing approximately 35% of the total sediment load for the period in less than 10% of the time. In drought years (such as 1980), the sediment contribution is small with the lowest year contributing only 0.3%. Approximately 50% of the sediment is transported in 20% of the years. Figure 9: Annual river flow and sediment loads for the Murrumbidgee River at Wagga Wagga Sediment load (t x 10 4 ) ML (x10000) Years since 1900 Variations in the annual sediment load for the period 1949 to 1992 (Olive et al., 1995). Annual flow at Wagga Wagga for the period 1900 to Summary statistics for annual rainfall, flow and sediment data for Murrumbidgee River at Wagga Wagga for period (after Olive et al. 1995) Rainfall (mm) Flow (ML) Sediment Load (t) Average Maximum Minimum Standard Deviation Co-efficient of Variation

19 Text Box 2: Calculating the contributions from the different land use types For simplicity Wallbrink et al. (1998) divided the sediment sources of the mid- Murrumbidgee into three broad categories: uncultivated lands, cultivated lands, and channels/gullies. If C u, C c, C b and P u, P c, P b represent the 137 Cs and 210 Pb ex concentrations from uncultivated, cultivated, and channel bank sources and C s and P s represent the respective total concentrations of 137 Cs and 210 Pb ex on suspended sediments, then A.C u + B.C c + C.C b = C s A.P u + B.P c + C.P b = P s A + B + C = 1 where A, B, and C represent the relative contributions from uncultivated lands, cultivated lands and channel banks, respectively. These are the unknown quantities. The radionuclide concentrations in <2 µm material from various land use and land form types in the tributaries of the mid-murrumbidgee catchments used to calculate the relative contributions are given in the table below. The subscripts are standard errors on the mean. Location Radionuclide Parameter Concentration (n) (Bq kg -1 ) Uncultivated lands 137 Cs C u Cultivated fields C c Channel/gully C b banks Suspended C s sediments Uncultivated lands 210 Pb ex P u Cultivated fields P c Channel/gully P b banks Suspended sediments P s Using these parameter values they calculated that >80% of the sediment in the Murrumbidggee River was derived from channel bank and gully wall erosion. Note: Wallbrink et al., assumed that the dominant hillslope erosion process on uncultivated land was sheet wash to a depth of 0.5 cm. If, however, rilling was a significant erosion process on hillslopes, the radionuclide concentrations from uncultivated land would be expected to be lower (due to the higher subsoil contribution), and the proportion of suspended river sediments derived from hillslopes would be higher. 18

20 The river at the time of settlement Early settlers and explorers describe the Upper Murrumbidgee catchment as having heavily wooded hills and large open grass covered alluvial plains (Starr et al. 1999). In the headwater reaches many of the valleys contained deep alluvium. In the less mountainous areas vegetated shallow valley floor depressions, swampy meadows, were common (Figure 10). Many of the creeks and streams consisted of deep pools linked by shallow vegetated riffle zones (Figure 11), commonly referred to as chains of ponds (Eyles 1977). It was at the Pack Inn, and afterwards at Lockyer 's Farm, that I first observed those highly characteristic chain of ponds, which would deserve a geological examination of months, as they are a phenomenon not to be found, to my knowledge, in any other part of the world. They are commonly round or oval basins, of from 20 to 200 feet in diameter, or length, excavated or sunk in the superficies of an alluvial soil, which is commonly of a rich kind, fed by subterraneous springs; often indeed generally very deep, and not at all to be confounded with water holes owing their origin to the accumulation of atmospheric water (Lhotsky 1835). Another common feature, often expressed as a complaint, was the fine texture of the sediment that formed the beds of the drainage systems. In the Molonglo valley Cunningham (1824) found that the banks of creeks were, in places, too steep for carts, and the channel itself too soft to cross. In contrast, the main channel of the river had beds of coarse material..we heard the Murrumbidgee murmuring over its gravelly bed (a rare occurrence in Australia). Near the hut (Ballebalaing) the river in some places hastes over a rocky or gravelly bed, in other places it presents several fine sheets of water, overhung with interesting species of Leptospermum, bordered with reeds and bull rushes... The river at this place was 30 to 40 feet broad with a very rapid current, but much interrupted by rocks of which the bed was composed... My first endeavour was to trace the Murrumbidgee upwards, which I was able to do to a distance Figure 10: Swampy Meadows sediment traps and sources of organic matter Prior to European settlement many of the headwater river valleys of the Murrumbidgee were swampy meadows rich in organic matter. Due to the high trap efficiency of these areas the sediment yield from headwater areas was low, and any mineral sediment is likely to have been derived from the surface soils. These swamp areas are highly productive in terms of bio-active carbon, including organic acids. Consequently, the organic/inorganic ratio of river sediments prior to European arrival is likely to have been high. 19

21 of about five miles. The first part of the way was rather annoying, the stream-bed of the river being covered with a high and stout grass, growing in large and thick solitary tufts. After passing this plain ground, I found the river flowing through a narrow valley... Its general direction is here N. W., although from its rather tortuous course, and the projecting angles of its banks, it is seldom visible to any considerable distance... The banks of the river are from 5 to 12 feet high, the left or westerly higher, there are plenty of fine alluvial patches, where as well as upon the adjacent rocks, a number of fine plants (Veronica n., s., &c.) are growing. The current of the stream is very rapid, although interrupted by rocky banks, with some fine Acacia, Leptospermum inclining over its pure and limpid waters (Lhotsky 1835). While noting the presence of tributary gullys, Lhotsky did not mention the confluence with the Numeralla River, now a major tributary, that would have been in the section he noted as this plain ground. When Lhotsky later encountered the Numeralla he noted that it was known as the Little River, a tributary stream of the Murrumbidgee. In dry weather, there is not a continual run, but it presents a number of different reaches of water. Ryrie (1840) noted the width of the Numeralla when he crossed it near the Murrumbidgee confluence. He wrote in his journal: The ford of the Umarally is good, with a pebbly bottom, the banks are low - width ten yards. It is occasionally flooded so as to prevent horse and teams from crossing. The Numeralla just upstream of the Kybeyan River confluence was noted by Ryrie as being very small, no more than a running creek, a yard and a half wide and there were many large and deep holes in it. Even some large streams, such as the Yass River, were chains-of-ponds. The ponds are always full of water, being supplied by springs: one of them was about Figure 11: Chain-of ponds 20 At the time of settlement many of the creeks and streams consisted of deep pools linked by shallow vegetated riffle zones, commonly referred to as chains of ponds (Eyles 1977). Sediment transport in these systems would have been inefficient with flood waters spreading out across the flood plain. Between 1840 and 1944 most of these systems incised and continuous networks of channels formed. More of the flood waters were confined within the channel banks, significantly increasing the efficiency of sediment transport.

