Techniques for Targeting Protection and Rehabilitation of Riparian Vegetation In the Middle and Upper Murrumbidgee Catchment

Transcription

1 Techniques for Targeting Protection and Rehabilitation of Riparian Vegetation In the Middle and Upper Murrumbidgee Catchment Scott Wilkinson 1, Amy Jansen 2, Robyn Watts 2, Arthur Read 1, Tristram Miller 1 1 CSIRO Land and Water, GPO Bo 1666 Canberra ACT Johnstone Centre, School of Science and Technology, Charles Sturt University, LMB 588, Wagga Wagga, NSW 2678 CSIRO Land and Water Technical Report No. 37/04 October 2004 CSIRO Land and Water

2 Copyright and Disclaimer 2004 CSIRO To the etent 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 ecept with the written permission of CSIRO Land and Water. Important Disclaimer: CSIRO Land and Water advises that the information contained in this publication comprises general statements based on scientific research. The reader is advised and needs to be aware that such information may be incomplete or unable to be used in any specific situation. No reliance or actions must therefore be made on that information without seeking prior epert professional, scientific and technical advice. To the etent permitted by law, CSIRO Land and Water (including its employees and consultants) ecludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, epenses 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. Cover Photograph: Umbango Ck upstream of Humula. Photographer: Scott Wilkinson 2004 CSIRO ISSN CSIRO Land and Water Page i

3 Techniques for Targeting Protection and Rehabilitation of Riparian Vegetation In the Middle and Upper Murrumbidgee Catchment Scott Wilkinson 1, Amy Jansen 2, Robyn Watts 2, Arthur Read 1, Tristram Miller 1 1 CSIRO Land and Water, GPO Bo 1666 Canberra ACT Johnstone Centre, School of Science and Technology, Charles Sturt University, LMB 588, Wagga Wagga, NSW 2678 Technical Report No. 37/04 October 2004 CSIRO Land and Water Page ii

4 Acknowledgements This work was funded by the National Rivers Consortium through Land & Water Australia, as part of the project Catchment Assessment Techniques to help determine priorities in river restoration. The authors thank Ian Prosser and Alistair Robertson for their leadership in the early stages of the project. We also thank Dr Jon Olley and Dr Gary Caitcheon for constructive reviews of this report. This report was prepared to assist the River Restoration project BG6_04, which is funded by the National Action Plan for Salinity and Water Quality (Murrumbidgee Catchment Management Authority 2004) CSIRO Land and Water Page iii

5 Eecutive Summary This report presents assessment techniques for setting catchment-scale priorities for protection and restoration of riparian vegetation in the Upper and Middle Murrumbidgee catchment. Erosion sources and sediment transport are assessed using the SedNet model, and the condition of eisting riparian vegetation is assessed using the Rapid Appraisal of Riparian Condition (RARC) method. The report describes and demonstrates several different ways in which the techniques can be used to set priorities, depending on the objective of the works. The SedNet assessment indicates that erosion control should target riverbank and gully erosion as the primary sediment sources. Two contrasting methods for spatially targeting bank and gully erosion control are discussed and demonstrated. One method addresses the objective of reducing total sediment supply to the river network, and the other addresses the objective of reducing suspended sediment eport from the catchment outlet. For each method, three levels of priority are defined. Tree cover mapping is used to assess riparian condition according to the RARC. Priorities for improving riparian condition follow in descending order of the amount of eisting riparian vegetation. The SedNet and RARC priorities are combined to produce overall priorities for protection and restoration of riparian vegetation. Implementing the techniques in the catchment should also involve field investigation of erosion hazard and riparian condition. CSIRO Land and Water Page iv

6 Table of Contents Copyright and Disclaimer Acknowledgements Eecutive Summary 1 Introduction 1 2 SedNet assessment of sediment budgets Method for constructing SedNet sediment budgets SedNet results for the Murrumbidgee catchment 6 3 RARC assessment of riparian condition Background on RARC Etending the RARC to enable assessment of riparian condition at catchment scales Assessment of riparian zone canopy cover in the upper and middle Murrumbidgee catchment Using riparian zone canopy cover to set priorities for protecting and restoring riparian condition 17 4 Scenarios that combine priorities from SedNet and RARC Introduction Scenarios with reducing sediment supply as the primary objective SedNet priorities for bank erosion control Gully erosion Lengths of bank and gully in each priority level Partial treatment of bank and gully erosion Predicted impact of the sediment supply scenarios Within-level priorities for improving riparian condition Within-level priorities for gully erosion control Scenarios with reducing suspended sediment eport as the primary objective SedNet priorities for reducing suspended sediment eport Lengths of bank and gully in each priority level Impact of the sediment eport scenarios Within-level priorities to improve riparian condition Scenarios with emphasis on improving riparian condition 29 5 Conclusions 30 6 References 31 i iii iv CSIRO Land and Water Page v

