Instream Flow Requirements for The Platypus (Ornithorhynchus anatinus): High Flows. Studies of Water Transfers from The Shoalhaven River System to The Hawkesbury-Nepean River System.

Abstract

During periods of low flows into the storages of the Hawkesbury-Nepean River system, Sydney Water pumps water from Lake Yarrunga, a storage at the junction of the Shoalhaven and Kangaroo Rivers. to the Wingecarribee Dam in the southern tablelands of New South Wales. From there it can be released into the upper Nepean River storages and/or the Wingecarribee River, which drains into the Wollondilly River and hence into the stored water behind Warragamba Dam. Prior to the formulation of an operational release strategy for this system, controlled releases of water were made into both the systems. During these releases the effects of flows on bank stability, water birds, benthic organisms and platypus populations and habitat were assessed, and a regime of maximum releases formulated. Potential impact on platypus populations was assessed by determining the height and/or area of bank left available for use by platypuses at the various flows. Subjective assessments were also made of the usable sections of the rivers for foraging at various flows. Broken white water was assumed to be unsuitable for foraging. The various investigations indicated that flows of 400 ML/day during September to March (water bird and platypus breeding and nesting season) and 600 ML/day at other times would have minimal impact on the ecology of the two river systems. Monitoring, studies involving netting and observations of platypuses in the upper Nepean River system indicated that operational releases within the suggested regime between July 1994 and May 1995 may have led to slightly reduced body condition in some animals during the higher flows in the winter of 1994. However the smaller discharges during the latter period of releases did not appear to result in platypuses entering the winter of 1995 in poor condition. Assessment of the usefulness of some sections of the river for foraging indicated that minor adjustments to the higher end of the flow regime may be necessary for future releases and that further monitoring needs to be done. Capture and observational monitoring studies indicated that releases of up to 500 ML/day in the Wingecarribee River between February and June 1995 had no noticeable effect on platypus activity or populations. Further monitoring needs to be done prior to, during and after higher flows and/or for longer periods in the Wingecarribee River.