22 fifty yards in length by twenty in breadth, but no great depth at any part. They form a chain along the plains during the wet season of the year; but during heavy rains they unite into a running stream... (Bennett1834, - referring to the Yass River). The Murrumbidgee River between Yass and Wagga Wagga is described as having a pebble and gravel bed, with sandy banks lined with trees. The view and country around are very picturesque and pretty, though not seen to most advantage from the cottage, I think. They consist of the banks of the Murrumbidgee River, here a running stream, with a good deal of water in it, even at present, flowing circuitously through a very broken country. There are a few flats, but the country is chiefly very hilly on each side, and bounded by rather high ranges; the hills are, however, very thinly timbered, and covered with grass, so that the whole view - which I think is best from the opposite range - of river meandering over a stony bed, through here and there flats, with hills and ranges such as I have described... The Murrumbidgee is a fine feature in this part of the country; dry as the season has been, it is still flowing on over a pebbly bottom and with some reaches presenting large sheets of water; on the banks and even in the bed, large trees of she-oak and flooded gums are growing. The sand in and about the banks of the Murrumbidgee river has a glittering appearance, which led many to report that gold dust abounded but in fact this was actually talc. Bennett (1834) "I now looked down upon a stream, whose current it would have been difficult to breast, and whose waters, foaming among rocks, or circling in eddies, gave early promise of a reckless course. It must have been somewhat below its ordinary level, and averaged a breadth of about 80 feet. Its waters were hard and transparent, and its bed was composed of mountain debris, and large fragments of rock. Sturt (1833). The Morumbidgee (sic) itself, from the length and regularity of its reaches, as well as from its increased size, seemed to intimate that it had successfully struggled through the broken country in which it rises, and that it would hence-forward meet with fewer interruptions to its course. It still, however, preserved all the characters of a mountain stream; having alternate rapids and deep pools, being in many places encumbered with fallen timber, and generally running over a shingly bed, composed of rounded fragments of every rock of which the neighbouring ranges were formed, and many others that had been swept by the torrents down it. (Sturt 1833).. with an increased breadth, averaging from 70 to 80 yards, presents a still, deep sheet of water to the view, over which the casuarina bends with all the grace of the willow, or the birch, but with more sombre foliage. To the west, a high line of floodedgum trees extending from the river to the base of the hills which form the west side of the valley... Below Wagga Wagga Charles Sturt noted that the bed of the river was sandy. "The river fell considerably during the night, but it poured along a vast body of water, possessing a strong current. The only change I remarked in it was that it now had a bed of sand, and was generally deeper on one side than the other. It kept a very uniform breadth of from ft - and a depth of from 4 to 20. Its channel, though occasionally much encumbered with fallen timber, was large enough to contain twice the volume of water then in it, but it had outer and more distant banks, the boundaries of the alluvial flats, to confine it within certain limits, during the most violent floods, and to prevent its inundating the country. Sturt (1833) He also described clear water at the Murrumbidgee-Murray confluence. 21

23 Sediment yield (t/yr) Its transparent waters were running over a sandy bed at a rate of two and a half knots an hour, and its banks, although averaging eighteen feet in height, were evidently subject to floods. Today river waters in this area would never be described as transparent. Before European settlement hillslope erosion rates are considered to have been low. Prosser et al. (1994) estimated that 28,500 m 3 of sediment had accumulated in the vegetated valley bottom of a 3.8 km 2 subcatchment of the Murrumbidgee River in the 3,000 years prior to European settlement; an accumulation rate of 9.5 m 3 /yr. This is equivalent to a rate of hillslope denudation, mainly by sheet and rill erosion, of ~2.5 m 3 /km 2 /yr. Neil and Galloway (1989) estimated a similar rate of soil loss of 2.8 m 3 /km 2 /yr from surveys of sediment in farm dams on the edge of forested areas that had been little disturbed. The natural pre- Figure 12: Quantitative estimates of pre- European sediment yields Catchment area (km 2 ) Pre-European sediment yields SY (t/yr) as a function of catchment area A (km 2 ) have been estimated to be: SY=1.6 A 0.79 (r 2 =0.85) This relationship was determined using stratigraphic records from 11 catchments (all less than 500 km 2 ) on the southern tablelands of New South Wales (Wasson 1994). European rate of sheet and rill erosion is therefore likely to be 2 to 3 m 3 /km 2 /yr, or 3 to 4.5 t/km 2 /yr (using a soil density of 1.5 t/m 3 ). As most headwater catchments (<100 km 2 ) had discontinuous or shallow streams running over deep alluvium with good vegetation cover (Eyles 1977; Prosser and Winchester 1996), it is likely that sediment delivery from these areas was low. Little of the material eroded from the hillslopes would have left these headwater catchments. Pre-European sediment yields as a function of catchment area (Figure 12) have been estimated by Wasson (1994). If it is assumed that this relationship holds for the entire upper catchment area then the pre-european yield at Burrinjuck Reservoir would have been very low ~0.23 t/km 2 /yr, or ~2,400 t/yr. While this relationship is based on the best available data, it is not well constrained for catchments > 500 km 2 and therefore the estimated sediment yield is only indicative. The early settlers and explorers reports of a bedrock, cobble and gravel bedded river with clear flowing water indicate that at the time of settlement the supply of fine suspended sediment was limited, and sediment transport capacity far exceeded the supply of sand. Further evidence for this comes from a 4 m deep slackwater flood deposit at the junction of Tuggeranong Creek and the Murrumbidgee River near Canberra (Figure 13). The flood deposits are derived from the Murrumbidgee River (Wasson et al., submitted) and are now being incised by Tuggeranong Creek. Dating, by optically stimulated luminescence, shows that these flood layers only began to accumulate in , some years after European arrival. Large floods were reported in 1824, 1832, 1852, 1867, 1875 and The lack of deposition associated with the earlier floods is attributed to a paucity of sediment (Wasson et al., submitted). 22