7 1 Introduction There is a growing worldwide interest in restoring rivers. In many cases the restoration work required eceeds the resources available. Consequently, management actions need to be targeted to achieve the greatest environmental benefit. In this report we describe two catchment-scale techniques for assessing river condition, and demonstrate their application to targeting protection and restoration of riparian vegetation in the Murrumbidgee catchment. This report builds on a considerable research effort, over more than a decade, into waterborne sediment sources in the Murrumbidgee catchment (e.g: Olley et al. 1993; Prosser and Winchester 1996; Wallbrink et al. 1998; Olley and Wasson 2003). Using sediment tracing and geomorphology techniques, this work has consistently identified riverbank and gully erosion in tributary streams as the primary sources of sediment in the Murrumbidgee catchment (Olley and Wallbrink 2004; Wallbrink and Olley 2004). This report seeks to build on the understanding of the erosion processes described above, by developing and applying the SedNet model (Sediment budgets for river Networks) to assess spatial patterns in the erosion processes within the catchment. The results are used to recommend priorities about where to implement erosion control measures, to improve riverine water quality and reduce habitat sedimentation (Section 2). There has also been a considerable research effort into the etent and causes of riparian zone degradation in the Murrumbidgee region (e.g. Robertson and Rowling 2000, Jansen and Robertson 2001). This has been accompanied by development of the Rapid Appraisal of Riparian Condition technique (RARC); a site-based method for assessing the condition of riparian zones (Jansen et al. 2004). This method has identified etensive degradation of riparian zones in the Murrumbidgee catchment due to clearing and grazing by domestic livestock. This report builds on the RARC, by using catchment-scale vegetation data to provide an assessment of riparian condition across the Murrumbidgee catchment (Section 3). This assessment is used to recommend priorities for protecting and restoring riparian vegetation to improve riparian condition. We demonstrate several ways in which the priorities from SedNet and RARC can be combined to produce overall priorities for protection and restoration of riparian vegetation across the Upper and Middle Murrumbidgee catchment (Section 4). The most appropriate way of combining these priorities will depend on the management objective. The report was written as part of the Catchment Assessment Techniques to help determine Priorities in River Restoration project. The Murrumbidgee is one of three focus catchments in which the assessment techniques are being developed and demonstrated. The NAP Project This report is intended to assist the Murrumbidgee Catchment Management Authority in setting priorities for the River Restoration Project BG6_04 (Murrumbidgee Catchment Management Authority 2004), hereafter referred to as the NAP Project. This report is also intended to assist working towards longer term management targets specified in the Murrumbidgee Catchment Blueprint (Murrumbidgee Catchment Management Board 2003). The specified outcomes of the NAP Project (Murrumbidgee Catchment Management Authority 2004) are: Primary Outcome: Improved water quality through reduced in-stream sediment (both suspended and bedload). CSIRO Land and Water Page 1

8 Secondary Outcomes: Reduction of in-stream nutrients. Improved riparian biodiversity and ecological connectivity. Increased protection of significant natural and cultural heritage sites. Increased community capacity to identify degraded river and stream reaches, develop options and implement Riparian Protection and Restoration Works Plans. The NAP Project will include approimately 150 km of protection (60%) and restoration (40%) of riparian vegetation; approimately 5 km of structural erosion control; and approimately 70 km of willow removal. In this report, we define protection to mean stock eclusion from the riparian zone by fencing, and restoration to mean revegetation of the riparian zone with trees and shrubs, in addition to stock eclusion by fencing. These are the definitions used in the NAP Project brief (Murrumbidgee Catchment Management Authority 2004). The report assists setting priorities for the primary outcome of the NAP project (improved water quality) through: SedNet assessment of sediment erosion. The report assists setting priorities for the secondary outcomes of the NAP project (improved water quality) through: Reduction of in-stream nutrients: Since a large proportion of in-stream nutrients are also delivered and transported in association with suspended sediment, the SedNet recommendations will assist meeting this outcome. Improved riparian biodiversity and ecological connectivity: RARC assessment of riparian condition has been demonstrated to be related with riparian biodiversity and ecological connectivity. The RARC will be used for setting priorities to meet this outcome. This report does not address protection of natural and cultural heritage sites. Increased community capacity to identify degraded river and stream reaches, develop options and implement Riparian Protection and Restoration Works Plans: The assessments and recommendations developed by this project will help improve the capacity of the NAP Project team and the community to undertake riparian protection and restoration. Scope of the report The recommended priorities for riparian protection and restoration are made for the entire Upper and Middle Murrumbidgee catchment (Figure 1), but not for the Lower Murrumbidgee catchment. This report presents catchment-scale priorities. Designing protection and restoration measures at the site-scale should rely on field surveys and established design procedures. No recommendations are made about priorities for structural erosion control. The scale of structural erosion control, potentially applied in sections of river as short as 100 m, is too small to be targetted on the basis of catchment-scale assessments. Field investigations should instead be used. The release of this report is timed to assist implementation of the NAP Project. Research into spatial patterns of erosion and sediment movement in the catchment is ongoing, and the recommended priorities represent our understanding to date. CSIRO Land and Water Page 2

9 JUNEE WAGGA WAGGA COOTAMUNDRA GUNDAGAI Murrumbidgee River JUGIONG LAKE BURRINJUCK YASS TARCUTTA TUMUT TALBINGO RESERVOIR Goodradigbee River CANBERRA LAKE BURLEY GRIFFIN QUEANBEYAN BENDORA DAM GOOGONG RESERVOIR CORIN DAM TANTANGARA RESERVOIR Murrumbidgee River BREDBO Legend COOMA Locations Streams Roads Reservoirs Murrumbidgee Catchment Kilometres Figure 1: Upper and Middle Murrumbidgee catchment CSIRO Land and Water Page 3

