Australian and New Zealand Guidelines for Fresh and Marine Water Quality

ion or diversion of surface water The increased consumptive use of water for agriculture, abstracted directly from rivers by pumps or diverted from a reservoir, has significantly reduced the volumes of annual flow, changed the seasonal distribution of flows through the year, and increased the length of low flow periods. For example, it has been estimated that the large scale diversions for irrigation that now occur in the Murray River produce drought-like low flow conditions in the lower Murray in 6 out of every 10 years, compared with 1 in 20 years under natural conditions (MDBC 1996). In rivers such as the Darling, excessive water abstraction for irrigation has severely disadvantaged downstream users, as well as having caused major physical and ecological impacts. For example, cotton farmers now have the means to pump such large quantities of water from the Darling River during high flows that more sudden falls in water level than would occur naturally can occur. These rapidly falling water levels have been implicated in massive bank slumping that has been observed over recent years in the Darling (Cullen et al. 1996). Ecological impacts of rapid recession rates include stranding, desiccation of fish and invertebrates, loss of floodplain inundation episodes and loss groundwater recharge opportunities. There have been many overseas studies of the effects of altered hydrological characteristics on riverine and floodplain ecology (National Research Council 1992, Calow & Petts 1992, Stanford et al. 1996). In New Zealand also, a number of investigations have attempted to better define the relationships between river flow and riverine ecology (NZ Ministry for the Environment 1997b). However, until recently few such studies have been undertaken in Australia (Arthington & Pusey 1993, AWWA 1994, Humphries & Lake 1996, Cullen et al. 1996, Arthington & Zalucki 1997).