Effects of regulation on the flow regime of the river Murray, Australia

Abstract

The flow regime of the River Murray has changed markedly over the last century, and especially the last 50 years, through increased diversions, construction of dams, weirs and levees and changes in operational procedures. A model developed by the Murray-Darling Basin Commission is used to compare simulated natural (unregulated) flows at eight stations with those at seven consecutive stages in the development of regulation. Monthly and annual average flows and coefficients of variation and skewness were computed, and the flow-duration, peak-flow and low-flow characteristics curves plotted. The results confirm that average monthly and annual flows are now considerably lower than those which prevailed under natural conditions. The seasonal distribution of flows has changed in the upper Murray, owing to the influence of dams. Flow-duration characteristics now vary considerably along the river, whereas there was little change under natural conditions. The effect of regulation on flow-duration characteristics is minimal at Albury and becomes more pronounced downstream; it is most apparent in regard to flows exceeded 20-80% of the time. The magnitude of average annual floods (annual exceedance probability 50%) has been reduced by over 50% at all stations, but big floods (average recurrence interval 20 years or more) are little affected. Further, the low flows for a given annual non-exceedance probability are higher under regulated conditions than those under natural conditions. These changes have profound implications for communities of native plants and animals in both riverine and floodplain environments, and also for the long-term utility of the river as a resource.

Analyse des écoulements dans le bassin du fleuve Sénégal de 1960 à 2008

“…-La méthode de « reconstitution » qui consiste à comparer les débits reconstitués en condition naturelle et les débits lâchés en aval d'un barrage (MAHESHWARI et al, 1995;PETERS et PROWSE, 2001). -La méthode de « station de contrôle » fondée sur la comparaison des débits mesurés en amont et en aval d'un barrage (ASSANI et al, 2002) ou sur la comparaison des débits mesurés sur une rivière non influencée par un barrage et en aval d'un barrage (BENN et ERSKINE, 1994).…”

Floodplain vegetation of the River Murray in 1987–1988: an important pre-drought benchmark for subsequent studies.

“…However, while some low-lying sites have become wetter, tree health has been affected more widely because the lower Murray floodplain in general has become drier as a result of the high level of water extraction, mainly for irrigation, throughout the Murray-Darling Basin. This has meant fewer, less extensive, less sustained floods, an effect that is most pronounced in the lower reaches of the river (Walker and Thoms 1993, Maheshwari et al 1995, Dyer 2002, Roberts 2004, CSIRO 2008. Reduced flooding has also meant less effective leaching of salt from floodplain soils in the lower Murray.…”

“…The findings of poor tree health and sparse regeneration below the Darling Junction coincide with the most heavily regulated part of the River Murray, where the reduction in flooding due to upstream storages and water extraction has been greatest (Walker and Thoms 1993, Maheshwari et al 1995, Dyer 2002. Reduced flooding, which means not only less water for the trees but also less flushing of accumulated salt from the saline Mallee Zone floodplain, is implicated as the primary cause of the observed decline of River Red Gum and Black Box on the lower Murray.…”

“…The reduction in flooding frequency has been greatest in the lower Murray, below the Darling Junction, corresponding to the part of the floodplain with the poorest tree health in 1987-88. Flows in the lower Murray have been approximately halved, and the frequency and duration of small, medium and large floods have all been significantly reduced (Walker and Thoms 1993, Maheshwari et al 1995, Dyer 2002, Roberts 2004, CSIRO 2008. Reduced flooding means not only less water for the trees but also increasing soil salinisation because of less flushing of accumulated salt from the saline Mallee Zone floodplain.…”

“…Water flow in the Murray is low compared with rivers of similar catchment size on other continents, and it is highly variable (Young 2001). Over the last two centuries, its waters have become increasingly regulated, especially since the 1920s, largely to support irrigated agriculture (Close 1990, Maheshwari et al 1995. The Murray system is now controlled by many dams and weirs, and water from the Snowy River has been diverted westward across the Great Dividing Range to augment the Murray flow.…”