10 2 SedNet assessment of sediment budgets 2.1 Method for constructing SedNet sediment budgets The SedNet model was developed for the National Land and Water Resources Audit (NLWRA; Prosser et al. 2001) and is a physically-based process model that identifies the major sources, sinks and loads of sediment. We have applied this model to the Murrumbidgee catchment, with some modifications described below. In the model, the river network is divided into a series of links which are the basic unit of calculation for the sediment budget. The links can be seen as the individual branches of the river network (Figure 1). Each link etends between adjacent stream junctions or nodes and has a sub-catchment, which drains into the link between its upper and lower nodes. ArcInfo TM AML scripts are used to define the river network and sub-catchments from a 25 m Digital Elevation Model (DEM). The catchment area at the upstream end of first order streams is set using a threshold; 20 km 2 is used in the Murrumbidgee catchment. The river network defined in SedNet is used throughout this report as the basis for setting priorities for riparian protection and restoration. Using the ArcInfo TM environment, separate budgets for bedload and suspended sediment are then calculated for each link. The two budgets have a similar structure. The total sediment yield ( Y ) from a link is given by: Y = H + G + B + T F C R (1) Sediment inputs to each link come from hillslope ( H ), gully ( G ) and riverbank ( B ) erosion, and from upstream tributaries ( T ). Both suspended and bedload sediments are deposited in reservoirs ( R ), suspended sediment can be deposited within the link on floodplains ( F ), bedload sediment can be deposited in channels ( C ), with the remainder transported downstream and delivered to the net link ( Y ). The link budget is illustrated in Figure 2, where HSDR is Hillslope Sediment Delivery Ratio. This process is carried out in each river link of the river network, from upstream to downstream. Sediment budgets for both pre-european and present-day conditions were determined, and the present-day or contemporary budget can be considered to be a meanannual budget for the period Hillslope erosion (t/y) Tributary supply (t/y) HSDR Riverbank erosion (t/y) Gully erosion (t/y) Bedload deposition Floodplain area Downstream yield (t/y) Figure 2: Sediment input and outputs in the SedNet river link sediment budgets CSIRO Land and Water Page 4

11 Hillslope Erosion: The input from hillslope erosion is estimated using the Revised Universal Soil Loss Equation, as applied in the NLWRA (Lu et al. 2001). Most of the sediment eroded on hillslopes is trapped on the hillslope and so the sediment delivered to streams ( H ) is modified by the hillslope sediment delivery ratio (HSDR). A uniform HSDR of 0.05 is used for this study. It is assumed that all hillslope sediment contributes to the suspended sediment budget. Gully Erosion: The linear etent of gully erosion in the Murrumbidgee catchment has been mapped by the New South Wales Department of Infrastructure, Planning and Natural Resources. We assumed gully erosion was negligible prior to the arrival of Europeans, the average rate of suspended sediment supply since, from gullies in each sub-catchment ( G ), is the product of gully length ( L ), cross-sectional area ( a =12 m 2 ), the proportion of fine sediment ( p f ; set at 0.5), and average dry bulk soil density ( ρ s =1.5 t/m 3 ), divided by the average time over which gullies have developed (τ = 120 years). A historical survey in the Murrumbidgee (Wasson et al. 1998) and recent measurements (Caitcheon 2004) indicate that current gully sediment generation rates have declined from their peak; and are of the order of 50% of the long term average. Consequently, suspended sediment supply from the gullies in each sub-catchment ( G ) is estimated as: p f ρ sa G = 0.5 L (2) τ Riverbank Erosion: Bank erosion rate, BE (m/y), is determined as proportional to stream power ρ gq S, where ρ is the density of water, g the acceleration due to gravity, bf Q bf is bankfull discharge in m 3 /s and S is the energy slope approimated by the channel gradient (Rutherfurd 2000). It is known that degradation of riparian vegetation, and flow regulation that confines a greater proportion of flow within the channel, have resulted in widening of Australian river channels. We assume negligible bank erosion in the proportion of the link length that has fully intact riparian vegetation ( PR ), as determined from LANDSAT imagery with 30 m piels (Barson et al. 2000). We also reduce the bank erosion rate in narrow valleys having eposure of rock and other unerodible materials; limited measurements suggest an eponential relationship between rock eposure and floodplain width F w, (m) (Hughes et al. 2003). Thus: BE =.0001ρ gq S (1 PR )(1 ep( 0.008F )) (3) 0 bf w The coefficient was calibrated to achieve a maimum bank erosion rate of 0.5 m/y; in accordance with observations of channel widening rates in some highly eroded, steep and unvegetated foot-hill streams. Thus, the amount of suspended sediment supplied from bank erosion, B = hp f ρ sl ( BE ) where h is bank height (3 m), L is link length, and p f, ρ s are as previously defined. Bank erosion is considered to be along one bank only, generally the outside of meander bends. Summary of improvements to SedNet The Catchment Assessment Techniques project has made several improvements to the original NLWRA version of SedNet, to make it more suitable for catchment-scale priority setting. These have been applied in developing priorities for the NAP Project and are listed below. CSIRO Land and Water Page 5

