Chemical and physical characteristics of a eutrophic reservoir and its tributaries: Long Lake, Washington—II

Abstract

Sampling of Long Lake, Washington and its tributaries was continued for additional information to facilitate a reliable understanding of the dynamics that account for the reservoir's eutrophic condition. The sewage effluent from the City of Spokane's primary treatment plant, via the Spokane River, is a major source of nutrients influencing the reservoir's trophic status. Existing loads of nitrogen and phosphate in the river increased in 1973 by 3.86 and 2.36 metric tons day −1, respectively, below the effluent. Of the influent phosphate, 32% was retained in the reservoir. The Spokane River occurred as a density interflow in the reservoir from mid-July to mid-September and effectively isolated a wedge of anoxic, nutrient laden and low conductive water between the dam and 24 km up-reservoir. The river, as an underflow, initiated fall homothermy. The reservoir had a mean retention time of 49 days and effected greater hypolimnetric oxygen depletion and nutrient accumulation than the previous year.