Effects of Reservoir Drawdown on Resuspension of Deltaic Sediments in Lake Powell

Abstract

ABSTRACT Due to prolonged drought conditions in the Upper Colorado River Basin, Lake Powell's water surface elevation declined steadily from 1988 to an early 1993 level approximately 27 m below the full pool level of 1130 m above MSL, resulting in a 60% reduction in reservoir storage. Associated with this drawdown was the exposure and resuspension of over 100 channel kilometers of deltaic deposits in the combined major tributary arms of the reservoir. This resuspension process maybe a significant source of nutrient and trace element loading to the reservoir due to the release of substances associated with these sediments. This may have implications to productivity processes and bioaccumulation of heavy metals in the aquatic ecosystem. In this study, 11 sites on the Colorado River and 3 sites in Dark Canyon, a small tributary in the inflow area of Lake Powell, were sampled for sediment concentration and size fractionation, nutrient concentrations, and total and dissolved trace element concentrations. Results showed most resuspension occurring in the lower part of the delta just before the river entered the lake. Significant increases in total nutrient and trace element concentrations were measured in a pattern similar to the increased sediment load. Dissolved nutrient and trace element concentrations remained very low but showed some increases in the active resuspension area. The inflow areas of the reservoir are typically the most biologically active and have shown increased biological activity during the recent drawdown years. In addition to the resuspension process, other physical and biological processes, along with the fate of the inflow current within the reservoir, play a significant role in making substances associated with the sediments in inflow areas available to the aquatic ecosystem.