Modelling the Effects of Reservoir Operation on Lower Capilano River Temperature: A Case Study

Capilano reservoir, located in the coastal mountains of southern British Columbia, is one of the three main sources of drinking water in the greater Vancouver region. The Lower Capilano River, located downstream of the reservoir, provides valuable spawning and rearing habitat for Salmonids and other fish species, and discharges into the Burrard Inlet. Construction of a dam has implications in changing the temperature regime of the downstream river, e.g. lower than normal water temperature in summer. Such a change tends to affect the growth of juvenile fish, which is largely regulated by the water temperature. The increasing water withdrawal and the change of future reservoir operation, which include a new pumped intake and a proposed hydropower plant operating during the wet season, could alter the reservoir thermal structure and hence affect the lower Capilano River temperature. A 1-D hydrodynamic model (DYRESM) is used to predict the consequences of future operation. The model prediction under the increased future demand shows that hydropower operation from late fall to spring contributes to the increase of overall reservoir water temperature by taking out the cold deep water and reducing the spilling of warmer surface water. This operation in conjunction with increased drinking water demand led to the lowest river temperature increase from 6o to 8oC, even in a cold and wet summer. Power generation might have more profound water quality implications by triggering earlier turnover in the fall and changing of reservoir stratification regime from the current dimictic to monomictic