Plunging inflows and the summer photic zone in reservoirs

Abstract

Kinbasket and Revelstoke Reservoirs are part of the Columbia River system in eastern British Columbia, Canada. Hydroelectricity from these large reservoirs represents about 30% of the province's generation capacity. Of interest to water use planning is the effect of reservoir operation on pelagic productivity. We address one small part of this question, namely, the supply of nutrients carried by inflows that plunge below the photic zone during the summer. Using an idealized water balance for the photic zone, three cases are considered: (1) a shallow outlet, (2) a deep outlet, and (3) no outflow. For a shallow outlet, all inflow that plunges below the photic zone is upwelled into the photic zone on its way to the outlet. For a deep outlet, inflow that plunges below the photic zone will short circuit or pass directly to the outlet unless entrainment generates upwelling of the inflow into the photic zone. For a reservoir with no outflow, such as a reservoir that is filling, inflow that plunges below the photic zone remains at depth unless either entrainment or a bathymetric effect generates flow into the photic zone; nutrients are then released when the reservoir is drawn down, often in winter.