Lake Hume Integrated Water Quality Study: Summer 2006/2007

"January 2008".

Project Number: Lake Hume Integrated Water Quality Study - M/BUS/215.

MDFRC item.

ISBN: 978-1-921257-97-1.

Hume Dam is the major regulating structure on the River Murray system and is located just upstream of Albury (Sherman, 2005). Construction of Hume Dam was commenced in 1919; the storage capacity was increased in 1924, a hydroelectric power station was incorporated in 1957, and storage capacity was again increased in 1961 (Murray-Darling Basin Commission, 2004a). At its full capacity of 3038 GL (Murray-Darling Basin Commission, 2004b), the dam has a surface area of 202.5 km2, a maximum depth of 41.4 m and the water surface is 192 m above sea level (Matveev and Matveeva, 1997). The lake has a catchment of 15280 km2 or 1.5% of the Murray- Darling Basin (Murray-Darling Basin Commission, 2004a). In normal rainfall years approximately one third of the inflow is derived from the Mitta Mitta River and the remainder is sourced from the upper Murray River (Murray-Darling Basin Commission, 2004a). An average net transfer of 580 GL of water is sourced each year from the Snowy Mountains Scheme (Murray-Darling Basin Commission, 2004a). Excluding flows from Dartmouth Dam and the Snowy Mountains Scheme, the average annual inflows are approximately equal to the storage capacity of the lake. Currently the main use is for the provision of irrigation water; flows >20,000 ML d-1 can be released from the dam to supply irrigation and South Australian water entitlements (Sherman, 2005). Because of irrigation demand, the water level of Hume Storage fluctuates on a fairly regular annual cycle. Typically the lake stores water in winter and spring and on average spills in about 50% of years, although it may go for many years without spilling during periods of low rainfall. Generally there is draw down between November and May to between 10 and 50% of capacity, mostly to supply irrigation requirements (Murray-Darling Basin Commission, 2004b). During periods of drought and high downstream demands, Lake Hume is drawn down to water levels of between 10% and 50% of its capacity. During these low water levels, water quality within Lake Hume can be quite poor. In particular, blooms of toxic cyanobacteria have been associated with low water levels within the lake (Baldwin et al 2006). However, our understanding of what drives water quality in Lake Hume during low-water levels has been far from complete. In order to address this knowledge gap, the MDBC commissioned the Murray-Darling Freshwater Research Centre to undertake a comprehensive study of water quality in Lake Hume during the summer and autumn of 2006/07. The objectives of the study were to: Identify drivers of water quality (including blue-green algae) in Lake Hume during periods of low storage levels; Identify downstream impacts of poor water quality releases from Lake Hume; Assess the impact of Dartmouth LLOW releases on Lake Hume water quality during low storage levels; Identify rehabilitation options in the Lake Hume catchment and determine their potential benefits; Evaluate management options for the improvement of water quality in Lake Hume during periods of low storage levels. Each of these objectives are dealt with in a separate section of this report. This report also includes a summary of new knowledge gaps identified in this study, a brief outline of activities that could be undertaken to address these gaps and, suggestions of management strategies for Lake Hume to mitigate poor water quality in the lake.