Officials blame the low level on three straight years of below normal snow and rainfall.
Local hydrologists expect this week's storms to help, but not significantly. The last time the water fell below the rim at the 6,200 foot level, was January 2005.
The lowest it's ever been was in 1992.
Once the water drops below the rim, water won't flow into the Truckee River, which means less water for downstream reservoirs.