Scientists: Lake Tahoe clarity slipped last year

RENO — The clarity of Lake Tahoe worsened a bit last year, but scientists say in the big picture it’s still positive news about a decade-long trend of stabilization in the azure mountain waters.

The UC Davis Lake Tahoe Environmental Research Center’s annual clarity report showed average clarity in 2013 at about 70 feet, a 5-foot decrease from 2012.

Clarity is measured by the depth at which a 10-inch white disk lowered into the lake remains visible from the surface. Measurements began in 1968, when the disk could be seen to depths of 102 feet.

Tahoe’s worst year for clarity was in 1997 at 64 feet. Scientists said the trend has been one of general improvement since then due in large part to efforts to control erosion and stormwater runoff into the lake.

Last year’s winter clarity —affected by heavy stream inflow from early winter storms — was measured at about 78 feet, 10 feet less than 2012. Summer clarity was measured at about 64 feet, about the same as last year.

“Clarity in Lake Tahoe largely reflected what we saw in the weather in 2013,” said Geoffrey Schladow, director of the research center in Davis, Calif. “At the beginning of the year, clarity was lowered by large stream inflows. At the end of the year, the low inflows resulting from drought conditions helped to improve clarity.”

The clarity level this year is the average of 25 individual readings taken throughout the year. The highest individual value recorded in 2013 was 90 feet, and the lowest was 49 feet, due to seasonal variations.

“Through the seasonal and annual fluctuations, the long-term clarity trend is good news, and it tells us that the investments being made on roadways and properties to infiltrate stormwater are working,” said Joanne Marchetta, executive director of the Tahoe Regional Planning Agency.