24 Figure 13: Optical dating at Tuggeranong Creek (after Wasson et al.,submitted) Optical dating provides a relatively new alternative for dating river sediments. The technique uses grains of quartz from the sediments as an indicator of how long ago they were deposited. The grains of quartz act as a clock, which begins to tick as soon as the grains are sealed away from daylight. The clock itself is read using a technique called optically-stimulated luminescence (OSL). This technique is used to estimate the time elapsed since buried sediment grains were last exposed to sunlight (Huntley et al., 1985; Aitken, 1998). Sunlight releases electrons from light-sensitive traps in the crystal defects of grains of quartz, which resets the optical stimulated luminescence (OSL) signal (the clock) to zero. When the grains are buried and hidden from sunlight, they begin to accumulate a trapped-electron population due to the effect of ionizing radiation emitted by the decay of radionuclides present in the sediments. If the dose rate is constant, then the burial time of the grains can be determined by measuring the size of the trapped electron population in the grains (equivalent to the burial dose), divided by the dose rate; such that: Burial-time (years) = Burial-dose (Gy)/Dose Rate (Gy year -1 ). Stratigraphic sections of the slackwater deposit (0 to 2.9m core 3), and underlying deposits at Tuggeranong Creek are shown above. Optical dating of the flood layers in the slack water deposit shows that these sediments only began to accumulate in AD, some 50+20years after European arrival. 23

25 Changes since settlement Europeans arrived in the 1820 s, beginning a process of change to the vegetation, soils and rivers that is still reverberating through the landscape. By 1825, sheep and cattle were grazing in the catchment (Starr et al. 1999). Millennia of Aboriginal land use practices were displaced quickly by new animals, and encroachment by new people with their settled ways. While grazing has always dominated, small areas of cereals, vegetables and orchards were planted to sustain the first settlers. Stock numbers grew rapidly, multiplying 30 fold on some properties within the first decade of settlement. Small towns developed early in the pastoral period, to service the grazing enterprise. Initially the major land-use change associated with European settlement in the catchment was the introduction of grazing stock. Clearing of the woodland areas only began in earnest after the Roberson Land Acts of 1861 (Figure 14). Shortly after the arrival of Europeans most of the vegetated headwater systems incised to bedrock and the extensive gully networks evident today formed (Figure 3). Prosser and Slade (1994) demonstrated that this incision was most likely due to disturbance of vegetation on valley floors. On grassy, undisturbed valley floors a 1 in 100 year flood has no erosional effect. However, degradation of the grass-cover by overgrazing made valley floors susceptible to erosion. These authors also argue that changes to critical shear stress for scour as a result of degradation of the vegetation had a far greater effect than increase in flow due to either climate variability or runoff increases following land use change. Domestic stock numbers in the upper catchment grew rapidly from the 1820 s, reaching a peak before 1900 (Wasson et al. 1998). The whole of the Murrumbidgee has long since been fully stocked (every four or five miles we come to a head station), at all events as far down as seventy or eighty mile below this, after which the country becomes dead level, and in wet seasons almost an entire Dense >30% cover Tall-medium trees Low trees and shrubs Grasses, forbes, herbs Open 10-30% cover Tall-medium trees Low trees and shrubs Grasses, forbes, herbs Sparse <10% cover Low trees and shrubs Grasses, forbes, herbs 24 Figure 14: Maps of vegetation in the Murrumbidgee catchment Before European settlement (top) and present day (bottom). Note the extensive clearing of open woodland area. Map source; Dyce and Dowling (1997).

26 swamp and unfit for pasturage. Walker (1838) referring to farms bordering the river in the mid Murrumbidgee catchment. The period 1830 to 1850 was a time of below average rainfall (discussed below). Given the lower rainfall and the higher stocking rates it is likely that widespread degradation of the vegetation cover along the valley floors commenced during this period. Degradation of the hillslope cover by grazing will also have occurred and this would have led to an increased the rate of hillslope erosion, possibly up to rates as high as 90 t/km 2 /yr, based on Neil and Fogarty s (1991) estimate of sediment yield from degraded native pasture. A large amount of hillslope erosion also occurred on cropping land as illustrated by this extract from a booklet by Max Leitch called Where the red gums are growing, Max was born in 1908, and in 1911 moved to a farm between Wagga Wagga and Narrandera. In 1914 the wheat growing fashion advocated by the Department of Agriculture was to grow wheat on long fallow. At the time the Junee, Ganmain and Coolamon area was regarded as the heart of the wheat farm area. About half the farm was sown to wheat on long fallow, about 30 to 50 acres was sown to oats on stubble, and an area around the farm building was set aside to run the horse team on. The oat crop was used to feed the horses, while the rest of the farm was ploughed and left to lie under long fallow, the longer the better. It was kept weed free by constant working with scarifier and harrows. Perhaps 20 to 50 sheep were also run for the farmer s meat supply. Houlaghan s Creek drains some of the area and flows into the Murrumbidgee. A heavy storm dumped over four inches of rain over the catchment area in a very short time and it is not hard to imagine the result with about half the area under long fallow. The rolling country eroded and the river virtually ran liquid mud, rose about six feet in a few hours and fell again almost as quickly. The mud killed every living thing in the river. Fish lined the banks with their heads out of the water in the morning, and were all dead floating upside down by lunch time. The fish probably died due to low oxygen levels in the water. This can happen after storm events if large quantities of organic matter are washed into the water and start decomposing, using up the dissolved oxygen and increasing the carbon dioxide levels. There is also the possibility that it was a fish kill caused by poison being washed into the river (eg. poison being used to kill rabbits). Surveyors notes and diary accounts by early settlers suggest that by the 1870s gullying was evident in many areas (Starr et al. 1999). One account by the Rev. W.B. Clarke (1860) describes. the masses of rock and earth that had been washed down to the berudba,(the Bredbo River, a tributary in the upper catchment) and also a mile or two to the southward, especially about Mr Cosgrove s, where the torrent had sought a way to the Murrumbidgee, were perfectly astonishing. It is likely that much of the early channel incision occurred at the same time as the large floods of 1852 and Once initiated most of the channel extension occurred within a couple of decades (Eyles 1977). Many of the gully networks were well developed by the early 1900s and there has been little change in the gully dimensions since aerial photography began in the 1940s (Prosser and Winchester 1996). Prosser et al. (1994) estimated that in the 50 km 2 Wangrah Creek catchment (in the upper Murrumbidgee catchment) the gully volume was 3.3 x 105 m 3 ; equivalent to ~500,000 t of sediment. Disturbance in the catchment began in 1842 and aerial photographs show that the gullies were fully formed by If all of the sediment was exported in this period the sediment yield was ~100 t/km 2 /yr, or 4,500 t/yr. Wasson et al. (1998) estimated for the 130 km 2 Jerrabomberra Creek catchment (also in the upper Murrumbidgee catchment, near 25

