ENVIRONMENTAL-EFFECTS OF FLOW REGULATION ON THE LOWER RIVER MURRAY, AUSTRALIA

Before regulation flows in the lower Murray were highly variable, as f or most rivers in semi-arid regions. Major floods promoted large-scale recruitment of flora and fauna in riverine and floodplain communities , and seasonal floods maintained lower levels of recruitment. The regi me changed with the construction of 10 low-level weirs in 1922 35, sup plemented by the effects of dams in upstream areas. Flows remain varia ble but are much reduced in volume (about 44%). Low flows (100-300 G1 per month) have decreased five-fold and moderate flows (500-1500 G1 pe r month) have increased two-fold. Although the magnitude of peak seaso nal flows has been diminished, the timing of flows is unaffected. The effects differ in the Valley and Gorge sections of the river, dependin g on local development of the floodplain and associated wetlands. The weirs have flooded once-temporary wetlands and contributed to problems of salinization. Weir operations cause daily stage fluctuations that diminish downstream, and the channel is developing a stepped gradient as a consequence of active deposition and erosion. Regulation has limi ted exchanges between the river and its floodplain, changed the nature of the littoral zone and generally created an environment inimical to many native species, notably fish. The key to rehabilitation may be t o restore a more natural balance of low and medium flows, but this may be unrealistic given the needs of irrigators and other water users. D espite its evolutionary history of wide spatial and temporal variation , the Murray river floodplain ecosystem evidently cannot accommodate t hese forms of disturbance.