Frequent Water Levels Fluctuations Drive Exchange Between a System of Coastal Embayments and a Large Lake

We show that in a harbour connected to large lake, frequent water level oscillations can drive significant exchange that is reminiscent of tidal flows. We use analysis of water currents, water levels and temperatures to determine the hydrodynamic characteristics of the lake-harbour system and demonstrate that strong, water level fluctuations with a period of one-hour drive most of the exchange flows in the shallow embayments of the harbour. Water velocities in these oscillating flows were as high as 0.4 m s^-1 and meant that most embayments had flushing times of order of days. This exchange sets the residence time in the numerous shallow embayments of the harbour, and thus the temperature differences between the shallow embayments and the colder and deeper harbour result from differences in thermal inertia. During summer, water temperature differences were as high as 15^oC between the embayments and Lake Ontario. Strong upwelling events in Lake Ontario and wind-driven internal dynamics at diurnal and synoptic time scales can determine additional changes to the thermal regimes in the harbour; however, these processes do not result in the water exchange being dominated by baroclinic forcing. Our observation that the exchange in these shallow is dominated by water level fluctuations is in contrast to the usual wisdom that water level fluctuations in the Great Lakes are too small to drive significant currents, and that water exchange with coastal embayments is always dominated by baroclinic process driven by differential heating or upwelling events. We will discuss how these observations could be used to optimize the creation of warm water fish habitat in shallow embayments located next to large, cold lakes.