Lake Tahoe's clarity improved by more than 6 feet last year, according to data released Wednesday by the University of California, Davis, and the Tahoe Regional Planning Agency.
The lake's average annual clarity in 2012 was 75.3 feet, a 6.4-foot improvement from 2011. It is the highest reading since 2002 when clarity was measured at 78 feet, according to a UC Davis press release.
Scientists measure the lake's clarity by the depth at which a 10-inch white Secchi disk remains visible when lowered beneath the water surface.
The annual clarity level is the average of 22 individual readings taken throughout the year.
While winter clarity has continued a long-term pattern of improvement, summer clarity has kept declining, according to the release.
The rate of decline for average annual clarity has slowed in the past decade, but even this year's value is 22 feet short of the 97.4-foot clarity restoration target set by federal and state regulators.
Urban stormwater runoff, most of which occurs during the winter and spring, contributes to reduced clarity. According to the release, researchers cited TRPA efforts to reduce urban stormwater runoff as a possible cause of this year's improved clarity.
More data is needed to make definitive conclusions though, according to the release.
"The improvement we see in both the summer and winter clarity during 2012 is very encouraging," Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center, said in the release. "The lake will continue to be subjected to a range of disturbances, each of which has the potential to impact clarity. There is now growing belief that managing for clarity is possible."
Clarity readings since 2000
* 2012: 75.3 feet (22.9 meters)
* 2011: 68.9 feet (21 meters)
* 2010: 64.4 feet (19.6 meters)
* 2009: 68.1 feet (20.8 meters)
* 2008: 69.6 feet (21.2 meters)
* 2007: 70.1 feet (21.4 meters)
* 2006: 67.7 feet (20.6 meters)
* 2005: 72.4 feet (22.1 meters)
* 2004: 73.6 feet (22.4 meters)
* 2003: 71 feet (21.6 meters)
* 2002: 78 feet (23.8 meters)
* 2001: 73.6 meters (22.4 meters)
* 2000: 67.3 feet (20.5 meters)
For a complete list of Annual Secchi Depth Data since 1968, visit http://terc.ucdavis.edu/research/SecchiData.pdf.