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Deep water renewal during wintertime stratification in a deep, freshwater, thermobarically stratified lake Smith, Lydia


Thermobarically stratified Quesnel Lake shows relatively high rates of hypolimnetic mixing and transport over its winter (inverse) stratification period. A decade-long data set reveals annual winter trends of warming and cooling at 500 m depth, field measurements record increasing hypolimnetic oxygen levels between fall and summer, demonstrating regular ventilation. This research describes processes responsible for the ventilation of the deep hypolimnetic water of Quesnel Lake. Temperature records from moored instruments, and conductivity, temperature, and depth (CTD) profiles were processed to assess transport and mixing in the lake. Local weather stations, regional government data, and modelled data were used to calculate atmospheric forcing at the surface of the lake. These data were compared to find links between atmospheric events and deep-water behavior over the winter of 2006-2007. The results show that the deep water is ventilated by wintertime thermobaric instabilities induced by baroclinic seiche, and by a prolonged spring turnover. Turbulent mixing from seiche homogenizes the bottom water (within 0.02ºC) to a maximum height of ~400 m above the lake bottom. Deepening of the surface mixed layer is aided by the homogenized bottom layer to produce a full spring turnover. This study supports previous research on Quesnel Lake, demonstrating that wind forcing is an effective mixing and transport mechanism even in the deepest region of the lake.

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