Downstream effects of the Gorden River Power Development, south-west Tasmania

Abstract

A survey of the physicochemical limnology of the Gordon River, designed to provide background environmental data for the proposed stage 2 of the Gordon River Power Development, has, ironically, revealed far-reaching consequences of the dams of stage 1, for which only rudimentary surveys were carried out. An obvious hydrological effect is the damping of seasonal flow variations so that the regulated river experiences higher summer flows and lower winter flows than under natural conditions. River levels are consequently affected and summer growth of photosynthetic algae in former shallows and riffle zones no longer occurs. Other biological effects are unknown. The effective headwaters of the river are now the hypolimnetic strata of a large stratified reservoir, Lake Gordon. Consequently, the river is fed with a relatively constant discharge of water of almost invariable temperature and chemical composition. The effects of influent tributaries downstream are now largely overridden by this constant flow. Temperatures vary little along the length of the regulated river, being above normal in winter and below normal in summer. Chemical variability has been reduced, both in respect of the lower salinity range now experienced and in the suppression of flow-dependent alternation between dominance by sodium chloride or by alkaline earth bicarbonate. An unfortunate hydrological effect is the elimination of an estuarine underflow of salt water on which the meromixis of three lakes depends. While two of them have shown little change in the short time since dam construction, a third became holomictic soon after discharge from the dam commenced. A temporary return to meromixis after a brief shutdown of the power station indicates that an appropriate operating regime, with brief summer shutdowns, could retain the affected lake in a meromictic condition of great scientific interest.