LAKES OF THE LAHONTAN BASIN DURING THE LAST GLACIAL MAXIMUM

The study reported here was designed to determine whether climate conditions forced by a southerly displaced jet stream, like those hypothesized for the Last Glacial Maximum (LGM), could cause a large lake to form in the Lahontan Basin of western Nevada.

LGM mean monthly maximum daily temperature (TMAX), the natural log of total monthly precipitation (LnP) and annual runoff (RUNOFF) were computed using a previously developed a climate model (LCM-2) and surface hydrology model (SHM) which were then used to calculate the geometry of lakes in equilibrium with LGM climate conditions.

Model validation using modern climate and gauging station observations suggest that both climate model and runoff model solutions are consistent with modern conditions measured within western Great Basin. However, a sensitivity analysis of LGM-scenario boundary conditions suggest that model bias was introduced when 500 mbar wind directions were computed using NCAR's Community Climate Model version 1 (CCM1) 21 ka wind field solutions.

Two alternative LGM scenarios were developed to compute the lake surface elevation, area and volume for the Lahontan Basin. The first, referred to as Glacial-A, used control (modern) wind field solutions to compute wind direction and were adjusted by a constant value (1.7) to compute 21 ka wind speed. The other scenario called Glacial-B used CCM1 21 ka and control run solutions reported by Bartlein et al., 1998 to compute temperature and precipitation anomalies which were then applied to control-scenario climate solutions of LCM-2.

Glacial-A results suggest that a large lake filled the Lahontan Basin to an elevation of 1327 m. This lake surface elevation is higher than previously reported lake surface elevation of 1265 m and is very close to the pluvial highstand elevation of 1332 m. In contrast, the Glacial-B lake surface elevation was much lower than previous reported estimates for the LGM suggesting that climate conditions influenced by a southerly displaced polar jet stream alone may not explain why large lakes formed in the Lahontan Basin 21,000 years ago.