12 Stream flow is predicted using a revised water balance approach (Wilkinson et al. submitted), rather than the method used in the NLWRA (Prosser et al. 2001). Riparian tree cover is mapped using 30 m resolution data from the BRS Land Cover Change Project (Barson et al. 2000), rather than the 250 m data used in the NLWRA. Bank erosion is based on bankfull stream power (Wilkinson et al. 2004) rather than bank full flow (Prosser et al. 2001). Floodplain etent is defined using terrain analysis of the 25 m DEM (Gallant and Dowling 2003) rather than hydraulic modelling of the 250 m DEM (Pickup and Marks 2001). Noise in channel slope predictions due to DEM irregularities has been removed. Gully density is determined directly from the mapped gully etent (DIPNR), rather than by modelling (Hughes & Prosser 2003). Suspended sediment generation from gully erosion has been reduced to 50% of the long-term average rate, because recent measurements indicate that most gullies in the catchment have declined in activity considerably since their peak of sediment production (Caitcheon 2004). 2.2 SedNet results for the Murrumbidgee catchment The defined stream network in the Upper and Middle Murrumbidgee catchment has a total length of 5,600 km. The network is composed of 745 separate river links, with 6.5 km average length. The total catchment area is 29,000 km 2, with each sub-catchment having an average area of 35 km 2. The network defines only large streams. The sub-catchments associated with each link may also contain smaller tributary streams that can be considered to have riparian zones. When each term in the bedload and suspended sediment budgets is totalled across the whole river network, the proportions of each source and loss term indicate the dominance of riverbank and gully erosion as sediment sources (Table 1). Bank and gully erosion can be grouped as channel erosion, with gullies representing small, incised streams that drain to the river network. Radionuclide tracer observations also support the SedNet prediction of channel erosion (riverbank and gully) as the dominant sediment source (Wallbrink et al. 1998). Reservoirs are the dominant sediment sinks in the catchment, trapping approimately 36% of total sediment ( %). Bedload deposition in the channel network (21%) and floodplain deposition of suspended sediment (9%) are also important sediment sinks. As a result of deposition, the amount of sediment eported from the catchment outlet is approimately one third of the sum of the total sediment input to the river network. The long-term average suspended sediment load at Wagga Wagga predicted by SedNet modelling is 578 kt/y, which is similar to the observed load of 600 kt/y (Olive et al. 1996). The SedNet stream network generally follows the topographically mapped stream network. However, there is one eception, where the trunk of the Murrumbidgee downstream of Wagga Wagga is defined as following an anabranch, rather than the main channel, for several km, due to the flatness of the DEM in this region. This error is the only one of its type detected in this catchment, and is retained in the results to illustrate the potential problem. The error results in erroneous riparian vegetation, and predicted bank erosion rate in this river link. CSIRO Land and Water Page 6

13 Table 1: Relative proportions of sediment sources and losses in the contemporary SedNet budget, totalled across the upper-mid Catchment Inputs % of total Outputs % of total Hillslope suspended supply 19 Floodplain suspended deposition Gully suspended supply 12 Channel bedload deposition 21 Gully bedload supply 24 Reservoir suspended deposition 20 Riverbank suspended supply 23 Reservoir bedload deposition 16 Riverbank bedload supply 23 Eport suspended sediment 24 Eport bedload sediment 9 Total supply 100% Total output 100 % 9 A map of bank vegetation (Figure 3) shows large areas of the river network have less than 40% tree cover along the riparian zone of each river link. For SedNet, the riparian zone etended 50 m either side of the river channel. The low levels of riparian vegetation contribute to high rates of predicted bank erosion (Figure 4), particularly in steep, non-vegetated foothill areas. The Monaro tablelands area upstream from Canberra was naturally grassy rather than tree-covered, and it could be argued that it is incorrect to predict high bank erosion rates along these naturally grassy streams. However, the channel network there is now incised, where it was once swampy meadows, and so fencing and vegetation cover is now required to prevent erosion. Hillslope erosion (Figure 5) is predicted to be low in the alpine and other forested areas, and high in the Monaro and intensively farmed tableland areas in the Murrumbidgee valley. Gully density (Figure 6) shows high rates on the Monaro upstream of the ACT, in the Yass, Jugiong and Tarcutta catchments. Across the catchment a total length of 7,500 km of gully supplies slightly less sediment than is supplied from bank erosion along the 5,600 km of river. Controlling gully erosion, particularly in areas of high gully activity, should therefore be an important part of efforts to improve water quality in the catchment, as discussed in Section 4. Suspended sediment load in the river network (Figure 7) is highest along the main Murrumbidgee channel, both upstream and downstream of Burrinjuck Dam. At the scale of individual links, there are considerable uncertainties in each of the budget terms. The sources of uncertainty include simplifications in the erosion process algorithms, and uncertainties in the data used to parameterise the algorithms. Over larger areas, the uncertainty is smaller, due to spatial averaging. Comparisons between predictions and measured observations suggest that the uncertainty in each erosion process is less than 30% for catchments larger than 3,000 km 2. Implications of uncertainty are discussed in Section 4. CSIRO Land and Water Page 7

14 COOTAMUNDRA JUNEE JUGIONG YASS WAGGA WAGGA GUNDAGAI TARCUTTA TUMUT CANBERRA QUEANBEYAN BREDBO COOMA % Riparian Vegetation Kilometres Figure 3: Proportion of riparian vegetation used in SedNet CSIRO Land and Water Page 8

15 COOTAMUNDRA JUNEE JUGIONG YASS WAGGA WAGGA GUNDAGAI TARCUTTA TUMUT CANBERRA QUEANBEYAN BREDBO COOMA Bank Erosion (m/y) Kilometres Figure 4: Mean annual bank erosion rate predicted by SedNet CSIRO Land and Water Page 9

