Impacts of River Regulation on the Terminal Lakes and Mouth of the River Murray, South Australia

Abstract

The River Murray is a highly regulated, low gradient stream. Prior to barrage construction near the Murray Mouth saline water was present sporadically far upstream. Barrage completion by 1940 has had geo‐morphic and ecological impacts. Estuarine Lakes Albert and Alexandrina became permanent freshwater bodies with elevated water levels. Exposed lake margins have eroded by up to 10 m yr', whereas along sheltered shorelines, sedimentation has accompanied reed growth. Upstream weirs have inhibited coarse sediment from reaching the lakes. In the Goolwa Channel, sedimentation changed from bioclas‐tic sands to muds, accumulating at up to 4.5 mm yr' over the past 50 years. Regulation has also transformed a migrating flood tidal delta at the Murray Mouth into a permanently vegetated island.

River-Dominated Coasts

“…The largest river system in the country is the Murray-Darling system that drains just over a million square kilometers. However, extrac tion of water for a variety of purposes means that the mouth of the Murray River has frequently been closed off by sand worked onshore by the strong swell (Bourman and Barnett, 1995;Harvey, 1996;Shuttleworth et al, 2005). Similarly the Snowy River, a once-mighty river draining from the Snowy Mountains, has experienced reduced flows, primarily as a consequence of a major diversion scheme constructed in the 1950s, and also as a consequence of reduced rainfall in its catchment.…”

Palaeolimnological evidence for the independent evolution of neighbouring terminal lakes, the Murray Darling Basin, Australia

“…This former shoreline exists today as a sill of sediments that separates Lakes Alexandrina and Albert from the Coorong and the Murray Mouth. The mouth has rapidly migrated to the north-west since the 1960s (Bourman & Murray-Wallace, 1991) and the sediments in the channel inside the mouth are accreting rapidly (Bourman & Barnett, 1995). Historically, the Goolwa channel carried *70% of the Murray River flow as it is considerably deeper and less sinuous than the other channels between Lake Alexandrina and the Coorong (Cann et al, 2000).…”

“…The Lake is shallow (Z max = 4.05 m; Z mean = 2.86 m) and highly turbid. The Darling River is known to carry a very highsuspended sediment load and this is thought to be the main source of abiogenic turbidity in Lake Alexandrina although Bourman & Barnett (1995) identified considerable erosion of the littoral zone of the Lake over the last century. The Coorong is a large coastal lagoon complex situated between a Holocene beach-dune barrier, the Younghusband Peninsula, and, on its north-eastern margin, a fossil shoreline formed during the last interglacial.…”

Anthropogenic acceleration of sediment accretion in lowland floodplain wetlands, Murray–Darling Basin, Australia

“…Human-induced increased flux of allochthonous sources down large rivers may come from surface erosion from the catchment, stream bank erosion and collapse and soil surface sodicity (Neave and Rayburg, 2006) and this may, at least in part, be balanced, in regulated systems, by sediment trapping behind impoundments (Olley and Wallbrink, 2004). Endogenic drivers that accelerate accretion rates include wave-driven erosion of exposed littoral zones (Bourman and Barnett, 1995) and increased sediment trapping by aquatic macrophytes promoted by accelerated nutrient fluxes (see Table 1). …”