Physical processes affecting epilimnetic turbidity in a glacier-fed hydroelectric reservoir

Turbidity from glacial meltwater limits light penetration in lakes and reservoirs with potential ecological consequences. Field observations, theory and hydrodynamic modelling were used to investigate changes in turbidity in response to changes in reservoir operation (e.g. water level, inflows and withdrawals), and to natural processes (e.g. particle settling, dispersion and upwelling) in Carpenter Reservoir, a long and narrow, hydroelectric reservoir situated in a glaciated catchment in British Columbia, Canada. Profiles of temperature, conductivity and turbidity, combined with meteorological measurements revealed that, during summer the relatively dense inflows into Carpenter Reservoir plunged into the hypolimnion, and despite the high glacial load entering the reservoir, the turbidity of the epilimnion declined due to particle settling. Occasionally, down-valley winds were strong enough to upwell turbid, metalimnetic water to the free surface. The upwelled water was blown down-valley, setting up a longitudinal turbidity gradient in the epilimnion. The dominant drivers of epilimnetic turbidity variations were particle settling out of the epilimnion, longitudinal dispersion along the length of the epilimnion, and wind-driven upwelling into the epilimnion. The relative importance of these drivers was investigated using scaling arguments and a mechanistic model based on the one-dimensional diffusion equation. Two nondimensional parameters were obtained: the epilimnetic inflow parameter, I, a measure of the turbidity flux into the epilimnion; and the dispersion parameter, D, a measure of longitudinal dispersion. In the case of Carpenter Reservoir, I << 1, indicating that turbidity declines over the summer, and D << 1, indicating a strong gradient in turbidity along the epilimnion. To determine the effect of reservoir operations on the turbidity of the epilimnion, a two-dimensional hydrodynamic model was used to simulate a wide range of scenarios based on 46 years of historical flows subject to two sets of meteorological conditions. The model results indicate that the water level and inflow rate in spring are the primary controls of epilimnetic turbidity at the beginning of summer, and this initial turbidity determines the turbidity and light penetration for the rest of the summer. These findings will help guide water use planning and reservoir management in dam-impacted systems.Applied Science, Faculty ofCivil Engineering, Department ofGraduat