Summary Progress Report on Available Data From 1999-2001, for Environment Australia
T Rose and R Bevitt
The Department of Infrastructure Planning and Natural Resources 2003
ISBN 7347 5370 5
About the report
The Federal, Victorian and NSW state governments agreed in October 2000 to release environmental flows of 21% mean annual natural flow (MANF) in the first ten years after corporatisation of the Snowy Mountains Hydro-Electric Authority. A further environmental flow release of 7% MANF will be reliant on cost savings by irrigators west of the great-dividing range. The environmental flow releases will be delivered to the Snowy River downstream of Jindabyne Dam to improve the physical and ecological integrity of the river. The first environmental flow was released from the Mowamba Weir on 28 August 2002.
The Snowy River Benchmarking and Environmental Flow Response Monitoring Project was developed to benchmark, then measure and monitor changes in the physical, chemical and biological features of the Snowy River as a result of environmental flow releases. The project's design and methods have been rigorously developed under the guidance of a Technical Steering Committee, and based on the first few years of sampling, considered capable of measuring changes after environmental flow releases. A summary of preliminary results for hydrology, geomorphology, water quality, aquatic macrophytes, macro-algae, macroinvertebrate and fish sampling are presented in this progress report for pre-flow release baseline data up to June 2001. Interpretation includes some of the effects of flow regulation from the Snowy Mountains Scheme and exploratory analyses into the adequacy of sampling design and methods.
Records from the Dalgety gauging station show that all aspects of the flow regime have been modified since the commissioning of the Snowy Mountains Scheme. Significant reductions have occurred in flow volume, magnitude and frequency of floods for all recurrence intervals, flow durations for all annual exceedance probabilities, and a complete loss of seasonal flow variability particularly the spring snowmelt.
Three geomorphic assessments are reported. First, post June 1998 sampling in the lower Snowy River at Sandy Point and Bete Bolong showed that floods with a peak discharge four times greater than the mean annual flood are important in mobilising sediment and hence, re-forming the channel boundary. Second, hydraulic modelling conducted in the upper Snowy River downstream Mowamba and Rockwell indicate that flows of 1,000 MLd-1 are theoretically capable of flushing unconsolidated fine-grained sediment laminae deposits in pools and unconsolidated very coarse sand in pools, and cobbles in riffles. This response is important because it is the size of the proposed flushing flow for the Snowy River, and therefore the model predicts channel change and sediment movement. A notable result from the hydraulic modelling is the development of a velocity reversal effect at the downstream Mowamba and McKillops Bridge sites under pre- Jindabyne Dam discharges of about 1 in 2 years on the annual maximum series. This is important because velocity reversals develop structural pools in bedrock riverbeds over geologic time, and prevent the deposition of bed load sediment in pools. Third, a maximum of 30,000 MLd-1 capacity outlet structure will provide both an adequate margin to manipulate the hydrograph shape and duration, and will satisfy the annual minimum peak flow recommendations of 20,000 MLd-1 developed by the Expert Panel to re-form the channel boundary.
Temperature and electrical conductivity (EC) were measured at Dalgety and Willis gauging stations, and as part of a pool stratification pilot study in summer 2000. Temperature exhibited strong seasonal patterns at Dalgety and Willis, with summer water temperatures high at Willis. EC levels generally correspond with discharge, increasing with flow events arising from local rainfall below Jindabyne Dam. Temperature and oxygen stratification was not prevalent in the Snowy River during the pilot study period but did occur at the sites nearest to the dam. Stratification was not present in the Thredbo or Deddick River reference sites, but strong thermal gradients occurred in the Delegate River reference site. Limited data indicate that there may be a combined effect of discharge, pool depth and pool size on stratification in the Snowy River sites nearer to the dam.
Four vegetation assessments are reported. First, the reach scale assessment allows seasonal and annual variation to be measured. It is important to have a measure of natural variability in vegetation communities to separate it from the effects of the flow releases. Native species data indicate a high component of macro-reach distribution in explaining the observed variation, and the weed flora, a strong seasonal component in explaining the observed variation. Second, emergent vegetation data support macro-reach classification and the adequacy of reference sites for comparing with test sites. Third, submerged vegetation data also support macro-reach classification and usefulness of the reference sites. Fourth, limited analyses indicate that the sampling design for macro-algae will enable the prediction of species groups that are expected to be flow-response indicators.
The macroinvertebrate fauna of pools and riffles in the Snowy River below Jindabyne Dam were very different to those sampled from reference rivers. Taxa found in the reference rivers reflected unregulated conditions, whereas the Snowy River taxa in the upland sites were more characteristic of still and slow flowing assemblages, thus reflecting the altered hydrology and habitat conditions below Jindabyne Dam.
Results of the fish assessment indicate that spatial, rather than annual variation explains the distribution of fish communities in the Snowy River. In particular, there was a clear spatial separation in the fish communities above and below Snowy Falls, in both the Snowy River test sites and reference rivers. Barriers to fish passage are detrimental to fish species in the Snowy catchment that require large -scale migration to complete their life cycle. Further studies are advised into the significance of natural barriers in the Snowy River under the reduced flows from Jindabyne Dam. Of concern is the stocking of trout and Australian bass for recreational fishing because of the potential to confound future results to the fish assessment unless specifically built into the design.
These preliminary results show that the sampling designs are adequate for detecting responses to environmental flow releases. The next progress report will include all analyses for pre-flow release data to the 28 August 2002 and will focus on the effects of Jindabyne Dam on all of the components measured, and compare these findings with the results from other scientific studies.