Transport of Glacial Meltwater to the Surface Layer of a Stratified Reservoir

Abstract:

Turbidity from glacial meltwater can limit light availability in receiving water bodies, which can have important ecological consequences. The extent to which glacial inflows affect the light regime within a reservoir depends, in part, on its pathway through the reservoir. Here we characterize the physical processes controlling the mixing and transport of glacial turbidity within Carpenter Reservoir, located in southwest British Columbia, Canada. Using meteorological data, a temperature mooring, and monthly surveys of the reservoir and tributaries, we describe the variability of turbidity within the surface layer of this hydroelectric reservoir from spring to fall of 2015 and 2016.

Profiles of temperature, conductivity and turbidity were collected at several locations along the length of the reservoir. In the spring, before the onset of summer stratification, turbidity was high (> 10 NTU). In the summer, thermal stratification kept the epilimnion relatively isolated from the cold, plunging, glacial inflows which passed through the hypolimnion to the deep outlets. As a result, the turbidity in the epilimnion declined through the summer to < 1 NTU, despite the high glacial load into the reservoir. This decline generally followed an exponential decay, which we attribute to the settling of suspended particles from the isolated epilimnion into the hypolimnion. Turbidity in the epilimnion only began to increase again in the fall when the epilimnion deepened, entraining cold and turbid water from below.

In addition to the decline in turbidity during the summer, we observed a variation in turbidity along the length of the reservoir. Turbidity was highest at the upstream end of the epilimnion nearest to the (plunging) glacial inflow, and lowest at the downstream end of the epilimnion near the dam. We describe a simple model of the epilimnion to estimate the longitudinal variation in turbidity during the summer. The model enables the estimate of a flux of turbid water into the epilimnion, as well as the relative importance of horizontal dispersion and particle settling. These observations provide a basis for understanding the pathways of glacial meltwater through a hydroelectric reservoir and the linkages between these pathways and the light regime within the reservoir.