Impoundment effects on the thermal regimes of Kootenay Lake, the Arrow Lakes Reservoir and Upper Columbia River

Abstract

The Arrow Lakes Reservoir and Kootenay Lake are comparable large reservoirs formed by the addition of storage onto natural lakes. Both have significant storage reservoirs upstream that are capable of subsurface withdrawal, but both show elevated outflow temperatures. For these reservoirs the presence of upstream dams capable of subsurface release of colder hypolimnetic water makes an observed 2–3 °C increase in spring/summer outflow temperatures unexpected. Mathematical models were applied to both reservoirs to examine hypotheses for changes in their outflow thermographs, namely, the effect of regulated changes to the natural volume and temperature of the inflows, water level rises and the influence of an oligotrophication-induced increase in water clarity on heat absorption. For the Arrow Lakes Reservoir, the increased transit time of near surface water during the heating season causes a demonstrable elevation of outflow temperatures. This effect was absent for Kootenay Lake since the transit time was greater than the heating period. The predominant cause of the limited outflow warming for Kootenay Lake is, therefore, more likely due to the decreased amount of cold water drawn into the outflow during the reduced spring freshet. Examination of the internal hydraulic controls within the Arrow Lakes Reservoir showed that some temperature mitigation might be possible by selective withdrawal of hypolimnetic water.