A sluggish recovery: the indelible marks of landuse change in the Loddon River catchment

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1 A sluggish recovery: the indelible marks of landuse change in the Loddon River catchment Bruce Abernethy 1, Andrew J. Markham 2, Ian P. Prosser 3, Tanya M. Wansbrough 1 1 Sinclair Knight Merz, PO Box 2500 Malvern, Victoria, 3143, babernethy@skm.com.au. 2 Hydrobiology, PO Box 2050 Milton, Queensland, 4064, andy.markham@hydrobiology.biz 3 CSIRO Land and Water, ian.prosser@lwa.gov.au Abstract The upper Loddon River Catchment is characterised by a complex geomorphology confused by the historical legacies of mining, clearing for agriculture and dam construction. This paper describes the processes acting on the catchment to erode the landscape and deliver sediments to the stream network. Our assessment of catchment processes is based on field observations and interpretation of a catchment scale sediment budget developed through application of the geomorphic process model, SedNet. Gully erosion has been the most important source of sediment input to the Loddon River (84% of sediment yield) with low combined sediment input from hillslopes and bank erosion (16%). Although the Loddon Catchment s sediment yield, per unit catchment area, is typical for southern Murray-Darling Basin (MDB) catchments and similar to the national average, the amount of sediment derived from gully erosion is some 50% more than the national average. Only a very small proportion of the sediment mobilised is exported from the study area. Of the sediment that remains, about half is permanently stored in reservoirs or on floodplains with the rest stored as bedload in stream channels. Most of inchannel sediment stores are sand that we classified as potentially mobile sand slugs. We predict that approximately 50% of stream length is impacted by sand slugs or similar instream deposition. This is approximately double the average proportion for MDB streams, making the Loddon River one of the most sand slug-impacted streams in the MDB. Although the effect of the combined disturbances of agriculture and mining has been profound, improvements in land-management practices have complemented the natural slowing of landscape adjustment and continue to assist in reducing sediment yield. The slow and episodic movement of sediment slugs in the watercourses of the upper Loddon Catchment will continue to influence channel form and stability for many years to come. Key Words Catchment management, erosion, Loddon River, SedNet Introduction This paper reports on an investigation of the upper Loddon River catchment conducted as part of ongoing natural resource management efforts in the region. Outcomes from the study have assisted the North Central Catchment Management Authority in their development of priority actions under the Loddon River Health Plan and other planning documents. The Loddon River drains a catchment of approximately 15,000 km 2. The river rises on the northern slopes of the Great Dividing Range, south of Daylesford, before flowing northward to join the Murray River 20 km north of Kerang. Prior to this study Davis (1999) undertook a scoping study of the geomorphological information resources of the study area. In her review, Davis found that despite the many works on various aspects of the catchment, there remained large gaps in our knowledge of the fluvial geomorphology of the Loddon River and its tributaries. Moreover, Davis pointed to the complex fluvial geomorphology of the catchment now confused by the historical legacies of mining, clearing for agriculture and dam construction. This paper describes the processes acting on the catchment to erode the landscape and deliver sediments to the stream network. Evolutionary phases are defined in terms of major shifts or changes in key factors that influence channel form and process such as landuse practices (e.g. gold mining), sediment yield and transfer processes and contemporary changes in rainfall patterns. From this background, the paper then considers the likely adjustment trends of the stream network and assesses the risks posed by contemporary erosion and deposition. Although we are unable to constrain the adjustment to absolute measures of change we can recognise the direction of change and make comparisons of the relative magnitude of adjustment across the catchment. Where appropriate, management issues associated with continued adjustment are discussed.

