For the first time since scientists began taking measurements of Lake Tahoe’s famed water clarity in 1968, UC Davis scientists reported today that the historical rate of decline in the lake’s clarity has slowed considerably in recent years.

In 2007, the waters of Lake Tahoe were clear to an average depth of 70.2 feet, a modest improvement from the 67.7 feet reading in 2006, based on how far below the lake surface researchers can see a white 10-inch plate known as a Secchi disk. In 1968, the lake was clear to an average depth of 102.4 feet.

But Geoff Schladow, Director of the UC Davis Tahoe Environmental Research Center, emphasized that the change in the long-term trend is far more significant.

“With new, more sophisticated models for detecting trends and factoring out the effects of annual precipitation,” said Schladow, “We can now say with confidence that the historic rate of decline in the lake’s clarity has slowed off since 2001.”

Schladow cautioned that the data doesn't pinpoint a specific cause for the recent improvements, but noted that new modeling results show that runoff of fine particles from urbanized areas and roadways around the lake are the primary factors that influence clarity levels. Fine particles scatter light and promote the growth of algae.

The report was welcomed by the Tahoe Regional Planning Agency (TRPA) and other agencies charged with protecting the lake, who suggested that the data provide evidence that years of investments in reducing and improving runoff may now be paying off. Federal, state, and local agencies, homeowners, and businesses have spent over half a billion dollars on water quality projects for reducing and improving runoff through Tahoe’s Environmental Improvement Program (EIP), which was launched in 1997 by President Clinton and other federal, state, and local officials.

“The new findings are encouraging,” said John Singlaub, TRPA’s Executive Director, “but they also suggest that we will need to increase our investments in reducing runoff to the lake to clearly reverse the decline and to meet our long-term clarity goals.”