27 Queanbeyan) that channel extension increased sediment yields by a factor of 245 to ~980 t/km 2 /yr for the period 1850 to 1890, that is 127,400 t/yr (Figure 15). If the Wangrah Creek incision occurred in the same period 40 year period, sediment yield was ~277 t/km 2 /yr, or 12,500 t/yr. Figure 15: Quantitative estimates of sediment yields in the period 1850 to 1900 for Jerrabomberra Ck Sediment yield (t/yr) b a. Pre Catchment area (km 2 ) Estimates of sediment yield (t/yr) against catchment area (km 2 ) for the periods; (curve a) pre-1820, and (curve b) 1850 to Solid lines are lines of best fit. There is an estimated 4520 km of gully networks upstream of Burrinjuck Reservoir. Using a conservative gully cross sectional area of 8 m 2, gives a total gully volume of ~36,000,000 m 3, or ~54,000,000 t of sediment generated (using a soil density of 1.5 t/m 3 ). Not all of this sediment would have reached the main channel of the Murrumbidgee River. Large deposits of post settlement alluvium are evident along many of the tributary channels (Wasson et al., 1998). However, if we assume (i) the sediment delivery ratio at Burrinjuck was similar to the present day ratio of 0.43 (Wasson, 1994), and (ii) much of the gullying across the catchment occurred in the period 1850 to 1900AD, then at Burrinjuck Reservoir the sediment yield during the period of gullying was ~43.5 t/km 2 /yr, or 460,000 t/yr. To this can be added ~5% for sediment derived from sheet and rill erosion of the hillslopes based on Wasson et al. (1998), giving 480,000 t/yr, ~200 times the pre-european rate. Upstream of Wagga Wagga there are ~7,500 km of gully networks; applying the same reasoning as above, the sediment yield at this point in the river would be ~780,000 t/yr. Channel extension increased the efficiency with which sediment could be moved through the drainage network. As well as the incision of the headwater valleys, channel widening occurred in the larger tributaries. The Numeralla River provides the most dramatic illustration of this. In 1840 the middle reaches of the river were described as being no more than 2 m wide (Ryrie, 1840). The channel in this location now has a width of ~40 metres at the highest level of the banks (Figure 16). Starr et al. (1999) attributes these changes to local disturbance [of the riparian vegetation] and changes in the catchment discharge resulting from upstream channelisation. Prosser et al. (2000) have argued that it was primarily the disturbance of the riparian vegetation that led to this type of catastrophic channel widening. Many of the tributary catchments experienced significant channel widening between 1840 and 1944 (wide channels are evident in aerial photographs). The total extent and volume of material liberated has not been assessed, but many 100,000 s of tonnes of material would have been delivered to the Murrumbidgee River. Several reaches of the main channel of the Murrumbidgee River have also experienced significant widening. For example anecdotal reports of the reach near Tharwa (south of Canberra) indicate that the channel doubled in width from ~50 m to ~100 m, in 1852 (Moore, 1982). Effects of the 1852 flood on the Murrumbidgee River at Lanyon Estate, Tharwa are described below: (from Moore 1982, based on article written for Queanbeyan Observer in 1892) 26

28 Figure 16 : Channel change in the Numeralla River In 1840 the middle reaches of the Numeralla River were described as being no more than 2m wide and probably resembled a small stream similar to that shown here. The channel along the middle reaches now has widths of up to 40 metres at the highest level of the banks. The Murrumbidgee at Lanyon, during the years of James Wright s occupancy, consisted of large deep holes, between which the stream flowed gently over gravel beds during normal summer flow. Some of the holes were so deep that when two of the longest bamboo sticks were joined together they would not touch the bottom. The river abounded with fish and water fowl, it is on record that Michael Gallagher landed a fish that was so large that when a sapling passed through its gills and carried on the shoulders of two men the tail of the fish dragged on the ground. It was a giant Murray Cod; they inhabited the river and were known to reach weights of over 100 lbs. Nowadays the only specimens of this giant native fish are to be found in the large reservoirs of Burrinjuck Dam and the Hume Weir. The river was lined with large gum and mimosa trees along its gently sloping banks. This peaceful scene altered dramatically after the record flood of 1852, the flood that has gone down in history as the Great Gundagai flood which caused the deaths of over ninety persons in that town. The bed of the river doubled in width, steep banks arose where formerly there had been only gentle sloping banks. Many of the giant trees that lined the stream were swept away and the pebbly bed of the stream disappeared, being covered by large quantities of sand which completely 27

29 filled the whole bed of the river for several miles. The deep holes disappeared and the large fish population were swept away and never returned to this stretch of the river. Another large flood in 1860, which rose to a height of only one foot below the 1852 height, completed the destruction of the original river structure. Channel incision increases flow velocities by reducing floodplain flows, and increases the size and speed of the peak discharges in downstream areas (Shankman and Pugh, 1992; Rutherfurd et al., 1996). Discharge records are not available for the first 80 years following settlement, consequently we cannot directly determine the degree to which channel incision changed the discharge peaks. Prior to European settlement floodwaters within the headwater catchments of the Murrumbidgee River would have flowed across wide vegetated valley bottoms. Channels through these areas were shallow and discontinuous. The effect of smaller channels and the vegetated headwater floodplains would be to dissipate the energy of the floodwaters and a significant proportion of the total floodwave probably occurred overbank. Rutherfurd et al. (1996) predicted that gullying in a 5km 2 headwater catchment is likely to have increased floodpeaks by 20% and 12%, and decreased the time to peak by 24% to 20%, over the original conditions, for the one year and hundred year floods respectively. A 20% increase in the hydrograph peak would produce an increase of ~30% in sediment transport capacity (see Text Box 3) of the peak flow. Channel incision in the headwaters also increased the confinement of the flow. Flows that previously spread over 10 to 20 m of vegetated floodplain would have been confined to 2 to 3 m of new channel with little or no vegetation. Changes in flow width from m across the vegetated floodplain to a 2-3 metre wide unvegetated channel would have produced a 2 to 2.5 fold increase in the sediment transport capacity. Similarly Text Box 3: Sediment transport capacity (after Prosser et al., 2001) Sediment transport capacity is a measure of the ability of the flow to transport sediment. It is a function of discharge, the width of channel, hydraulic roughness and slope of the channel. Sediment transport capacity (Q s ) is given by: β Q s Q γ k S w w = where the constant k includes parameters describing the sediment (eg. size and density) and the hydraulic roughness, and γ and β are empirically derived exponents, Q is discharge, w is channel width and S is channel slope (to approximate the hydraulic gradient). Using exponent values from Prosser and Rustomji (2000), the sediment transport capacity of a river reach is given by Q s = kw 0.4 Q 1.4 S 1.4 channel incision along the main tributary streams, such as the Numeralla, would have increased the confinement of the flow (from a 200m width across the floodplain to a 40m wide channel) increasing the sediment transport capacity by a factor of about 2. In contrast, channel widening in the main channel of the Murrumbidgee River, where the flow was already well confined, is likely to have decreased the sediment transport capacity. In the example described above, the channel widening in the reach at Lanyon would have produced a 25% decrease in the sediment transport capacity of that reach. There have been significant variations in annual rainfall over the last 180 years with multi-decadal periods of below average and above average falls. Analysis of the records of annual rainfall at Queanbeyan (1860 to 1998), Cooma (1861 to 1989) and Wagga Wagga (1891 to 1998) using cumulative deviations from the long term annual mean, show mean annual rainfall in the period since the mid-1940s has been higher than for the 45 28