16 COOTAMUNDRA JUNEE JUGIONG YASS WAGGA WAGGA GUNDAGAI TARCUTTA TUMUT CANBERRA QUEANBEYAN BREDBO COOMA Hillslope Erosion (t/ha/y) Kilometres Figure 5: Mean annual hillslope erosion rate used in SedNet CSIRO Land and Water Page 10

17 COOTAMUNDRA JUNEE JUGIONG YASS WAGGA WAGGA GUNDAGAI TARCUTTA TUMUT CANBERRA QUEANBEYAN BREDBO COOMA Gully Density (km/km2) Reserves Kilometres Figure 6: Gully density in each sub-catchment, calculated from DIPNR gully mapping. CSIRO Land and Water Page 11

18 COOTAMUNDRA JUNEE JUGIONG YASS WAGGA WAGGA GUNDAGAI TARCUTTA TUMUT CANBERRA QUEANBEYAN BREDBO COOMA Suspended Sediment Load (kt/y) Kilometres Figure 7: Mean annual suspended sediment load predicted by SedNet CSIRO Land and Water Page 12

19 3 RARC assessment of riparian condition 3.1 Background on RARC The Rapid Appraisal of Riparian Condition (RARC) was developed as a tool for on-ground assessment of riparian habitats. It is simple and relatively quick to apply in the field, and studies have shown there is a clear relationship between the RARC and more detailed measures of riparian zone biodiversity and function (Jansen and Robertson 2001a,b; Jansen et al. 2004). The RARC uses indicators to reflect functional aspects of the physical, community and landscape features of the riparian zone, as defined by Naiman and Decamps (1997). The RARC inde is made up of five sub-indices, each with a number of indicator variables (see Table 1). The indicators chosen reflect a variety of functions of riparian vegetation, including reducing bank erosion, providing organic matter and habitat for fauna, and providing connections in the landscape. Each sub-inde is scored out of 10, with a total possible score of 50 representing best condition. For details of the RARC, see Jansen et al. (2004). Table 2: Sub-indices and indicators used in the Rapid Appraisal of Riparian Condition Sub-inde Indicators HABITAT (Habitat continuity and etent) COVER (Vegetation cover, structural compleity) DEBRIS (Standing dead trees, fallen logs, leaf litter) NATIVES (Dominance of natives vs eotics) FEATURES (Indicative features) Width of riparian vegetation Longitudinal continuity of riparian vegetation Canopy (>5 m tall) Understorey (1-5 m tall) Ground (<1 m tall) Number of layers Leaf litter Standing dead trees (>20 cm dbh) Fallen logs (>10 cm diameter) Canopy (>5 m tall) Understorey (1-5 m tall) Ground (<1 m tall) Leaf litter Native canopy species regeneration Damage to regeneration Native shrub/sub-canopy regeneration Reeds 3.2 Etending the RARC to enable assessment of riparian condition at catchment scales Since many catchments are large and field time is epensive, for catchment-scale assessment we have developed a method of assessing riparian condition that does not require on-ground visits. Development of the method involved testing whether eisting vegetation cover mapping, derived from satellite imagery, and could be used to assess riparian condition as measured by the RARC. The indicators which potentially could be measured from remotely sensed data included canopy cover, riparian vegetation width and longitudinal continuity of riparian vegetation. On-ground measurements were made at 57 sites on the main channel of the Murrumbidgee River and 15 sites on tributaries in the Upper and Middle catchment (Figure 8). Using these site assessment data we investigated the relationship between the total RARC score for a site and the scores for those three indicators. CSIRO Land and Water Page 13

20 Figure 8: Location of 57 sites on the main channel and 15 sites on the tributaries of the Murrumbidgee River where on-ground measurements of RARC scores were made. Site assessments of canopy cover eplained 50% of the variance in the total RARC score on the Murrumbidgee main channel and and 86% of the variance in the total RARC score on its tributaries. Adding riparian vegetation width and longitudinal continuity of riparian vegetation increased this to between 85 and 90% respectively (Figure 9). Continuity score <1% Murrumbidgee main channel Remainder 15% Canopy cover score 50% Continuity score 2.7% Riparian width score 5.8% Murrumbidgee tributaries Remainder 9.7% Riparian width score 35% Canopy cover score 86% Figure 9: Variance eplained in the total RARC score by adding additional components of the score which could be measured remotely in the Murrumbidgee (main channel 57 sites, tributaries 15 sites). Given the ease of measuring canopy cover automatically using spatial datasets, and the large proportion of the variance in the total RARC score that it eplains, we used canopy cover alone to assess riparian condition at catchment scale. As a test of this approach, we derived canopy cover measures from satellite imagery, at the sites where on-ground measurements were made. The imagery we used for the Murrumbidgee was the BRS landcover classification of LandSat TM data with 30m piels (Barson et al. 2000). Figure 10 shows the relationship between the on-ground total RARC score, and the canopy cover derived from the satellite imagery at the same sites. Measurement of canopy cover eplained 45% of the variance in the on-ground RARC scores for the tributaries but only 5% of the variance for the main channel sites. CSIRO Land and Water Page 14

