Sierra droughts not uncommon

Lake Tahoe's water level is falling back into the red zone this summer, with at least two boat launching facilities already closed, long before Labor Day. After last winter's great ski season, many believed that regional drought conditions were over, but now hydrologists are warning that once again water levels are likely to fall below the natural rim.

Last year's El Nino-influenced winter consistently delivered enough snowfall to produce some of the best skiing and snowboarding conditions in years. The active weather pattern, along with some aggressive price reductions for season passes, helped increase skier visits which boosted the bottom line at many local resorts.

Finally, a damp, cool spring with several powerful winter-like storms delayed the snow melt, extended the ski season, and pushed peak stream and river flow into early summer. In the final analysis, however, the actual water content in the Tahoe Basin snowpack was insufficient to overcome three previous below average winters.

For 2010, precipitation in the Central Sierra region averaged between 110 percent and 120 percent of normal, not enough to alleviate current water deficits. Research indicates that it takes roughly two or three years of above normal precipitation to significantly increase Tahoe's water level, and often longer.

Occasionally a big winter can erase water deficits in one fell swoop, like the drought-busting snowfall of 1995 which raised Tahoe's water level nearly 6 feet in one season.

Ironically, the high water levels that resulted from the wet winter of 1995 would ultimately force government agencies to dump excess water from Lake Tahoe during the devastating 1997 New Year's Flood.

Lake Tahoe is a reservoir controlled by the dam at Tahoe City. There is a federal limit on the maximum level of the lake so that it may not exceed 6,239.1 feet in elevation, but when the lake drops to 6,223 feet no more surface water can feed into the Truckee River. Since construction of the Lake Tahoe Dam in the early 20th century, yearly water levels have fallen below the natural rim about 20 percent of the time.

For Tahoe, the historic 1930s "Dust Bowl" drought was eclipsed less than 15 years ago, when below normal winters plagued the region from 1987 to 1994. In 1992, when the Tahoe Basin was experiencing the peak of its worst dry spell on record, the lake level fell to 6,220 feet.

For nearly three years in the early 1990s, no surface water from Lake Tahoe entered the Truckee River, a scenario that drastically cut down a vital water source for Reno and the Truckee Meadows.

• Tahoe historian Mark McLaughlin is a nationally published author and professional speaker. You can reach him at mark@thestormking.com.