30 Figure 17: Variation in rainfall since European settlement 8 6 no data a Depth (m) b Rainfall (mm) Queanbeyan Cooma Wagga Wagga Year (a) Water level in Lake George since 1820 (after Jacobson and Schuett, 1979); (b) Cumulative deviations from the long term mean annual rainfall at Queanbeyan (between 1870 and 1998), Cooma (between 1871 and 1982), and Wagga Wagga (between 1891 and 1998) years prior to the mid-1940s (Figure 17b). This change is consistent with the changes in mean annual rainfall that occurred across much of eastern New South Wales (Pittock 1975; Cornish 1977). The water level in Lake George, a closed basin adjacent to the Upper Murrumbidgee catchment provides a longer term integrated record of changes in rainfall (Jacobson and Schuett 1979). The water level in the lake (Figure 17a above) is the product of rainfall minus evaporation and a small loss to groundwater. High lake levels are only sustained during periods of above average rainfall. The earliest part of the record from 1820 to 1839 is a period of generally high but declining water levels, suggesting below average annual rainfalls. In 1839 the lake was dry enough to drive a team across the middle (Russell 1887). This dry period lasted until about Lake levels then generally increased to a peak in 1875 and then declined, increasing again in about 1892, then a long dry spell continued to the mid 1940s, with a few wet spells lasting only a few years. For the periods 1904 to 1912 and 1930 to 1945 the lake was dry or nearly dry. A wet period then set in until the 1970s. 29

31 The variations in rainfall produce variations in stream flow (Figure 18). Riley (1988) compared mean annual flow for the dry period and the wet period for three gauging stations in the upper catchment; Cotter River, Queanbeyan River, Molonglo River. Mean annual flows for the two periods were comparable at the Cotter gauge. At the other two gauges, however, mean annual flows during the period were twice those of the earlier dry period. The ratio of mean annual rainfall at Queanbeyan for the wet and dry periods was Variations in the annual rainfall produce more marked variation in flow. Annual rainfall (Queanbeyan bowling club) and runoff data (gauge ) illustrate this point (Figure 18). Annual runoff varies as an exponential function of annual rainfall. The non-linear relationship between rainfall and runoff, and between runoff and sediment transport capacity, means that small variations in rainfall can produce a marked Figure 18: The effect of variations in rainfall on sediment transport capacity Deviation from the mean Discharge (Ml X 106) Rainfall (mm) This figure shows deviations from the long term mean annual rainfall at Queanbeyan (between 1870 and 1998). Variations in the annual rainfall produce marked variation in flow, and even more marked changes in the sediment transport capacity. Deviations from the mean annual discharge (1913 to 1975) at the Queanbeyan River gauge Year Discharge (Ml x 10 5 ) Rainfall (mm) Annual discharge varies as an exponential function of annual rainfall (left). Data from the Queanbeyan River catchment. The non-linear relationship between rainfall and discharge, and between discharge and sediment transport capacity, means that small variations in rainfall produce marked variation in sediment transport capacity. Cumulative portion of the total STC For the period 1913 to % of the total sediment transport capacity occurred in 10% of the time Fraction of time

32 variation in sediment transport capacity. Over the last fifty-five years there have been significant improvements in the way the catchment is managed. The formation of the NSW Soil Conservation Service in 1938 began a program of educating land managers in ways of minimizing soil erosion. The introduction of myxomatosis in the 1950s greatly reduced rabbit numbers, improving the vegetation cover. Many of the gullies have now stabilized (Figure 19) with more Figure 19: Revegetation of the gully networks than 50% of the gully networks in the Wagga Wagga region now well vegetated (Caitcheon, pers. comm.). In many tributary catchments in-stream wetlands have formed (Figure 16). These wetlands trap significant amounts of sediment derived from upstream (Zierholz et al. 2001). The construction of water storage reservoirs and farm dams in the catchment will also have decreased the supply of sediment along the river. Construction of water storage An actively eroding gully near Wagga Wagga NSW. Prior to European settlement, many headwater valleys with catchment areas up to 100 km 2 had no continuous streams or had a shallow stream perched above extensive alluvial deposits. Much of the gully erosion occurred soon after the valleys were first cleared and valley floor vegetation was disturbed by grazing. There is good evidence that sediment yield declines with gully age (Prosser and Winchester 1996). Colonization of the gully walls and floor by vegetation (as shown in this photograph) will significantly decrease the supply of sediment. More than 50% of the gully networks in the Wagga Wagga region are now well vegetated (Caitcheon, pers. comm.). 31

33 Figure 20: In-stream wetlands new sediment traps and sources of organic matter Over the past few decades typha, and in some instances phragmites, have been aggressively colonising tributary stream channels in the upper and mid Murrumbidgee catchment. These instream wetlands trap significant quantities of sediment (Zierholz et al., 2001), and may be serving a similar function to swampy meadows. In 1944 about 7% of the channel network in Jugiong Creek was covered by instream wetlands; this has expanded to about 25% at present. Encouraging and developing these in-stream wetlands is perhaps the best way of returning water chemistry of the Murrumbidgee River to what it was prior to European settlement. reservoirs began with the Cotter Dam in Tantangara Dam, built in 1960 as part of the Snowy Mountain Hydroelectric Scheme, now diverts 99% of the flow from the upper part of the catchment (470 km 2 ). Upstream of Burrinjuck Dam 25% of the total catchment area is now separated from the main channel by dams with a combined storage capacity of 500 GL (Figure 21). Volume of storages (GL) Years Figure 21: Changes in reservoir storage (GL) in the upper Murrumbidgee catchment upstream of Burrinjuck Dam since 1820AD. 32

