HOLOCENE PALEOHYDROLOGY AND DROUGHT IN THE UPPER COLORADO RIVER BASIN FROM ALPINE LAKE SEDIMENT ON THE WASATCH PLATEAU, CENTRAL UTAH, USA

Historic droughts in the Upper Colorado River basin (UCB) are compounded by reductions in snowpack, which is the primary water source for the region. This study investigates drought within a snowpack dominated UCB headwater catchment at 3050 m elevation, from the Holocene sediments of Emerald Lake, Utah, a cirque lake located in the freshwater Flagstaff limestone formation of late Paleocene to early Eocene age. Sediments were analyzed for organic and inorganic carbon content, mineralogy, carbonate isotopes, and radiocarbon. Basal radiocarbon ages from deep and shallow water cores of ~10,000 and 7,000 cal yr BP (calibrated year before present as 1950), respectively, indicate lake level rise during the early Holocene in response to rising precipitation-evaporation balance. At ~7000 cal yr BP, the onset of endogenic carbonate sedimentation within a shallow marl bench initiates the modern lake configuration and when the carbonate oxygen and carbon isotope proxy record of lake water values begins. Relatively low oxygen isotope values indicate wet conditions at that time. An abrupt change in sedimentary texture and mineral composition that occurs at ~5,700 cal yr BP likely reflects the evident landslide movement on the cirque face into the western shoreline of the lake. The mid-Holocene Emerald Lake landslide may coincide with extensive landslide movement on the Wasatch Plateau. Possible triggers include higher soil moisture with rising precipitation-evaporation balance (and rising snowpack) and seismic activity on nearby faults. Between ~3500 and 2500 cal yr BP, the inferred lake water oxygen isotope values indicate a prominent period of sustained drought that is also recognized in the Great Basin. After ~2500 cal yr BP to present, results indicate generally wetter conditions and recurring periods of lower lake level and drought on multidecadal to century time scales. Comparison with records of drought from alpine elevations at UCB headwaters in northwest Colorado suggest spatial climate patterns that mirror those of the historic period and suggest similar climate drivers, namely ENSO and Pacific Decadal Variability. Earlier droughts of significantly longer persistence are not readily explained by modern analogues.