2 Study area Squatters first settled parts of the upper Loddon River Catchment (Figure 1) after Mitchell passed through the area in By 1840 a large proportion of the upper Loddon Catchment was used for pastoral activities (Schoknecht, 1988). In 1851 gold was discovered at Clunes and Bendigo, beginning the gold rush. Much of the area s forest was removed for agriculture and timber production to support the mining activities (LCC, 1978). The environmental impacts of early development during this period have been long lasting. Figure 1: The upper Loddon River catchment. The catchment is now an important agricultural region, with highly productive irrigation districts, mixed dryland cropping/grazing and increasing viticulture. Some 80% of the native vegetation in the catchment area has been cleared for grazing sheep, beef and dairy cattle, cropping (wheat, oats and barley) and fruit, grapes and oil production. Forestry (both native and plantation) occurs throughout the southern and central parts of the catchment. There are three major storages in the area: Cairn Curran; Tullaroop; and Laanecoorie Reservoirs. Chronology of adjustment Our review of the historical and anecdotal information of the upper Loddon River catchment provides a general chronology of geomorphologically significant events (Table 1, Figure 2). These events would have affected the rate of sediment yield and delivery to the channel network and sediment transport within the channel. The chronology relates to the main independent controls on channel form and indicates punctuation points in the evolving morphology of the stream network. The combined effect of rainfall/flow patterns and the level of catchment disturbance largely determined the rate of change of fluvial processes.

3 Table 1: Chronology of adjustment. Date Event Significance 1830 s First settlement. Establishment and expansion of sheep farming. Initial catchment disturbance. Increasing rates of sediment delivery to the channel network from gully and sheet erosion, and failing banks s-1850 s Damming of creeks for stock and domestic water. Local siltation, increased vegetation density and local flooding (e.g. Bullock Creek) s Start of gold mining. Timber milling commenced. Extensive clearing, destruction of riverbanks, floodplain disturbance and mobilisation of sediments to watercourses, extensive siltation s Introduction of willows and poplars. Localised stream choking and siltation, localised increase in flood frequency (e.g. Upper Loddon) Major flood. Cultivation and extensive clearing had occurred by Probably extensive sheet erosion and transport due to de-stabilised soils. Large geographic disturbance footprint. this time Generally wet years. Product of higher than average rainfall and disturbed and highly mobile soils likely caused high rates of sediment delivery and transport (e.g. anecdotal reports of crops smothered by mud in Bullabul Creek catchment during the 1870 flood). Gullying and extension of the channel network likely to have occurred s-1940 s Below-average rainfall years. As above. Below average rainfall likely resulted in lower rates of sediment delivery and transport Completion of Laanecoorie Weir. Flow regulation Failure of Laanecoorie Weir. Localised downstream flooding s Evidence of sand slugs and headward incision. Cessation of mining. Renewed clearing for agriculture. Sediment from early disturbance now in the watercourses as mobile slugs, or semi-transient deposits. Some stabilisation of side bars by vegetation. General decline in the rate of gully development and reduction in the rate of sediment yield to the watercourses although clearing continues for agricultural purposes Completion of Cairn Curran Dam. Flow regulation Completion of Tullaroop Dam. Flow regulation s Slow rate of gully development. Wetter years but an improvement in land practices including soil conservation (eg. Bradford Creek catchment). Probably overall reduced sediment loading to the watercourses s-1980 s Below average rainfall. Declining rates of sediment yield and transport s Above average rainfall. Declining rates of sediment yield and transport. Following early European occupation, very high rates of soil erosion and sediment delivery to the channel network occurred. The above-average rainfall that occurred at this time may have exacerbated the effects of catchment disturbance. A significant impact on the morphology of the catchment occurred in 1850 with the commencement of gold mining. The various methods of mining collectively had a devastating impact on the stream network causing extensive siltation and destruction of banks and inner floodplain areas. Sediment delivery to the stream network probably peaked during these years, as did the rate of gully development and stream network extension. Extensive flooding occurred in 1870 and was likely a significant event for sediment transport and river channel change. The years 1870 to 1940 were generally drier years and we speculate that at some time during this period the rate of soil erosion and sediment yield to the channel network started to decline although massive sediment deposition occurred in the watercourses. Mining generally ceased during the 1950 s and although these were wetter years it is likely that the delivery of sediment to the watercourses continued to decline. Evidence of sand slugs appears in the literature during the 1950 s; Forbes (1950) notes their existence in the upper Loddon River whilst also noting stabilisation of side bars by vegetation. We conclude from this that part of the sediment load became locked in the system in permanent or semi-transient bars, while other material may have been transported more quickly as bed material load. The gully network is thought to have stabilised by the 1960 s at which time land practice improvements and soil conservation works were noted in the literature. While the rates of sediment delivery continue to decline, slugs of sand in the tributary network continue to move along the system. Although there are limited data available with which to validate the interpretations presented in Table 1, Davis (1996) presents sequential survey data from Laanecoorie Weir, which show a consistent reduction in capacity between 1932 and Davis used these data to infer the unit rates of catchment sediment delivery to the reservoir necessary to account for this reduction. The data show a much higher rate of sediment delivery for earlier years of operation (1932 to 1945) than for subsequent years. Davis results are generally consistent with the interpretations contained in Table 1, which suggests initially high and subsequently declining rates of sediment delivery that are insensitive to phases of below-average and aboveaverage rainfall. Hence, physical disturbance to the catchment is the key driver of sediment yield and

