Anomalous stratification behaviour of Lake Gorden, headwater reservoir of the lower Gorden River, Tasmania

Abstract

Lake Gordon, a large hydro-electric reservoir in south-west Tasmania, holds water of low pH, high colour and low dissolved solids content. It stratifies each year, with consequent anoxia below the thermocline. The nett effect of the inflow pattern, combined with the deep-V morphometry of the reservoir, is to prevent holomixis. For 6 months of the year, the cold inflow of the upper Gordon River follows its old course and rolls down to the dam, intruding first as an underflow, then rising slightly to become an interflow as it warms and finds its density level. This inflow displaces older water upwards, sandwiching the former hypolimnion between the inflow and the epilimnion. The anoxic, sulfuretted water of the sandwiched stratum gradually mixes with water above and below it, leading to its ventilation and dispersion. Meanwhile; the inflowing water stagnates and anoxia develops afresh in bottom waters. This occurs at a time when the upper strata are restratifying, leading to a multiple sandwich of several strata of discrete lineage. The early history of the reservoir indicates that it was predisposed for partial meromixis of the Lake Barrington type, preconditioned by its morphometry and by a mass of flooded vegetation. That a lasting monimolimnetic pool did not form, as expected, is attributable to the annual inflow of dense, oxygenated water. The hypolimnion of Lake Gordon is now the effective headwater of, and greatest contributor to, the lower Gordon River. Because of the stratification pattern, the discharge stratum may be oxygenated or anoxic and laden with hydrogen sulfide, iron and manganese. Any undesirable effects of the latter are rapidly dissipated in a short stretch below the dam, but the relatively invariable nature of the temperature and major ion chemistry of this stratum impose an unwonted uniformity on the river, to its mouth.