HOLOCENE PALEOHYDROLOGY OF THE UPPER COLORADO RIVER FROM LAKE SEDIMENT STUDIES ON THE WHITE RIVER PLATEAU, COLORADO

The headwaters of the Colorado River are a major source of water for the southwestern U.S. Tree-ring based paleohydrologic reconstructions have identified periods of drought during the last four centuries and suggest that the 20th century was anomalously wet. Concern for the potential of increasing severity and extent of future droughts is driving efforts toward a better understanding of past climate variability of the region. Lake sediment studies within headwater regions of the Upper Colorado River have the potential to extend our understanding of the spatial patterns and severity of Holocene droughts. The White River Plateau in west-central Colorado is a limestone massif at ~3000 m a.s.l. within the central Rocky Mountains. Numerous lakes on the plateau are the headwaters of the White River and other tributaries of the Upper Colorado River. Preliminary sedimentary analyses of cores spanning the Holocene along depth transects from two hydrologically-closed basins on the plateau, Yellow Lake (39.652°N, 107.453°W, 3140 m a.s.l.) and Bison Lake (39.764°N, 107.346°W, 3255 m a.s.l.), suggest Holocene lake level variations and sensitivity to changes in effective moisture. Oxygen and hydrogen isotope ratios of modern lakes, streams, springs and snow throughout the area indicate that both basins currently undergo significant evaporation during the summer. Lake waters contain abundant bicarbonate and calcium, and the Holocene sediments are rich in organic matter and calcium carbonate. Detailed multi-proxy sediment-based lake-level reconstructions and oxygen and carbon isotope analyses of endogenic carbonate are underway.