4 rainfall is less important. Based upon the preceding interpretation, and following the method presented by Brooks and Brierley (2000), dimensionless trend lines for key catchment processes can be drawn (Figure 2). Rainfall Above average Below average Rate of disturbance Gully development Sediment supply Sediment transport Figure 2: Dimensionless trend lines. Processes This section describes the spatial and temporal distribution of geomorphic processes at a catchment-wide scale. Our assessment of catchment processes is based largely on our field observations and interpretation of a catchment scale sediment budget. The sediment budget was developed through application of the geomorphic process model, SedNet (see Prosser et al., 2001). We used SedNet to determine the spatial distribution of sediment supply and movement through the catchment and to identify the relative importance of different geomorphic processes in different parts of the catchment. The sediment budget provides an estimate of mean annual values for sediment inputs from hillslopes, gullies and bank erosion. It does not include sediment directly input to the stream system from gold mining. Eroded sediment is either exported from the catchment, or it is stored within the study area in artificial reservoirs, on floodplains or in channel bed deposits. Values in the sediment budget are averaged over a 100 year period.

5 Sediment budget Our sediment budget for the study area indicates that, over the past 100 years, gully erosion has been the most important source of sediment input to the Loddon River system, accounting for some 84% of the sediment yield (Table 2). Only a very small proportion of the sediment mobilised from source areas within the catchment is exported out of the study area. Of the sediment that has not been exported and remains in the study area, about half is permanently stored in reservoirs or on floodplains. A critical conclusion from this analysis is that half of the sediment liberated by 150 years of catchment disturbance remains stored as bedload deposition. Most of this material is sand. Over engineering timescales, inchannel sediment stores should be treated as potentially mobile. Table 2: Summary sediment budget. Budget item Mean annual rate (kt/y) (%) Bank erosion Gully erosion Hillslope erosion Total inputs Net floodplain and reservoir deposition Net bedload deposition Export Total outputs The results presented in Table 2 are consistent with the results of sediment budgets calculated for adjacent Campaspe River catchment (Davis, 1996) and the Avoca River catchment (Rutherfurd and Smith, 1992). In fact, the total amount of sediment, per unit catchment area, delivered to the Loddon River over the last century is typical for southern Murray-Darling Basin (MDB) catchments and similar to the national average. However, the amount of gully erosion is well above the national average. Nationally, the proportion of riverbank to gully erosion to hillslope erosion is 0.25:0.35:0.40 (NLWRA, 2001), while in the Loddon River catchment the proportions are 0.10:0.85:0.05. That is, gully erosion makes up 85% of sediment supplied to the Loddon River, 50% more than the national average. We predict that approximately 50% of stream length has been impacted by sand slug deposition or similar instream accumulation of sediment. This is approximately double the average proportion for MDB streams (NLWRA, 2001), making the Loddon River one of the most sand slug-impacted streams in the Murray- Darling Basin. Suspended sediment loads of the Loddon River, per unit catchment area, are below the national average but typical of the southern MDB streams that have good vegetation cover and low rainfall intensity (Wallbrink et al., 1998). Sediment inputs Hillslope erosion refers to soil erosion by sheetwash and rills. SedNet calculates the amount of sediment delivered to the stream as 10% of the gross amount predicted by the universal soil loss equation (USLE). Work by Prosser et al. (2001) showed that 90% of eroded sediment, measured at the plot scale (with the USLE), remains on the hillslope in transient storage and is not delivered into the stream network over engineering timescales. Hillslope inputs occur across a range of lithologies and soil types in the middle part of the upper catchment where landuse is relatively intense on sloping land with moderate rainfall intensity. High sediment input is largely restricted to Management Units 2-8 (MUs, see Figure 1). Gully erosion is the dominant sediment source in the catchment. A gully density map based on aerial photograph interpretation by Milton (1971) shows a very high density of gullies within the catchment; almost all gullies had developed fully by the 1960 s. High sediment inputs from gully erosion were predicted for streams in MUs 3, 6, 7, 9, 10, 11, and the upper parts of MU 14 (Figure 3). Generally, the distribution of high gully inputs coincided with the Castlemaine Group (marine sandstones) and the Shepparton Formation (fluvial sands and silts) in cleared country. Bank erosion is moderately low in comparison to other catchments because of the good riparian cover. The model predicts the rate of bank erosion to be greatest in the lower (alluvial) reaches of the upper Loddon River catchment. The occurrence of high bank inputs is largely restricted to duplex soils with its distribution within these soil types largely a reflection of local riparian condition.

