210Pb-determined sedimentation rates and trace-metal concentrations, upper Klamath Lake and Lake Euwana, Oregon

Abstract

Rates of sedimentation calculated from analyses of 210 Pb activities in cores from Upper Klamath Lake and Lake Euwana (Klamath County, Ore.) indicate that the lakes, whose mean depths are 2.4 m, are filling at approximately 3.5 mm/yr. In Upper Klamath Lake, average accumulation rates range from 0.9 mm/yr at a site in Howard Bay to 12.5 mm/yr at a location near the input from Agency Lake. Divers collected cores, approximately 50 cm long, from eight locations in Upper Klamath Lake and from one location in Lake Euwana; coring sites were chosen to represent a variety of sedimentary environments. Plots of excess 210 Pb activity versus depth show that a sediment-mixing layer, ranging from 5 to 20 cm in thickness, is below the sediment-water interface at all coring locations; the mixing is due to biological activity, wave and current action, and gas generation. Two cores show changes in accumulation rates during the past 100-150 years that are probably related to changes in land use. X-radiographs indicate the presence of pumice layers and allow assessment of biological activity; these two kinds of information are important to the interpretation of 210 P b activity data. Trace-metal contents were determined to establish baseline information. The eight cores from Upper Klamath Lake contain much lower concentrations of all metals measured (Cd, Cu, Cr, Fe, Pb, Mn, and Zn) than are found in other lacustrine environments. The core taken from Lake Euwana contained higher concentrations of metals than the cores collected from Upper Klamath Lake. This study was initiated in response to a concern that possible rapid accumulation of sediments in Upper Klamath Lake poses a threat to the lake's ability to support economically critical activities. The findings indicate that the lake is not filling at a rate that will cause an immediate threat to powerplant, agricultural, and recreational lake use.