Scientists say the clarity of Lake Tahoe has improved, and more importantly, the rate of decline has slowed considerably in recent years. In 2007, the waters of Lake Tahoe were clear to an average depth of 70.2 feet, a slight improvement from the 67.7 feet reading in 2006, according to a report by researchers with the University of California, Davis. 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," Schladow said. He said 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. 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, which was launched in 1997 by President Clinton.