KIRK BRYAN AWARD: OLDER PLUVIAL LAKES IN THE GREAT BASIN: INSIGHTS FROM A DECADE OF FIELD WORK

Our recognition of lake shorelines that are higher and older than the late Pleistocene lakes of the Great Basin has seeded several new studies that have addressed Pleistocene climate change in the Great Basin, effects of tectonics and drainage captures on lake level, processes of drainage-basin integration, and effects on the distribution of endemic aquatic species. Many closed lake basins in Nevada (e.g. Lahontan, Columbus-Rennie, Newark) show nested shorelines that decrease in age and elevation. Dating based on 14C, U-series, cosmogenic nuclides, tephrochronology, paleomagnetism, amino-acid racemization, and soils shows that the highest shorelines likely correspond to oxygen-isotope stage (OIS) 16, a time of maximum global glacial extent; intermediate shorelines are mainly of OIS 6 age, also a time of large ice extent (deposits of OIS 8-12 are locally present), and the lowest shorelines are OIS 2. Calculations using lake and drainage-basin area related to temperature, precipitation, runoff, and evaporation suggest that the highest levels of these pluvial lakes required a regional increase in effective moisture of 1.2 to 3 times late Pleistocene pluvial amounts. However, some lake basins (e.g. Diamond, Madeline, Walker Lake subbasin of Lake Lahontan) exhibit shoreline records that reflect episodic threshold incision by lake overflow, and in others (e.g. Wellington, Russell) fault displacement or volcanism altered the threshold altitude.

Drainage-basin changes have caused profound alterations in aquatic migration routes. Deposits of Pliocene lakes suggest connections among proto-Lake Lahontan and basins to the south. Lake Russell, now tributary to the Owens River, overflowed to the Walker River prior to 1.4 Ma, permitting southward migration of fish along the east side of the Sierra Nevada. The Humboldt River shows evidence of piecemeal assembly of several internally drained basins and eventual integration with the Lahontan basin during the Pliocene to middle Pleistocene. Although many connections were established by progressive threshold incision, others occurred by large catastrophic floods (Lakes Coyote, Adobe, and probably Manix). Collectively, these studies demonstrate how much there is yet to learn from field work in the Great Basin.