Study: Lake Tahoe's Decline Is Slowing

For the first time since researchers began continuously measuring Lake Tahoe's famed water clarity 40 years ago, University of California, Davis scientists have reported that the historical rate of decline in the lake's clarity has slowed considerably in recent years.

Scientists at the UC Davis Tahoe Environmental Research Center say that by using new, more sophisticated models for detecting trends and, by factoring out the effects of annual precipitation, they have concluded that the historic rate of decline in the lake's clarity has slowed since 2001.

"From 1968 to 2000 there was a near-continuous decline in lake clarity. There were several years at a time when things seemed to improve, but invariably we returned to the same trend," said Geoffrey Schladow, a professor of civil and environmental engineering who directs the research center. "But since 2001, we have had seven years in which the clarity has consistently been better than the long-term trend would have predicted. This is unprecedented."

Schladow cautioned that the data do not pinpoint a specific cause for the recent improvements but noted that new modeling results show that runoff of fine particles from both urbanized areas and roadways around the lake are the primary factors that influence clarity levels. 

In addition, Schladow and his colleagues cautioned that it is difficult to use data from a small number of years (2001 to 2007) to draw conclusions about when the trend might change from a slowdown in clarity decline to an improvement in clarity. "Only with the commitment to long-term monitoring can we truly evaluate environmental changes over time," he said.

Even so, the report was welcomed by the Tahoe Regional Planning Agency and other agencies charged with protecting the lake, who suggested that the data provide evidence that years of investments in reducing runoff may now be paying off.

Federal, state and local agencies, as well as local homeowners and businesses, have invested more than $500 million in a coordinated effort to reduce runoff through Tahoe's Environmental Improvement Program, which was launched in 1997 by President Clinton and other officials.

In 2002, the states of California and Nevada cooperatively began to develop a water-clarity restoration plan for Lake Tahoe, known as the total maximum daily load or TMDL. The agencies now are in the process of completing the TMDL, as well as a 10-year update to the Environmental Improvement Program and a new regional plan for the Tahoe basin.

As part of those efforts, the California Lahontan Regional Water Quality Control Board and the Nevada Division of Environmental Protection recently issued a Clarity Challenge that calls for an improvement in clarity to between 77 and 80 feet in 15 years. (In 2007, the waters of Lake Tahoe were clear to an average depth of 70.2 feet; in 1968, the depth was 102.4 feet.)

A recent report for the TMDL program demonstrates that this challenge is achievable, said Harold Singer, executive officer of the Lahontan Water Board.

"Source-control measures are the most effective means to reduce fine sediment reaching Lake Tahoe," Singer said. "Such efforts include restoring disturbed lands to increase infiltration and minimize soil erosion, along with more attention to the nature and amount of applied road abrasives. Efforts to treat stormwater runoff also need to focus on removing these very fine particles."

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