21 Murrumbidgee main channel Murrumbidgee tributaries Total RARC score Total RARC score Proportion vegetated Proportion vegetated Figure 10: Measurement of canopy cover (proportion vegetated) derived from satellite imagery in relation to total on-ground RARC scores at 57 sites on the main channel of the Murrumbidgee River and 15 sites on its tributaries. The poor relationship found for the main channel of the Murrumbidgee River is likely due to the relatively low resolution of the imagery used. In other catchments where we have access to higher resolution data, there is a strongly significant relationship between on-ground RARC scores and canopy cover measured from remotely sensed data. This suggests that the low resolution imagery is limited in its ability to detect narrow strips of trees (as they are likely to occur in riparian zones) and particularly sparsely scattered trees that are characteristic of old River Red Gums on the Murrumbidgee floodplain. It is also apparent from Figure 10 that most of the error appears at the low end of the canopy cover scale. Given these limitations, it is likely that predictions of riparian canopy cover from the BRS data will be poor for the lower, floodplain reaches of the river which have little riparian zone vegetation. However, the majority of the Murrumbidgee catchment being assessed for this study more closely resembles the tributaries than the main channel. Thus, it is likely that prediction is much better in the upper tributaries and areas with larger patches of vegetation. 3.3 Assessment of riparian zone canopy cover in the upper and middle Murrumbidgee catchment We used the BRS landcover data to assess riparian zone canopy cover for each link in the SedNet stream network (Figure 11). As for the site-scale RARC, the width of the riparian zone was defined as four times the width of stream channels, or 40 m wide for channels less than 10 m wide. Stream links entirely within conservation reserves were ecluded from the analysis. CSIRO Land and Water Page 15

22 JUNEE COOTAMUNDRA JUGIONG YASS WAGGA WAGGA GUNDAGAI TARCUTTA TUMUT CANBERRA QUEANBEYAN BREDBO % Riparian Vegetation in Riparian Zone COOMA Reserves Kilometers Figure 11: Assessment of riparian zone canopy cover across all stream links in the upper and middle Murrumbidgee catchment within riparian zones four times the channel width (or 40 m wide for channels less than 10 m wide). CSIRO Land and Water Page 16

23 3.4 Using riparian zone canopy cover to set priorities for protecting and restoring riparian condition For prioritising protection and restoration to improve riparian condition, we recommend giving highest priority to links with large proportions of eisting riparian vegetation cover. The relationship between proportion of canopy cover and RARC score described above means that this approach is targeting works to links with generally the best eisting riparian condition, as assessed by RARC. This approach is in line with the protect and conserve first, restore and rehabilitate second approach outlined by others (eg. Rutherfurd et al. 2000, Bennett et al. 2002). It is based on the principle that it is usually more cost-effective and efficient to protect the sections of rivers that remain in good condition, rather than to undertake epensive restoration works to fi badly damaged reaches. The following priorities are recommended: River links having the highest riparian zone canopy cover (>80% canopy cover) are given the highest priority. This follows the principle that we should protect the streams that are in the best general condition before trying to improve those that are in poor condition (Rutherfurd et al. 2000, Bennett et al. 2002). The prioritisation then continues into each subsequent category: 60-80%, 40-60%, 20-40%, and finally <20% canopy cover. It follows the principle that it is better to protect and improve deteriorating reaches rather than re-vegetating reaches that are in very poor condition and have little chance of recovering without intervention over time (Rutherfurd et al. 2000). We have assumed reserves to be low priority for protection and restoration by the NAP Project. However, reserves equate to the category zero (Rutherfurd et al. 2000), and there may be threatening processes in these areas, such as feral animals and fire, which need to be managed. Limitations of this assessment: The priorities are based on the assumption that highly vegetated links are in better condition. In reality, there may be valuable remnants of riparian vegetation in links with a low vegetation proportion. Small amounts of vegetation in one link may abut larger areas in adjacent links, and so gain increased priority for protection. The priorities are based on present condition only and do not consider the trajectory of condition (deteriorating or recovering). There is no scope to identify rare or endangered organisms or communities that require protection (Category 1 and 2 reaches, Rutherfurd et al. 2000) The assessment is based on tree cover and will not identify areas of heath or grassland that should be protected by fencing. The assessment only identifies eisting protection in conservation reserves. Outside of reserves, the recommendations identify priority areas for protection and restoration, assuming that fencing or revegetation has not already been carried out. The vegetation data used is also limited in its ability to detect narrow strips of trees or sparsely scattered trees. Of course, narrow strips of vegetation have less integrity and functional value than wider strips. The imagery used in this assessment does not provide information on size of trees or density of tree cover. CSIRO Land and Water Page 17

24 4 Scenarios that combine priorities from SedNet and RARC 4.1 Introduction This section demonstrates several ways to combine the priorities recommended by SedNet and RARC assessments, to provide overall priorities for protection and restoration of riparian vegetation. We have developed separate priorities for erosion control, and for improving riparian condition. The most appropriate way to combine these in the Murrumbidgee catchment will depend on the management objectives that are being pursued. There are three types of scenario described. Section 4.2 describes scenarios with reducing total sediment supply to the river network as the primary objective, and improving riparian condition as a secondary objective. Section 4.3 describes scenarios with reducing suspended sediment eport from the catchment as the primary objective, and improving riparian condition as a secondary objective. Section 4.4 describes scenarios with greater emphasis on improving riparian condition. The scenarios are based on the following assumptions: 1. The recommendations etend to a much larger amount of riparian revegetation than may be achieved by the NAP Project and they are intended to be useful for a significant proportion of the Murrumbidgee Catchment Blueprint, which aims to revegetate 1,500 km of riparian zone over 10 years. 2. The vegetation cover data does not indicate whether vegetation is fenced or unfenced. Therefore, the priorities indicate on which links protection and rehabilitation is required, assuming it has not already occurred. 3. We assume that protection and restoration inside conservation parks and reserves is of lower priority than elsewhere, given the protection already in place here. 4. We assume that willow removal will be followed by planting of native vegetation, and that it therefore has no long-term effect on erosion rates. 5. There is some uncertainty in the recommendations caused by differences in actual vegetation etent from the dataset used. This is due to inadequate data resolution, or clearing and revegetation since the data was constructed (1995). 6. It is assumed that restoration will return riparian zones to good condition. It is not known how long this will take or whether restoration can lead to fully functional riparian zones. 4.2 Scenarios with reducing sediment supply as the primary objective These scenarios assume that the channel erosion control function of riparian vegetation is always of higher priority than its biological function. We define primary priority levels to reduce total sediment supply from riverbank and gully erosion. We then use RARC to define secondary priority levels within the primary bank erosion control levels. Such scenarios may be appropriate for the NAP Project, given its Primary Outcome is to reduce in-stream sediment and the secondary outcome is Improved riparian biodiversity and ecological connectivity. The scenarios in this section assume that the value of reducing CSIRO Land and Water Page 18

