PLUVIAL LAKE LEVEL RECORDS OF THE NORTHWEST GREAT BASIN SINCE THE LAST GLACIAL MAXIMUM: WHAT WE KNEW THEN AND NOW, AND WHAT WE COULD KNOW BETTER

Paleoshorelines are ubiquitous features of the closed watersheds of the Great Basin, which record the presence of vast lakes during past wet climate conditions. These wet conditions, occurring most recently during the last glacial cycle, resulted from low temperatures and movement of the mean position of the cool season westerly storm track southward. However, geochronologic lake level histories suggest timing of maximum wetness was diachronous across the Great Basin during the last deglaciation. In particular, lake systems of the northwest Great Basin (northern California and Oregon) show evidence that as mean storm track position moved poleward, expansive lakes persisted up to millennia later than those further to the south. These pluvial hydrologic conditions, subsequent drying of lakes and wetlands, and the changing regional climate that supported them, affected the lifeways of the earliest inhabitants of the northern Great Basin. However, evidence from geological and archaeological sources constraining lakes of the northwest Great Basin provide a conflicting picture of the timing of hydrologic change. Interpretations based on scarce data have interpreted peak lake levels coincident with the last glacial maximum, or with cold periods of the last deglaciation like Heinrich Stadial 1 and Younger Dryas. Recently developed lake histories based on radiocarbon dating of lake deposits, in contrast, indicate deep lake conditions may have occurred during Heinrich Stadial 1, but also during the warm Bolling/Allerod. They also suggest lakes were greatly reduced in size by the beginning of Younger Dryas time. Even so, data gaps prevent a continuous picture of lake level history, which hampers the ability to reconstruct paleoclimate and fully constrain the landscape which was important to early Great Basin inhabitants. Here we review existing and present new constraints on lake levels since the Last Glacial Maximum from geological and archaeological archives, focused on the Chewaucan and Fort Rock basins of southern Oregon.