34 Sediment yield (t/yr) Figure 22: Quantitative estimates of sediment yields c b a. Pre Catchment area (km 2 ) y=38x 0.91 Estimates of sediment yield (t/yr) against catchment area (km 2 ) for the periods: (curve a) pre-1820 (Wasson, 1994); (curve b) 1850 to 1900 (Olley and Wasson, in press) and 1945 to 1994 (a subset of the southern tablelands data, Wasson, 1994). Solid lines are lines of best fit. Dashed lines show the 95% confidence limits on the 1945 to 1994 regression line. Burrinjuck Dam constructed in 1925 and enlarged in 1956, and Blowering Dam, built in 1968, now prevent most of the sediment derived from the upper areas of the catchment being transported downstream. Over the last 20 years a large number of farm dams have been constructed in the tributary catchments; 4,376 in Jugiong Creek catchment alone. Between 10-20% of the tributary area is now separated from the main channel of the Murrumbidgee River by farm dams. Wasson et al., (1998) estimated that the sediment yield for the period 1945 to 1960 from the Jerrabomberra Creek catchment (a headwater catchment in the Murrumbidgee) declined by a factor of about 40, from the peak rates in 1850 to 1890, to ~24 t/km 2 /yr or 3100t/yr. This suggests a marked decline in sediment yield from these now incised valleys. Deposition at the Tuggeranong Creek site near Canberra, ceased in the s (Wasson et al., submitted). Floods of sufficient magnitude to cause deposition occurred in 1974 and The lack of deposition associated with these floods and Figure 23: Declining rate of sedimentation in Burrinjuck Dam (Starr et al. 1999) Sediment accumulation rate g/cm 2 /yr Years The sediment record in the dam begins in The sediment accumulation rate (g/m 2 /yr) shown has been derived from the analysis of many cores collected from the main basin; ~ 1km upstream of the dam wall. The cores were cross-correlated using litho-stratigraphy and diatoms. The sedimentation rate curve has two major features: three major inflow events which reworked previously deposited sediment and a declining trend in the sedimentation rate to the present (indicated by the regression line). The earliest inflow peak was in 1925 during a very large flood. The second, in 1945, was produced by a relatively small flow, after a drought, which reworked exposed reservoir sediments. The peak in 1983 was a medium flow event that also followed a drought. All three events reworked either reservoir sediment or, in the case of the 1925 flood, pre-european river terrace sediment. 33

35 erosion of the deposit suggests a decline in the amount of sediment in the main channel. Estimates of sediment yield derived from surveys of farm dams and storages constructed since ~1945 in the local region are shown against catchment area in Figure 22 (this is a subset of the southern uplands data; Wasson, 1994). The current specific sediment yield to Burrinjuck Dam is ~18.6 t/km 2 /yr, or 250,000 t/yr; a factor of ~2 lower than the rate and ~100 times higher than the pre-european rate. Sediment cores collected from Burrinjuck Dam indicate that the overall rate of sediment accumulation in the dam is declining (Figure 23). The peaks in the sediment accumulation record in 1925, 1945 and 1983 are associated with inflow events which reworked upstream dam sediment following dry periods. The rates of accumulation were higher in the early to mid 1940s, the late 1950s to early 1960s and the late 1960s to early 1970s. The construction of 500 Gl of water stored upstream of Burrinjuck Dam which now Figure 24: 50 years of rainfall and turbidity data effectively traps material derived from 25% of the catchment area is likely to be a significant factor in the declining sedimentation in the dam. Stabilisation of the gully networks and better land management are also likely to have contributed. Daily turbidity measurements have been made at the Wagga Wagga water treatment plant since Measurements are made by visual comparison with a set of standards. Turbidity has been measured at Gundagai since 1990 using a Hach turbidity meter. Turbidity at both sites is linearly correlated with sediment concentration (Olive et al., 1996). Conversion of the turbidity data to sediment concentration introduces another variable into the analysis of trends in the records. Consequently monthly flow weighted turbidity has been used. This is equivalent to the average turbidity observed per unit volume of water in the month, and equates to the mean monthly sediment concentration. The two records have been cross calibrated and are shown together in a) Rainfall (mm) a) Total monthly rainfall at Wagga Wagga for the period since December 1948 (dark blue lines). The mean monthly rainfall for this period is shown as the blue line. The light blue line represents a 2.5year running average. b) Turbidity Sediment accumulation g/cm 2 /yr b) Monthly flow weighted turbidity for Wagga Wagga (brown) and Gundagai (green). The thick black line shows the 2.5 year running average on the combined turbidity data. For comparison the light blue line shows the 2.5year running average monthly rainfall and the grey line the sediment accumulation rate in Burrinjuck Dam. months since

36 Figure 25: Turbidity and mid-region tributary catchment flows since Turbidity Trib. flow (Mlx1000) Proportion of flow derived from the dams months since Jan Top: Monthly flow weighted turbidity for Wagga Wagga and Gundagai (thin brown line) for the period since The thick brown line shows the 2.5 year running average on the combined turbidity data. The recent decline in turbidity is correlated with a decrease in flows derived from the tributary catchments below the dams (thin blue line). Note also that the high turbidities occur at the same time as flow events in the tributary catchments. The thick blue line shows the 2.5 year running average on the tributary flow data; the light blue line shows the 2.5 year running average on the rainfall data. Bottom: The recent decline in turbidity is also related to an increase in the relative proportion of the water derived from the dams. Figure 24b. As with the Burrinjuck sediment accumulation record the turbidity data shows an overall decline in sediment supply over the last 50 years (indicated by the 2.5 years running average). The 2.5 year running average turbidity and rainfall data tend to covary with increases and falls in the turbidity associated with increases and falls in the local rainfall. The recent decline in turbidity is correlated with a decrease in flows derived from the tributary catchments below the dams (Figure 25) and a related increase in the relative proportion of the water derived from the reservoirs. The high turbidities occur at the same time as flow events in the tributary catchments indicating that the tributaries are the primary source of sediment causing the higher turbidity. The decrease in flows from the tributary catchments over the last 20 years may be related to a decrease in regional rainfall (evident in Figure 25). Or it may be a result of the construction of more than 20,000 farm dams in the catchment. This is under further investigation. 35

