Patterns of Circulation and Nutrient Supply in a Medium Residence-Time Reservoir Kootenay Lake, British Columbia

Kootenay Lake is a long, deep lake in the Columbia River System. It is a medium residence-time system in which the physical and chemical characteristics are influenced by the two major rivers entering the lake. To understand seasonal circulation patterns it is necessary to account for the interactive effects of three separate processes; these are mixed-layer dynamics, internal wave behaviour and the riverine circulation. Incoming river water may move laterally through the lake as a surface overflow, an intermediate depth interflow, or a near-bottom underflow. Vertical redistribution of riverine water masses is dependent upon surface-driven processes. The combined effect of both of the above processes may be altered still further by internal seiche motions, which can reposition various layers within the lake relative to the surface, where wind mixing is more intense. Construction of dams on both upstream rivers have led to changes in the timing and temperature of discharge to the lake. To predict the effects of alterations in riverine nutrient loadings on the primary producers, the nutrient supply of phosphorus and nitrogen was compartmentalized on the basis of physical processes controlling distribution during various times of the year. The compartments were the epilimnion starting load, river inputs during the summer growing season and inputs from entrainment of metalimnetic water during autumnal cooling.