Forests and Tahoe clarity: Managing one to enhance the other
October 1, 2006
You may not be able to see more than 100 feet below Lake Tahoe’s surface as you could 120 years ago, but for now it remains one of the clearest large lakes in the world.Since I began studying lake clarity in 1959, Lake Tahoe has lost a third of its remarkable transparency. Nitrogen and phosphorous levels have both increased, and development along its shores has accelerated the nutrients and sediments delivered to the lake.There is growing understanding that everyone loses if Tahoe’s water quality deteriorates. We have also witnessed increased efforts to conserve the water’s clarity.But one major wildfire in the Tahoe Basin could wipe out everything we’re trying to do. One big fire would reduce the lake’s clarity enormously.If a high-intensity fire burns through the basin, significant erosion is inevitable. Post-fire rains and snowmelt will wash mud, ash and debris into the lake. Toxic pollutants and particulate matter from the smoke that rises above catastrophic wildfires largely would be trapped in the basin and ultimately settle into the lake itself.For centuries, forests in the Tahoe Basin helped keep the lake clear. Root systems held soils on the slopes and healthy forests helped clean the air. Then, in 1857, gold and silver were discovered in the Comstock Lode at Virginia City, Nev. Forests were cleared, the wood used to shore up mines and build railroads.Erosion increased dramatically during the Comstock era and heavy sediments loads degraded Lake Tahoe’s clarity. Fortunately, the immediate impact of the Comstock mining subsided and sediment loads eventually dropped to about 20 percent of their peak levels, but the effects of the era may still haunt the lake.When the Comstock mining ended, forests returned to the basin. White fir and brush grew back in dense stands. Human fire suppression efforts over the past century extinguished the lightning-ignited fires that could have naturally thinned the forests, and now the forests around Lake Tahoe are so overcrowded and riddled by bark beetles they pose a major fire hazard.Catastrophic fire, in turn, poses the most significant, immediate threat to Lake Tahoe’s clarity. A single wildfire could put Lake Tahoe back to Comstock conditions. It could stay that way for quite a while, too.Because of the lake’s tremendous depth, small particles of dust and sediments that get washed into the lake remain suspended in the water for years. When fine particles are suspended in the water, they reflect light and decrease light penetration, both of which diminish the lake’s transparency.Imagine what would happen should rain wash 50 million cubic yards of post-fire topsoil and debris into Lake Tahoe like it washed into Lake Isabella following the McNally Fire in and around the Giant Sequoia National Monument in 2002. Or, if 700 million cubic yards of debris clogged Tahoe’s watershed as it did the Santa Ana River watershed after the Old, Grand Prix and Padua Fires in Southern California in 2003.Clearly, some trees and excess fuels must be harvested in overly dense forests to reduce the fire risk and keep Lake Tahoe blue. Better we should manage forestland than watch it burn and devastate Lake Tahoe’s clarity in the process.With UC Davis expecting the grand opening of its world-class research facility in Incline Village in October, the increase in public awareness about what’s happening in the lake and increased conservation efforts, there is cause for optimism regarding the lake’s clarity.However, we must take responsibility for the factors that we can control. Fuel-efficient automobiles that reduce air pollution would help. But the most immediate impact can be had from managing the forests around Lake Tahoe to reduce the threat of catastrophic fire.Well-managed forests can help keep the lake a national treasure. Burning forests could foul the water for generations. If we truly want to keep Tahoe blue, the choice is clear.Charles Goldman, Ph.D, is a professor of limnology at UC Davis and director of the Tahoe Research Group.