GLACIAL INFLUENCES on WATER RESOURCES OF THE EKLUTNA BASIN, ALASKA

The Municipality of Anchorage relies on the Eklutna Glacier for drinking water and hydroelectric power. Monitoring and predicting changes in glacier volume, and the distribution and timing of glacial meltwater and sediment runoff, which affect reservoir storage in Eklutna Lake, is critical to developing feasible and effective long-term water-resource management plans. The U.S. Geological Survey studied the Eklutna watershed from 1985-1988 and concluded that glacier melt water contributed 9-19 percent of recharge to Eklutna Lake; that fine sediments were accumulating in the reservoir at 74 acre-feet/year; and that Eklutna Glacier remained in an equilibrium state. Since that study, the Eklutna Glacier has diminished in size considerably. Ground-based GPS and airborne laser altimetry data from 2007/2008 document an area weighted average of 42 m of surface lowering for the glacier over 50 years. These data, combined with measured terminus retreat of 1.7 km, document substantial volume reduction since 1957 topographic mapping and suggest accelerated volume reduction in the last 20 years. Here, we present the results of our efforts to document the magnitude of these changes, the modern rates of water and sediment discharge to the lake, and the possible reduced storage capacity of the lake itself. Compiled monthly time series present the distribution of water and sediment inputs to Eklutna Lake during the 2008-2010 field seasons, current bathymetric survey map of Eklutna Lake, and glacial volume assessment through ice penetrating radar surveys. Our analysis reflects substantial loss of glacial volume through thinning and earlier peak runoff from the glacierized basin. Data from the non-glacierized East Fork are intended to provide insight into what the runoff characteristics of the West Fork Eklutna River may be in the future as the percentage of glacier cover in the basin diminishes. With detailed bathymetric mapping of Eklutna Lake we will create an accurate model for analysis of how sediment inputs influence lake volume. Using records of municipal water withdrawal from the lake and projected water use demand, we will consider the future water budget across a variety of scenarios of diminished glacial ice volume.