HYDROLOGICAL IMPACTS OF FLOW REGULATION ASSOCIATED WITH THE UPPER NEPEAN WATER-SUPPLY SCHEME, NSW

The upper Nepean River has been progressively regulated for water supply to Sydney and Wollongong since 1886 by the Upper Nepean Water Supply Scheme which consists of four large dams, two small dams and two diversion weirs. Secular rainfall changes produced periods of high rainfall and large floods (flood-dominated regimes) between 1857 and 1900 and 1947 and the present, and an intervening period (1901-46) of low rainfall and small floods (drought-dominated regime). Upstream impoundment and flow regulation significantly reduced flood magnitudes for most return periods during both types of flood regimes. The probability distribution of mean daily flows was also changed significantly by flow regulation such that during the drought-dominated regime, the high and low frequency flows were reduced substantially but the moderate frequency flows were increased due to dam releases; the change from a regulated drought-dominated regime to a regulated flood-dominated regime resulted in a substantial increase in discharge for most durations; and increased water diversions to Wollongong during the current flood-dominated regime produced a marked downward shift in the whole flow duration curve, Nepean Dam reduced downstream suspended sediment yields by two orders of magnitude because it traps in excess of 99 per cent of the inflowing suspended sediment load. Streamflow releases are urgently required from the two diversion weirs to improve downstream water quality and to ensure the viability of the resident 'potentially threatened' eastern Macquarie perch (Macquaria nov. sp.).