Lake Tahoe’s clarity improves over last year |

Lake Tahoe’s clarity improves over last year

Dylan Silver
Dylan Silver / Tahoe Daily Tribune

Lake Tahoe’s clarity improved in 2011, but its famed waters are far from historic values and goals set forth by federal and state regulators, according to a Wednesday report from the UC Davis Tahoe Environmental Research Center.

In 2011, the average depth a 10-inch white disk, called a Secchi dish, remained visible was 68.9 feet, about 4.5 feet deeper than the previous year’s average.

“We’re encouraged that lake clarity is improving and seems to be responding to the substantial restoration investments we’ve collectively made through the Environmental Improvement Program,” Joanne Marchetta, executive director of the Tahoe Regional Planning Agency, said in a statement.

Though TRPA is hopeful lake clarity is improving, the numbers tell somewhat of a different story.

In the 44 years since measuring began in 1968, last year’s average is the seventh lowest on record. Though higher than 2009 and 2010, the 2011 measurement is lower than 2007 and 2008. The deepest average recorded was 102.4 feet in 1968; the shallowest was 64.1 feet in 1997.

The wet winter of 2010-11 is believed to have played a role in the poor summer measurements, which averaged to just 51.4 feet, the second worst summer average on record. Winter clarity averages, at 84.9 feet, were the third deepest they’ve been in 20 years.

Federal and state regulators want to restore Lake Tahoe to an average clarity depth of 97.4 feet.

In collaboration with federal, state and local agencies, TRPA hopes to contribute to that and further protect the lake with its Lake Tahoe Environmental Improvement Program, first implemented in 1997. The agency lists treating stormwater from hundreds of miles of highway, managing the implementation of “best management practices” on thousands of private properties, creating more than 120 miles of bike and pedestrian paths and decontaminating thousands of watercraft as the project’s successes.

But it’s unclear exactly what impact the program is having on Lake Tahoe’s clarity.

“The factors that contribute to lake clarity are complex, and are not necessarily linked to factors occurring in the current year,” Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center, said in a statement. “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.”

With wet winters usually meaning a decline in clarity, TRPA hopes last year’s measurements are a signal its efforts are paying off. But officials are careful about claiming victory just yet.

“The scientific community feels that the cause and effect is still somewhat nebulous,” said TRPA spokeswoman Kristi Boosman. “And we strongly support that.”

More information about environmental factors affecting Lake Tahoe will be included in the 2012 State of the Lake Report, expected this summer.

BOX: More info:

For a complete list of Annual Secchi Depth Data since 1968, visit:

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