Lake Tahoe Gets Clearer

Lake Tahoe clarity improved in 2011, but overall has remained nearly stable since 2000, according to UC Davis scientists who study the lake and reported on the UC Davis site.

Data released by the UC Davis Tahoe Environmental Research Center and the Tahoe Regional Planning Agency (TRPA) reported the average annual clarity level for 2011 at 68.9 ft, a 4.5-ft improvement over 2010, when average clarity levels were the second-worst on record.

Winter clarity last year continued a decade-long pattern of improvement, while summer clarity continued to decline at the same rate that it has since the late 1960s, when monitoring began.

Average annual clarity in the past decade has been better than in recent decades. In 1997-1998, annual clarity reached an all-time average low of 65.1 ft. From 2001-2011 the average clarity was 70.6 ft.

Yet this year’s value is still more than two dozen feet away from the clarity restoration target of 97.4 ft set by federal and state regulators.

Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center, said it is important to remember that long-term trends are a better indication of Lake Tahoe clarity than year-to-year comparisons.

“The factors that contribute to lake clarity are complex, and are not necessarily linked to factors occurring in the current year,” said Schladow. “For example, the 2011 clarity improvement followed a winter that was one of the wettest in recent years, something that is usually associated with clarity declines. Understanding what controls the long-term trends is at the heart of what we are attempting to do.”

Researchers provided measurements for both winter (December-March) and summer (June-September) months. The winter average of 84.9 ft in 2011 was well above the worst point seen in 1997 and a 12-ft improvement over 2010.

Urban storm water runoff has long been one contributor to reduced clarity at the lake. Most of that runoff occurs during the winter and spring, when rain and snowmelt carry small, inorganic particles from the land into the lake.

Yet despite a wet winter during 2011, clarity improved. Researchers say this could indicate that efforts led by TRPA, other management agencies, local jurisdictions and private property owners to reduce urban storm water runoff are having an impact. However, they emphasize that they need more data on storm water to make definitive conclusions.

Summer clarity levels continued to show a decline. The 2011 value of 51.5 ft was the second worst on record, which the UC Davis data suggest may be due to the large and late spring snowmelt, which carried enormous amounts of fine sediment and nutrients from the watersheds surrounding the lake.

Clarity is measured by the depth at which a 10-in. white disk, called a Secchi disk, remains visible when lowered beneath the water’s surface. The measurements have been taken since 1968, when the Secchi disk could be seen down to 102.4 ft.