37 3. The Namoi The river today The Namoi River is situated in the central north of New South Wales. The catchment (43,000 km 2 ) extends generally westward for 350 km from The Great Dividing Range to the Barwon River (Figure 26). The highest point in the catchment is 1400 m above sea level, falling to about 120 m at its junction with the Barwon River. Physiographically the catchment can be divided into two main regions; the mountainous eastern region and the low slope western plains. Most of the western half of the catchment and the areas south of the Namoi River, in particular the Liverpool Plains have land slopes of less than 3 degrees. (Figure 27). The catchment is a highly productive agricultural and pastoral area and has been extensively cleared. Approximately half of the catchment is grazed. Cropping occurs over approximately 25% of the catchment, centred mainly around the townships of Narrabri and Gunnedah. Cropping is particularly intense on the highly fertile basalt-derived-soils which form the Liverpool Plains (Figure 28). Cropping occurs right up to the streams in this area and little riparian vegetation remains. In his report on The condition of the Namoi River system, Thoms (1998) found that the upland sites he examined on the Macdonald, Cockburn and Peel rivers were in excellent or good condition. River conditions deteriorate downstream with all of the lowland sites being assessed as in poor condition. Degradation of the riparian zone, or complete lack of riparian vegetation, and channel erosion particularly on the Mooki River, which crosses the Liverpool Plains, and the Namoi River bordering the Liverpool Plains area (i.e. downstream of Keepit Dam), were identified as significant problems. In the steep upland areas most of the river reaches are cobble and gravel bedded; much of the Macdonald River, Peel River and upper reaches of the Coxs Creek and Mooki River are cobble bedded. Sand is present in the Peel Figure 26: Map of the Namoi catchment. 36

38 Gullies not mapped Gullies mapped Legend Elevation (m a.s.l.) Main river network Gully network Reservoirs Resevoirs Kilometers Figure 27: Map of the Namoi catchment showing elevation and extent of the gully networks. (Source; NSW DLWC) River downstream of Tamworth. In the lower reaches the beds of the Coxs Creek and Mooki River are generally clay and silt. Often the clay is present as small sand sized aggregates. Flows in the Namoi Catchment are regulated by three large water storage dams (Figure 26). Keepit Dam (427,000 ML) was constructed in 1960 on the Namoi River upstream of its confluence with the Peel River; Chaffey Dam (62,000 ML) was constructed in 1979 on the Peel River upstream of Tamworth; and Split Rock Dam (397,000 ML) was constructed in 1988 on the Manilla River. Much of the eastern upland is now separated from the middle and lower reaches of the Namoi River by these storages, which are considered to trap most of the sediment derived from these upland areas (Caitcheon et al. 1999). There is a strong rainfall gradient across the Figure 28: The Namoi catchment is a highly productive agricultural and pastoral area. Cropping is particularly intense on the highly fertile basalt derived soils which form the Liverpool Plains. 37

39 catchment with the annual averages ranging from 470 mm in the west to more than 1000 mm on the Great Dividing Range. The summer months average about 40% more rain than the winter months. Much of the summer rainfall occurs in short duration storm events. It is common for more than half of a month s rainfall to be delivered in a single event lasting one or two days. This pattern is predisposed towards flooding. Annual rainfall is highly variable, with variability increasing towards the west. Rainfall in the period 1900 to 1945 was lower than in the period 1945 to the present (Figure 29). This pattern is also present in the stream flows (Riley, 1988). Mean annual discharge for the period , in the middle reaches of the Namoi (where the changes in flow are the greatest) were 0.6 that of the subsequent period Rainfall (mm) ΣDeviations from the Mean Year Figure 29: There have been significant variations in annual rainfall over the last 110 years. Annual rainfall at Gunnedah (1892 to 2000) shown in the top figure has ranged from 248 mm in 1946 to 1134 mm in Cumulative deviations from the long term annual mean shown in the bottom figure, show mean annual rainfall in the period since the mid-1940s has been higher than for the 45 years prior to the mid-1940s. This change is consistent with the changes in mean annual rainfall that occurred across much of eastern New South Wales (Pittock 1975; Cornish 1977). 38

40 Figure 30: Bundella Creek which drains onto the Liverpool Plains. Creeks and streams in the upper catchment tend to be cobble bed and are often dry for much of the year. Most of the river flow is generated in the steep eastern upland areas where rainfall is the highest. Over 90% of the runoff comes from only 40% of the catchment area above Gunnedah. The average annual flow at Gunnedah is 770,000 ML/yr; about 6% of the average annual rainfall. Under natural conditions, river flows were summer dominated when the greatest number of storms occur. Dam operations have reduced the downstream variability of flow in the Namoi, with periods of low flow being virtually eliminated. Prior to the construction of Keepit Dam, flow in the main river ceased at least once in 24 different years over the period 1890 to The largest period of no flow, 252 days, occurred in 1902 during one of the most severe droughts experienced in New South Wales. Most of the tributary streams are also characterised by periods of no flow (Figure 30). Over the last 10 years the Department of Land and Water Conservation has measured turbidity at sites along the river. The results, biased towards the summer months when most of the samples were collected, show a pattern of increasing turbidity downstream (Figure 31). Median turbidity values increase from 12 NTU at the upper most site on the Peel River to 65 NTU at the site furthest downstream on the Namoi River. Turbidity (NTU) Coxs Creek Distance along the river (km) Figure 31: Median turbidities along the Namoi River for the period 1991 to Samples were collected weekly during the December to February period, with fortnightly samples collected for the two-month period either side of this period and monthly samples collected during the rest of the year. The numbers (1-7) shown refer to sampling locations shown on Figure

