Upper Klamath basin nutrient-loading study: Estimate of wind-induced resuspension of bed sediment during periods of low lake elevation

Abstract

At low lake elevations for Upper Klamath Lake (below 4,140 feet, Bureau of Reclamation datum), 75 percent or more of the areal extent of the lake is affected by resuspension of bed sediment, under prevailing summer wind conditions when critical water quality conditions occur. At the minimum lake elevation of 4,137 feet, 89 percent of the lake is affected. Winds of 10 miles per hour or greater from the northwest can createa bottom shear stress great enough to resuspend bed sediment in Upper Klamath Lake. Winds of this speed occurred for more than 1 hour on 101 days out of 153 days from May to October 1991 at a weather station located at Agency Lake. At a wind speed of 10 miles per hour from the northwest, on Upper Klamath Lake, mean bottom shear stress is approximately four times greater for a lake elevation of 4,137 feet than for a lake elevation of 4,140 feet. Using relations developed between shear stress and resuspension rates for other lake studies as a guide, resuspension rates will be more than an order of magnitude greater for the lower lake elevation. In Upper Klamath Lake, resuspension of bottom sediment may be a major source of the internal phosphorus loading. It is estimated that the contributing phosphorous load sorbed on suspended sediment resuspended from a single wind episode occurring October 26-27, 1991 ranged from 330-1,800 tons with a median of 780 tons. Need for further study is indicated by our inability to predict (1) overall phosphorous- loading conditions, (2) the contribution of phosphorous load from the release of interstitial water in the lake bed sediment during resuspension, and (3) a more narrow range in calculation of phosphorous loading for single wind episodes.