SURFACE WATER FLUCTUATIONS IN THE WESTERN UNITED STATES OVER THE PAST 30,000 YEARS

Sixty-two closed basins in the western United States periodically contained pluvial lakes during the past 30,000 years. Variations in the amount of water stored at the surface and available to the hydrologic cycle are important considerations for understanding past climate fluctuations. For example, 16,000 years ago (cal. yr BP) nearly 10,000 km3 of fresh water was stored in western U.S. basins (Lake Michigan today contains 4900 km3), greatly altering regional circulation and precipitation patterns. Qualitative estimates of past lake level fluctuations for the western U.S. were last compiled in 1994 with the publication of the Oxford Lake Level Database. In the Oxford database, a standardization procedure was applied in order to render each of the 62 basins comparable, regardless of catchment size. The procedure resulted in three intervals of lake status: high (water level at 70-100% of the total range of fluctuation), intermediate (15-70% of the total range of fluctuation), and low (1-15% of the total range of fluctuation). We are currently updating the lake status database for the western U.S. by compiling and evaluating both sediment core and shoreline data from nearly 500 publications. Pertinent data from each manuscript is being recorded and organized in a Microsoft Access database. As part of this effort, the geochronology for each basin is being documented (including 14C age, lab number, material dated, locality, stratigraphic context, and original citation). Well-developed paleo-shorelines and shoreline deposits (when present) are being used to generate quantitative estimate of past lake levels, which (in a GIS format) can be used to calculate lake area, lake volume, and to better understand late Quaternary changes in surface water hydrology across the western U.S.