41 Downstream trends in discharge and sediment transport Caitcheon et al. (1999) developed a discharge and suspended sediment budget for the Namoi River for the years ; the longest common period of flow records for sites along the river. Most of the average annual discharge is derived from the tributaries on the steeper eastern side of the catchment (Figure 32), with 79% of the flow at Gunnedah coming from Keepit Dam and the Peel River. The western tributaries, the Coxs and Mooki Rivers, only contribute 25% of the average annual flow at Boggabri. The average annual flow increases along the river to below Narrabri. At Narrabri the river enters the Riverine Plain and the carrying capacity of the channel decreases. Near Wee Waa there are several anabranches, which leave the main channel and rejoin the river further downstream. Between Narrabri and Walgett about 22% of the average annual discharge at Narrabri is lost to the Riverine Plains. While most of the water is derived from the eastern tributaries, most of the sediment comes from the western tributaries. The Mooki River is the largest source of suspended sediment (308 kt/y) to the Namoi River. Coxs Creek also contributes a significant load (94 kt/y) of suspended material. Suspended sediment transport along the Namoi River is very inefficient. Caitcheon et al. (1999) estimate that of the 400 kt/yr of sediment delivered to the Namoi River from Coxs Creek and the Mooki River, only ~100 kt/yr passes Narrabri and less than 35 kt/y is 62 (Pian Ck) 553 (Goangra) 785 (Mollee) 21 (Maules Ck) 749 (Boggabri) 86 (Coxs Ck) 291 (Keepit) 254 (Peel R) 698 (Gunnedah) 108 (Mooki R) Figure 32. Annual Namoi River water flow budget for the longest period of common record ( ). Values are in gigalitres 40

42 Figure 33: Lower Coxs Creek after the 1997 floods. Note the extensive mud deposits along the sides of the channel. conveyed to the Barwon River. During floods on the Namoi River, large volumes of water flow across the floodplain. There is little water storage on the floodplain so most of the water that leaves the channel returns during the flow recession leaving the sediment on the floodplain (Figure 33, 34). After the floods thick deposits of sediment are evident on the floodplain and along the channel banks. Figure 34: Flood waters returning to the Namoi River near Narrabri. During floods, water which leaves the river and enters the floodplain is highly turbid. Most of the suspended sediment is deposited on the floodplain and the water which returns to the river during flow recession, has a much lower turbidity. 41

43 Erosion Sources in the Namoi Catchment Concentrations of the fallout nuclide 137 Cs (Text Box 1) have been measured in sediment samples (<10 µm) collected from throughout the stream network (Figure 35). Concentrations of 137 Cs are higher in the samples collected from the eastern upland areas, which are dominated by grazing. In samples collected from the mid to lower reaches of the Peel and Mooki Rivers and Coxs Creek the concentrations are low <1.5 Bq/kg. This has been interpreted as indicating channel bank and gully erosion dominates the supply of sediments in these rivers (Text Box 4). This finding is supported by measurements of channel erosion and suspended sediment concentrations in the Mooki River (Green et al. 1999). Eroding channel banks on Bundella Creek, Upper Namoi Catchment Kilometers 0.5 +/ / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / /_ / / / / / / / / / / /- 0.3 Figure 35: Concentrations of the fallout nuclide 137 Cs in sediment samples (<10um) collected from throughout the stream network. 42

44 Text box 4: Sediment contributions from different landuses to rivers in the Namoi basin. (from; Wallbrink, pers com.) The following presents previously unpublished data from the Namoi catchment. The method described in Text box 2 was used to calculate the different contributions of erosion from cultivated land, uncultivated land and channel/gully to sediment samples collected from major rivers across the basin (Figure 35). The 210 Pb ex and 137 Cs concentrations from the different sources, as well as that on sediments taken from each river, are presented below. Mac - Donald Peel Mooki Coxs Namoi Above Narrabri Namoi Above Walgett Tracer Activity Activity Activity Activity Activity Activity (Bq kg -1 ) (Bq kg -1 ) (Bq kg -1 ) (Bq kg -1 ) (Bq kg -1 ) (Bq kg -1 ) Uncultivated 137 Cs Cultivated Subsoils Sediments Uncultivated 210 Pb e Cultivated Subsoils Sediments Using the equation in Text Box 2; and the parameters for the different input terms given above, the relative contributions of uncultivated lands, cultivated lands and gully-walls/channel banks has been be calculated for each river. The results are given below. Landuse Macdonald Peel Mooki Coxs Namoi Above Narrabri Namoi Above Walgett (% ) (% ) (% ) (% ) (% ) (% ) Uncultivated Cultivated Channel bank This analysis assumes that sheet erosion occurs from the uppermost surface of cultivated land (top 1cm). However if it is from deeper rill erosion, say to 30cm depth, then the 210 Pb ex and 137 Cs values of the eroded sediment would be 15.5 and 4.3 Bq kg -1 respectively. This increases the contribution from that source by up to 5%. The uncertainties on all these estimates are in the order of 10%. 43

45 The river at the time of settlement The pre-settlement condition of the Namoi River catchment was described by the first explorers who entered the region in the early 19 th century. For instance, John Oxley provided the following description of the Peel River valley; This was the largest interior river (with the exception of the Macquarie and Castlereagh), which we had yet seen. It would be impossible to find a finer or more luxuriant country than it waters: north and south, its extent is unknown, but it is certainly not less than sixty miles, whilst the breadth of the vale is on a medium about twenty miles. This space between the bounding hills is not altogether level, but rises into gentle inequalities, and independently of the river is well watered; the grass was most luxuriant; the timber good and not thick: in short, no place in the world can afford more advantages to the industrious settler, than this extensive vale. The river was named Peel's river, in honour of the Right Hon. Robert Peel. Oxley (1820). the stream consisted of marl, fragments of red quartz, and other rocks. Oxley (1833) also observed turbid flows in the streams draining the plains This stream had been very recently flooded, and the water, yet muddy, had not subsided within its proper level. As in the Murrumbidgee Catchment many of the headwater areas in the less mountainous areas of the catchment consisted of vegetated shallow valley floor depressions, swampy meadows. Channels crossing the plains were smaller and less active than they are today (Figure 36). Observation of channels in less disturbed sections of Cox s Creek indicate that they did not have vertical banks and were well vegetated with native grasses (Wallbrink et al. 1999). The Namoi River itself, was lined with trees and shrubs. The channel contained numerous fallen trees, to the extent that Thomas Mitchell (1839) had to abandon attempts to sail down it. The rivers and creeks in the upper catchment are described as having cobble beds, and the beds of the streams were of every variety of pebble (Oxley 1820). Thomas Mitchell (1839) also noted that the Namoi just downstream of the junction with the Peel River was cobble bedded; the alluvial bed of Figure 36: Channels crossing the Liverpool Plains were smaller and less active than they are today. Observation of channels in less disturbed sections of Coxs Creek indicate that they did not have vertical banks and were well vegetated with native grasses. 44