In the clear

Despite an abundance of invasive fish threatening to cloud Lake Tahoe’s waters (“Full of carp,” Feb. 28), the lake’s water clarity improved for the second year in a row. Last year, the clarity reading for the lake was 75.3 feet; in 2011, clarity was at 68.9 feet. Before that, clarity had been declining from 2008, reaching a low point of 64.4 feet in 2010. The highest clarity level in the past 12 years was in 2002, which had a clarity reading of 78 feet. While 2012’s numbers improved, researchers at the University of California, Davis Tahoe Environmental Research Center say that the trends still indicate a decline in clarity during the summer months. But the researchers say that the overall improvement is encouraging.

Several factors impact lake clarity, including runoff of urban stormwater. Heavy runoff from snowmelt during the winter and spring seasons carries “small, inorganic particles from the land, roads and other developed areas into the lake,” according to a statement from University of California, Davis.

A Secchi disk, a 10-inch white disk, is used to measure clarity when it is lowered into the water. This method has been used since the late 1960s, when the average clarity depth was 102.4 feet. For an archive of Secchi data from 1968, visit