MILLENNIAL-SCALE HYDROCLIMATE VARIATION AND PROLONGED EPISODES OF EPHEMERAL RIVER FLOW IN THE ROCKY MOUNTAINS DURING THE LATE-QUATERNARY

Water is a vital resource, and climatic changes can dramatically alter the availability of water. Networks of paleoclimate data are required to understand the full range of hydrologic variability and the underlying processes involved. In particular, little is known about hydroclimatic variations on centennial to millennial time-scales, but well-dated sedimentary lake-level records with century-scale resolution can provide direct evidence of such variability. Few lake-level records exist in the critical water source region of the Rocky Mountains, where many of the most contested rivers in the United States originate. Therefore, to evaluate the potential that abrupt and short-lived (100-1000 yr) moisture variations have affected the Colorado River and adjacent watersheds in Colorado and Wyoming, we reconstructed the Late-Quaternary water levels of several lakes in the region: Hidden and Upper Big Creek Lakes, Jackson Co., Colorado, Little Windy Hill Pond, Carbon Co., Wyoming, and Lake of the Woods, Fremont Co., Wyoming. Geophysical surveys and transects of sediment cores collected from the lakes show a series of paleoshorelines submerged below the lake surfaces, which consistently date from ca. 8000-1200 cal yr BP. One particularly prominent paleoshoreline spans a 1500-yr period from 3700-1200 cal yr BP at all of the study sites. An earlier paleoshoreline spans the Younger Dryas interval (12,900-11700 cal yr BP). Sediment cores collected from near the shore of each lake contain the sandy, inorganic paleoshoreline deposits, which interrupt deep-water organic-rich mud deposits. Ground-penetrating radar profiles show these layers associated with prominent near-shore unconformities ringing each basin. At Upper Big Creek Lake, the unconformity extends basinward from a submerged, wave-cut bench that also rings the lake. Moisture-balance calculations for the watersheds indicate that when the documented paleoshorelines were occupied, persistent flow of major regional rivers would have been substantially reduced; the lake-level reconstruction of Upper Big Creek Lake, which overflows into a significant tributary of the North Platte River, confirms our inference because the wave-cut benches are >1 m below the lake’s modern overflow point.