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The heat budget of Quesnel Lake, British Columbia Potts, Daniel John

Abstract

Quesnel Lake, a long, narrow fjord lake in the Interior Plateau, is the deepest lake in British Columbia (506 m). This pristine, oligotrophic lake and its watershed are a cornerstone of the province's salmon fishery, and were historically home to up to 30% of the Fraser River sockeye run. The transport processes and spatial distribution of oxygen and nutrients in the lake are controlled by temperature stratification; therefore, a solid understanding of the lake's thermal structure and of the dynamics controlling that structure is vital to ecological management decisions. Water temperatures in the lake were measured with two thermistor moorings, one of which reached a depth of 283 m. Weather data recorded at Williams Lake airport were used to estimate heat and mass fluxes at the lake surface. The temperatures of the three largest inflowing rivers were measured, and river flow rates were measured or estimated. Inflows estimated from historic flow data balanced the water budget well except during the summer of 2003, which was unseasonably dry. In fall as the surface waters cooled towards 4°C, the mixed layer deepened until the entire water column briefly became isothermal. Because of pressure effects on water's temperature of maximum density, 1-D transport processes cannot explain this isothermal condition. Episodic cooling of the deep water during winter was likewise inexplicable by 1-D principles. During spring warming, the mixed layer deepened only to 153 m, and complete turnover did not occur. The heat content of the lake reached a maximum on 23 August 2003 and a minimum on 11 March 2004. The heat budget, or difference between minimum and maximum heat content was 1.72 GJm-2 (41.1 kcal cm-2 ). Heat flux estimates overpredicted the lake's heat budget by 3.5%, and indicated that shortwave and longwave radiation and evaporation were dominant. Flows across a sill into the lake's West Basin were estimated by two methods, using conservation of volume and conservation of heat energy. The estimated flow rates were comparable to the limits imposed by 2-layer inviscid hydraulic control. Temperature fluctuations in Quesnel River were due to basin- and lake-scale internal motions.

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