9417963 Freidel Rapid and extreme climatic fluctuations have occurred commonly during the past 250 thousand years, although infrequently during the Holocene. Information regarding the mechanisms that controlled past climatic fluctuations and the hydraulic response to such events will aid in understanding possible future climate change. This study will investigate the timing and climatic significance of an oscillation in lake level of two paleolake sub-basins that occurred during the Pleistocene-Holocene transition of Lake Chewaucan, south-central Oregon. Two hypotheses will be tested concerning possible causes of the lake-level oscillation: (1) The oscillation was caused by increased precipitation during a brief period when the mean position of the jet stream core was located over the study area, as it migrated north following the rapidly melting Pleistocene ice sheets, sometime after 13,800 yrs B.P. (radiocarbon years before present) but before 11,000 yrs. B.P. (2) The oscillation was caused by cooler temperatures and increased effective moisture associated with the Younger Dryas reversal in deglaciation, after 11,000 yrs B.P. but before 10,000 yrs B.P. Information to test these two hypotheses will be obtained by excavating across a fan of sediment that was deposited along the shore of one sub-basin lake when an adjacent sub-basin lake filled and overflowed across the sill between the two Lakes. Radiocarbon- datable materials will be collected in the fan sediments, as well as along the shorelines associated with the1 overflow in both sub- basins. This study is part of on-going research to investigate the past climate of the Pacific Northwest during the Late Pleistocene and Holocene, in order to learn more about the relationship between climate, atmospheric circulation patterns, and surface hydrology during a period of rapid climate change.
