Tahoe's clarity is measured by how far below the surface scientists 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, we can now say with confidence that the historic rate of decline in the lake's clarity has slowed off since 2001," he said.

The data doesn't pinpoint a specific cause for the recent improvements, but Schladow noted that new modeling results show that runoff of fine particles from urbanized areas and roadways are the primary factors that influence clarity.

Fine particles scatter light, promoting algae growth.

The report was welcomed by the Tahoe Regional Planning Agency.

"The new findings are encouraging," John Singlaub, Tahoe agency executive director, said in a written statement.

"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."