River rehabilitation from the hydrogeomorphic impacts of a large hydro-electric power project: Snowy River, Australia

Abstract

Large interbasin water transfers for hydro-electric power generation have substantially changed the hydrology of the Snowy River below the Snowy Mountains Hydro-Electric Scheme in southeastern Australia, since 1967. Annual, monthly, mean daily and peak instantaneous flows have been significantly reduced and flood variability greatly increased along 352 km of channel. The first 70 km of river has responded by substantial contraction through the reworking of tributary mouth bars into downstream pools, bars and benches. The tributary mouth bars formed where actively eroding gullies and incised channels debouched into the main stream. These recent deposits have been invaded and stabilized by a number of exotic and native tree and shrub species but particularly willows. Lichens have also colonized the formerly inundated bedrock and boulder surfaces since 1967 in the first 20 km of channel and are now found at elevations up to 2.15 m closer to the river bed. Exotic plants are also invading the riparian corridor downstream of the contracted sections due to reduced disturbance caused by flood suppression. The interaction of reduced flows with high sand and gravel loads has reduced the amount of available habitat. Alternating, bank-attached side bars and their associated pool–riffle sequence have been replaced by transverse and longitudinal bars with a flat featureless bed on the most downstream reach due to a significant reduction in the duration of formative flows between 23.15 and 138.9 m3/s. The rehabilitation of the Snowy River involves the adoption of an environmental flow strategy that includes channel maintenance flows, habitat maintenance flows, minimum acceptable flows, habitat maximizing flows and flow seasonality. This strategy will increase the total amount, duration and variability of releases. A Water Inquiry has been set up as part of the corporatization of the Scheme and will address the issue of environmental flows. Copyright © 1999 John Wiley & Sons, Ltd.