6 Figure 3: Sediment inputs from gully erosion. Figure 4: Bed deposition. When inputs are combined (Table 3), we can see that MUs 3, 5, 6 and 7 have been the most affected by sediment deposition over the past 100 years. The modelled results are consistent with our field observations of large sediment deposits in the lower reaches of these MUs. SedNet predictions indicate that these reaches have only low to moderate capacity to move this sediment through the system. Table 3: Distribution of high sediment inputs 1. MU Bank Gully Hillslope Intersection 2 Union 3 km km % km % km % km % km % All Where high sediment inputs are represented by the third of streamlink length affected by the highest input values and total streamlink length assessed by SedNet is 1,131 km. 2 Length of stream affected by high ratings of all of the input types. 3 Length of stream affected by high ratings of any of the input types.

7 Sediment transport The southwest of the catchment (MUs 6 and 7) along with the mid-east (MU 3 and upper MU 14) generates much of the load observed in the lower reaches. However, the vast bulk of suspended sediment sourced from the catchment is now stored in the downstream reservoirs. Otherwise, the overall loads increase downstream as expected. Bed deposition is the mean annual excess of supply of bedload over mean annual sediment transport capacity (STC). SedNet assumes that there is no deposition where STC exceeds supply and that the river has the capacity over historical times to push through all of the bedload supplied. The model further assumes that the natural situation is one where compared to the present, the bed is in equilibrium with the load and there is no net accumulation. Importantly, Figure 4 shows the distribution of sites for the potential build up of sediment slugs. The model predicts that the Loddon River system is particularly prone to sediment slugs, because of the extensive areas of gully erosion combined with low STC. Low STC occurs in areas of low gradient and moderate annual discharge. High rates of bedload deposition and bed material transport are predicted in the western part of the catchment, particularly MUs 7 and 10. Low rates of deposition are predicted for the south and southeast part of the catchment: MUs 1, 2, 4. 5 and 8. Management While the headward extent of most gullies in the catchment has stabilised, the width depth ratios of the gully network will continue to relax overtime. Continued adjustment will occur sporadically to deliver additional sediment to the channel network. Improvements in land-management practices over recent decades complements the natural slowing of landscape adjustment and will further assist in reducing the sediment yield from hillslopes, riverbanks, and gullies. Although the rate of degradation has slowed, many aspects of catchment physiography are now irrevocably changed. Over 95% of the sediment that has been delivered to streams in the catchment over the past 100 years has remained in the study area. Much is deposited on floodplains, or trapped in reservoirs, where it will have little impact but there is also massive deposition of sand and gravel from gully and streambank erosion on the catchment s riverbeds, forming sediment slugs. The slow and episodic movement of sediment slugs will continue to influence channel form and stability for many years to come. From a whole-of-catchment perspective no single issue, or location, stands out as an immediate or obvious priority for management intervention. Hence, the management strategy for the Loddon River catchment should seek to improve river health across the entire study area (see also DNRE, 2002). Improving the health of the streams in the catchment would require the early identification and management of local bed or bank instabilities that threaten assets and instream values and will reduce the need for later more expensive interventions. Over the longterm, gradual improvement in riparian condition will see other improvements in stream condition. For the most part, improvements in stream health will not alter the long-term adjustment trajectory of the steam network. However, works