Pyrites Creek is a tributary of the Werribee River that lies to the west of Melbourne.  Pyrites Creek downstream of Merrimu Reservoir has been highly modified as a result of flow regulation. Prior to the construction of the reservoir, Pyrites Creek had an ephemeral flow regime that was punctuated by infrequent high magnitude flow events. The reservoir has effectively eliminated all major flow events in the downstream reaches. The first few kilometres of river channel downstream of the reservoir now has perennial low flow due to a constant leak from the reservoir that is needed to maintain the structural integrity of the dam wall. Further downstream, the stream is more ephemeral, but still lacks large flow events. These hydrological changes have allowed vegetation such as Phragmites, Typha and Tea Tree to encroach into the main channel and have contributed to a greater accumulation of fine sediment and filamentous algae on the streambed.
An environmental flow study conducted for Pyrites Creek recommended more frequent freshes of 10 ML/day and high flows of approximately 100 ML/day to reduce vegetation encroachment and clean the streambed.. In 2012, Melbourne Water engaged SKM to assist them evaluate the effectiveness of trial environmental flow releases. High flows delivered in July and August 2012 failed to scour or drown established macrophyte beds within the channel. However, they did scour fine sediments and algae mats from cobbled surfaces in shallow pools and riffles that provide macroinvertebrate habitat. Smaller magnitude freshes released in late 2013 were also effective at cleaning the streambed, although they are probably only needed in the warmer months when algal growth and biofilm development are more prolific.
This paper describes the environmental flow trial and associated monitoring in Pyrites Creek and discusses how the results are being used to revise environmental flow objectives and the recommended flows that are needed to meet those objectives. Future monitoring will now focus on the response of macro-invertebrates to the improved environmental flow regime