25 sediment supply (t/y) to the stream network is spatially uniform; that is there are no areas in the catchment that should receive higher priority for improving water quality. The SedNet assessment shows that bank and gully erosion are equally dominant erosion processes, supplying a similar amount of sediment to the stream network (Table 1). Therefore, we recommend that a similar proportion of investment, or approimately kilometres of erosion control, be allocated to controlling bank and gully erosion. The allocation between bank and gully erosion control should also be based on field inspections of the current severity of bank and gully erosion. We define separate spatial priorities for bank and gully erosion below, to achieve the largest reduction in sediment supply from each of these erosion processes SedNet priorities for bank erosion control To determine priorities for controlling bank erosion that are independent of eisting woody vegetation cover, we calculate the predicted rate of bank erosion in the absence of riparian vegetation ( PR = 0 in equation 3). This is termed bank erosion hazard and is determined by stream power (product of channel slope and bankfull flow rate), and the amount of erodible soil along each link (Wilkinson et al., 2004a). Bank erosion hazard represents the potential erosion rate, whether or not it has been realised to date. Bank erosion hazard is mapped in Figure 12, in three priority levels. Reserves make up a 4 th priority level, since protection and restoration is generally not required in these areas (discussed in Section 3.4). The vast majority of links are either fully inside, or fully outside reserves and links with >50% of length inside reserves are classified into the 4 th priority level. Uncertainty in bank erosion hazard It is likely that the predicted bank erosion and bank erosion hazard are under-estimates in incised streams. The bankfull stream power used to calculate bank erosion hazard assumes a constant recurrence interval for bankfull flow, while incised streams contain larger flows, and have a relatively greater stream power available to cause bank erosion. The larger bank height in incised streams will also contribute to elevated sediment supply. Bank erosion control should therefore receive higher priority in areas with deeply incised streams. Variations in erosion resistance between soil types are not accounted for in calculating bank erosion hazard. For eample, the trunk of the middle-murrumbidgee, from Burrinjuck reservoir to the catchment outlet, passes mainly through soils that are fine-grained, stable, and well consolidated compared with those along some tributary streams. This may result in relative over-prediction of bank erosion and bank erosion hazard in lowland reaches. Counter to the potential for over-prediction of bank erosion in lowland areas, the regulated flow in the trunk of the middle-murrumbidgee results in reduced flooding, and an elevated proportion of flow being transported within the channel. The constant elevated in-channel flow also kills bank vegetation, reducing the resistance to bank erosion. It is recommended that site-scale design of bank erosion control is also based on field reconnaissance of bank activity. Accounting for uncertainties in bank and gully erosion Defining only three priority levels of bank and gully erosion control is appropriate given the uncertainty in link-scale predictions. Although significant differences in the rate of bank and gully erosion are likely within each level, there is a high level of certainty that as a group, the 1 st level has considerably higher priority than the 2 nd level. CSIRO Land and Water Page 19

26 JUNEE COOTAMUNDRA JUGIONG YASS WAGGA WAGGA GUNDAGAI TARCUTTA TUMUT CANBERRA QUEANBEYAN BREDBO COOMA Bank Erosion Hazard (m/y) Reserves Figure 12: Priority levels for bank erosion control using bank erosion hazard; the mean annual bank erosion rate in the absence of riparian vegetation. CSIRO Land and Water Page 20