will tend to reintroduce geomorphic diversity, which will increase the habitat diversity of the stream and hence improve its biodiversity (e.g. Piégay et al., 1997). Conclusions The upper Loddon River catchment has been subject to a number of landuse changes over the past 200 years. The effect of the combined disturbances of agriculture and mining has been profound. Eroded hillslopes, gullies and degraded riverbanks have left indelible marks on the landscape that have changed the channel system forever. Changed hydrology, riparian degradation, sediment slugs, stock access, and the spread of woody weeds continue to alienate or simplify habitats along the rivers and creeks of the study area. Sediment sources are now relatively stable and are not producing sediment in the quantities of the past. Just over half of the sediment mobilised over the past 200 years has entered permanent storage on the catchment s floodplains (48%) or has been exported from the study area (4%). The remainder is now in semi-permanent storage in the beds of the Loddon River s tributaries (48%). The local movement of this material remains the subject of smaller scale investigations but, for the most part, can be passively managed.

8 As the streams are now generally stable, recovery of stream health values should form the priority for treatment. The management challenges for the catchment are to improve stream health and to ensure that future local adjustments do not further threaten catchment assets. These management aims will rely on the re-planting of riparian zones throughout the catchment. Revegetation should be planned to protect or extend reaches that already support healthy riparian zones. Acknowledgments The study was initiated and funded by the North Central Catchment Management Authority. The first author also received a writing-up grant from Sinclair Knight Merz. We thank Greg Peters, Rohan Hogan and Angela Gladman, of the NCCMA, who provided valuable advice and local knowledge during the study. References Brooks, A.P. and G.J. Brierley, The role of European disturbance in the metamorphosis of the lower Bega River. In S. Brizga and B. Finlayson (eds.), River Management, the Australasian Experience. Wiley, Chichester. Davis, J.A., Catchment Management for the Control of Sediment Delivery: the Case of the Eppalock Catchment, Victoria. PhD Thesis. Department of Civil and Environmental Engineering, The University of Melbourne, Melbourne. Davis, J.A., Loddon Fluvial Geomorphological Scoping Study. The Centre for Environmental Applied Hydrology at the University of Melbourne for the North Central CMA. DNRE, Healthy Rivers, Healthy Communities and Regional Growth: Victorian River Health Strategy. Department of Natural Resources and Environment, Melbourne. Forbes, I.G., The catchment of the Cairn Curran Reservoir. Government Printer, Melbourne. LCC, Report on the North Central Study Area. Land Conservation Council, Victoria, Melbourne. Milton, L.E., A Review of Gully Erosion and its Control. Soil Conservation Authority, Melbourne. NLWRA, Australian Agriculture Assessment. National Land and water Resources Audit, Canberra. Piégay, H., M. Cuaz, E. Javelle and P. Mandier, Bank erosion management based on geomorphological, ecological and economic criteria on the Galaure River, France. Regulated Rivers: Research and Management, 13(5): Prosser, I.P., P. Rustomji, W.J. Young, C.J. Moran and A.O. Hughes, Constructing River Basin Sediment Budgets for the National Land and Water Resources Audit. CSIRO Land and Water, Technical Report 15/01. On line: Rutherfurd, I.D. and N. Smith, Sediment Sources and Sinks in the Catchment of the Avoca River, North Western Victoria. Department of Conservation and Natural Resources, Melbourne. Schoknecht, N.R., Land inventory of the Loddon River Catchment: A reconnaissance survey. Land Protection Division, Department of Conservation Forests and Lands, Victoria, Melbourne. Wallbrink, P.J., J.M. Olley, A.S. Murray and L.J. Olive, Determining sediment sources and transit times of suspended sediment in the Murrumbidgee River, NSW, Australia using fallout 137 Cs and 210 Pb. Water Resources Research, 34: