A Review of the Existing Literature on the Environmental Effects of Wyangala Dam

"August 2005".

Project Number: Wyangala Dam Literature Review; Project PNO: OD163B / Contract No. 3539.

MDFRC item.

The Lachlan Valley is one of the most important agricultural regions in Australia; it covers only 10% of New South Wales yet accounts for 14% of the state’s agricultural production. Lake Wyangala is the major water storage in the Lachlan Valley and is located downstream of the junction of the Abercrombie and Lachlan rivers, about 45 km south-west of Cowra. Wyangala Dam was originally built in 1935, but storage capacity was tripled in 1971 primarily to meet projected water demands from an expanding agricultural industry. Irrigated agriculture in the Lachlan Valley has increased substantially in the last 60 years, from a little over 13 000 ha in 1944 to about 88 000 ha in 2001. State Water Corporation is currently reviewing safety and environmental improvement options for Wyangala Dam, as part of the Wyangala Dam Upgrade Project. Safety or remedial options are concerned with meeting ANCOLD requirements for dam safety and flood handling capacity. This report investigates the environmental effects of Wyangala Dam on the surrounding environment by reviewing all available relevant literature. The evaluation of current environmental condition enables the development of a conceptual model of how Lake Wyangala functions and identification of the major environmental effects of Wyangala Dam on the surrounding environment. This report recommends environmental improvement options aimed at ameliorating the current impacts of Wyangala Dam on the surrounding environment. We also recognise, in this report, that the operational constraints under the current Water Sharing Plan limit environmental improvement options for Wyangala Dam. The literature review is a substantial and integral part of this report: it was the basis for the assessment of current condition and the development of the conceptual model. Over 110 documents are cited, most of which are ‘grey’ literature — agency reports — as very few peer-reviewed published papers specific to Lake Wyangala and/or the Lachlan River exist. Analyses of unpublished data provided important information regarding storage levels and water quality (pH, salinity, phosphorus, nitrogen) in Lake Wyangala, and fish communities in the lake and in the Lachlan River downstream. The development of the conceptual model was not possible without a number of assumptions. Most assumptions stem from the fact that many aspects of current environmental condition have not been adequately studied and exist as knowledge gaps. For the most part, Lake Wyangala functions similarly to other headwater storages in the Murray–Darling Basin. There are, however, some aspects of environmental condition — namely aspects of water quality and the biological community — specific to Lake Wyangala and the Lachlan River downstream. The key functional aspects are: Nutrient enrichment of Lake Wyangala. The lake is a depositional zone for sediments to which nutrients are attached. Phosphorus and nitrogen levels, particularly in the deeper waters, are frequently above ANZECC default trigger levels for lakes and reservoirs. Stratification of Lake Wyangala. The lake stratifies during summer and autumn each year, reducing water temperature and dissolved oxygen levels in the deeper waters (hypolimnion). The flow regime of the Lachlan River has been dramatically altered by the operation of Wyangala Dam. Of particular consequence to the biological community are the reversal of seasonal trends and the removal of high and low flow events. Cold water pollution in the Lachlan River is caused by the combination of stratification of Lake Wyangala, the fixed-level intake tower in the lake and the timing of irrigation releases. Blue-green algal blooms occur frequently in Lake Wyangala between October and May each year. They are caused by the combination of high nutrient inputs, stratification, calm waters, low turbidity and warm water temperatures. Salinity levels in Lake Wyangala are rising and consistently exceed ANZECC trigger levels for lakes and reservoirs. pH levels in Lake Wyangala are increasing. Native fish populations have declined and introduced species dominate in Lake Wyangala and the Lachlan River. Macroinvertebrate diversity in the Lachlan River directly downstream of Wyangala Dam is lower than elsewhere along the river. The depauperate condition of aquatic fauna, particularly native fish, in Lake Wyangala and the Lachlan River downstream is probably due to a combination of processes created by the existence and operation of Wyangala Dam. Cold water pollution in the Lachlan River, experienced from Wyangala Dam to downstream of Cowra, restricts the spawning period for some native fish and denies suitable spawning conditions for others (catfish and silver perch). Regulation of the Lachlan River also affects spawning cues and the extent and availability of nursery habitats critical for the recruitment of juvenile fish. Wyangala Dam is a significant barrier to the upstream and downstream movement of aquatic fauna; the impact on native fish is likely to be significant given that upstream movements are critical for the breeding of some species, and larvae of most species are distributed downstream by drift. The health and diversity of native fish populations in the Lachlan River is clearly a major priority of environmental improvement options for the management and operation of Wyangala Dam. Reducing the prevalence and severity of cold water releases will certainly improve conditions for native fish, but substantial improvements are not expected to occur unless some other issues are addressed; namely, altered hydrology, habitat restoration, control of alien species and Wyangala Dam as a barrier to fish movement. There is some uncertainty predicting the outcomes of environmental improvement options, such as the construction of a fish passage, given that rehabilitation measures of this magnitude have not been previously implemented in Australia. Also, in Australia at least, restoration ecology is a developing branch of ecology and the results of many restoration efforts are, as yet, unknown (Lake 2001). Predicting the outcomes of environmental improvement options is further compounded by the fact that much of the assessment of current environment condition, of Lake Wyangala and the Lachlan River, is based on sketchy information. The following are recommended as environmental improvement options for the management and operation of Wyangala Dam: Recommendation 1 State Water develop a modified flow regimen, that fits within the current Water Sharing Plan, aimed at improving in-stream production, water quality and habitat diversity based on increased variability in flows over the whole year. Recommendation 2 A. Determine the status of the fish community in Lake Wyangala and the Lachlan River downstream of Wyangala Dam: collect data on presence/absence, breeding and recruitment of native species. It is critical to establish whether there is, or isn’t, a viable fish community before undertaking significant works. B. Before the potential benefits of any management action can be determined, a wider monitoring program is established that describes the current condition of the river: including fish, invertebrate, vegetation condition, hydrological assessment, geomorphic assessment. This monitoring program should continue after any changes to the management of Wyangala Dam are implemented. Recommendation 3 Carry out a more detailed investigation into the suitability of localised destratification of Lake Wyangala compared to the construction of a multilevel offtake, taking into consideration not only thermal pollution but also the potential effects on water quality. Scenario testing (based on thermal dynamic models) should provide a solid basis for examining the relative merits of localised destratification compared to the construction of a multilevel offtake. Recommendation 4 State Water does not undertake a feasibility study into the design and construction of a fish passage until other factors affecting downstream ecological condition have been addressed. Recommendation 5 State Water commissions a study to determine the historical sources of sediments currently in Lake Wyangala. Recommendation 6 State Water encourages the relevant catchment management agency to undertake an analysis of the major current sources of sediments and nutrients in the catchment (eg. SedNet modelling) and prioritise rehabilitation of those sites. Recommendation 7 In addition to thermal modelling outlined above (Recommendation 3), the study be expanded to include the impacts of internal loading of nutrients on water quality in Lake Wyangala.