27 4.2.2 Gully erosion The linear etent of gully erosion in the Murrumbidgee catchment has been well mapped by DIPNR. We assume that all small streams that have significantly incised below the land surface are included in this mapping, so that all channel erosion is assumed to occur either along the defined stream network, or along the mapped gullies. The model assumes that all gullies produce sediment at the same rate per kilometre, and so all gullies are regarded (in the model) as having equal priority for stabilisation. We recommend that gully erosion control should target areas in decreasing order of gully density for several reasons: 1. Treating a large number of gullies in a small area, and across a few land-holdings, is more efficient than treating isolated gullies. 2. There is anecdotal evidence that some gullies are naturally stabilising, with a flowon effect to sediment generation. It is likely that areas of high gully density will contain a higher density of actively eroding gullies. 3. Gullies in high-density areas may also more likely to be well connected to the stream network. Gully density for each sub-catchment is illustrated in Figure 6. The highest gully density class defined in Figure 6 (>1.13 km/km 2 ) represents the first priority level for gully erosion control, and the second class is the second priority level. The third priority level (<0.78 km/km 2 ) contains all other gullies outside of reserves, and due to its size this priority level has been split into two classes in Figure 6. Only 10% of all gully length eists in the bottom class (<0.2 km/km 2 ) of the two classes that make up the 3 rd priority level Lengths of bank and gully in each priority level Table 3 shows the length of gullies and river links, and eisting riverbank vegetation in each priority level. The priority levels were defined so that the 1 st priority level contains approimately 1,000 km of river link, or 1,000 km of gully erosion, and the 2 nd priority level approimately 1,500 km of river link, or 1,500 km of gully erosion. Table 3: Lengths of gully and stream in each priority level for reducing sediment supply Priority level Vegetated river length requiring protection (km) Non-vegetated river length requiring restoration (km) Total river length (km) Total gully length (km) 1 st nd 396 1,169 1,565 1,528 3 rd 350 1,553 1,903 4,946 Total outside reserves 1,116 3,332 4,448 7,450 Reserves , Partial treatment of bank and gully erosion Ecluding reserves, the total length of riverbank is approimately 4,600 km. In addition, there is a total of 7,450 km of mapped gullies outside reserves. Together, this totals more than CSIRO Land and Water Page 21

28 eight times the 1,500 km of riparian protection and restoration proposed in the Murrumbidgee Catchment Blueprint ( ), emphasising the need to target high-priority areas to achieve maimum impact. There are several practical reasons why 100% protection and restoration of riverbank and gully, even in high-priority areas, may be unnecessary: Some of the eisting riparian vegetation may be already protected, or has a lower level of threat. Field assessment may show that some gullies have stabilised, and that stabilising only a portion of gullies will have a large impact on gully sediment supply. Access to sections of the channel banks may be difficult due to topography or lack of landholder support, and the etra cost involved per kilometre of treatment may reduce the priority of these areas. Within-link variations in the bank erosion hazard, due to variations in channel slope, channel depth, soil erodability, and location relative to the inside or outside of channel bends, means that revegetating only a part of the bank length will be sufficient to treat the highest-eroding portions, after which it may be appropriate to move to the highest-eroding channels in the net sub-catchment. The objective of protection and restoration programs under the Blueprint may include demonstrating the value of these activities across the catchment, and encouraging voluntary involvement to complete the remainder of these activities in each area. We therefore suggest scenarios where 50%, or 20% of protection and restoration are implemented in each sub-catchment. That is, 50% (20%) of eisting riparian vegetation is protected, 50% (20%) of presently non-vegetated riparian zone is restored, and 50% (20%) of gullies are stabilised, before proceeding to the sub-catchment of net priority. The lengths of riverbank protection and restoration required for each of these scenarios can be determined from Table Predicted impact of the sediment supply scenarios The impact on sediment supply from bank erosion for the 100%, 50% and 20% scenarios is shown in Figure 13. The first priority level gives greatest reduction in sediment supply from bank erosion because it targets links with high bank erosion hazard. The total length or protection and restoration for each scenario is represented by the right-hand end of the relevant line. The predicted bank erosion response is subject to some assumptions: The SedNet model assumes a 95% reduction in bank erosion when woody vegetation (tree) cover is present (Wilkinson et al. 2004). Protection (fencing) of eisting riparian vegetation has no effect on the predicted responses. In reality, ecluding stock from an already vegetated riparian zone will increase the cover of ground vegetation, and so cause a (small) reduction in bank erosion. A well grassed riparian zone will also act as a sediment filter and help to reduce delivery to the stream of sediment from hillslope erosion (Prosser and Karssies 2001). The predicted reductions in bank erosion are relative to the present-day condition. They do not account for other changes that may occur to riparian vegetation etent, from natural dieback, voluntary revegetation, landuse or climate change. Reductions in bank erosion will occur progressively over time as restored vegetation grows. Several decades will be required to achieve full response. CSIRO Land and Water Page 22

29 Bank Erosion t/y 1,200,000 1,000, , , , , st priority level 2nd priority level 20% protection/restoration 50% protection/restoration 100% protection/restoration 3rd priority level 0 1,000 2,000 3,000 4,000 5,000 Length of Works (km, both sides) Figure 13: Predicted change in sediment supply from bank erosion for implementing the protection and restoration over different percentages of each link; the response in each priority level is labelled on the 100% line. In the SedNet model, gully cross sectional area is assumed constant. This simplification means that the predicted reduction in sediment supply from gully erosion following restoration of gullies will be linear; stabilising half of all gullies will reduce the sediment supply from gullies by half. In reality, gully size and activity will be variable, and targeting stabilisation to the largest, most active gullies will produce a disproportionately large impact on sediment supply, in a similar way to bank erosion. It is therefore difficult to determine the reduction in sediment per kilometre of erosion control Within-level priorities for improving riparian condition Within each bank erosion control priority level, protection and restoration of riverbank vegetation should proceed in order of the RARC priority by starting with links having the largest proportion of eisting vegetation and proceeding to those links with least eisting vegetation (Section 3.4). For the 1 st priority level, the proportion of riparian vegetation is illustrated in Figure 14. This figure uses the same data as Figure 11, with the levels other than the 1 st bank erosion priority level greyed out. A strip of riparian vegetation, of minimum width 15 m increasing to 1.5 times the channel width for channels greater than 10 m wide, should be protected or restored to best improve riparian condition, as measured by RARC. CSIRO Land and Water Page 23