Lake Tahoe clarity increases 6.4 feet

By Kathryn Reed

For the second year in a row the water in Lake Tahoe is clearer than the preceding year.

As noted last year by the UC Davis scientists who take the measurements, the lake’s clarity has been stable for the last decade. Still, the last time the white dinner plate looking Secchi dish could be seen more than 70 feet below the surface was in 2007.

The Secchi disc is ready to be used to test Tahoe’s clarity. Photo/LTN

The 2012 reading of 75.3 feet is a 6.4-foot improvement from 2011. The disc is dropped over the side of a boat and eyeballed by the scientists on deck. Twenty-two readings are taken throughout the year to come up with the average clarity figure. In 2012, 107 feet was the best reading and 57 feet the worst.

“We are very excited about the results from 2012, especially within the context of the long-term record for annual and winter clarity,” John Reuter, associate director of the UC Davis Tahoe Environmental Research Center, said in a statement. “It is particularly encouraging to see clarity improve during wet years when the amount of fine sediments and nutrients going into the lake is high.”

In the world of science a trend is established at about the five-year mark, according to Kristi Boosman with the Tahoe Regional Planning Agency.

Lake Tahoe clarity readings

• 2012: 75.3 feet
• 2011: 68.9 feet
• 2010: 64.4 feet
• 2009: 68.1 feet
• 2008: 69.6 feet
• 2007: 70.1 feet
• 2006: 67.7 feet
• 2005: 72.4 feet
• 2004: 73.6 feet
• 2003: 71 feet
• 2002: 78 feet
• 2001: 73.6 feet
• 2000: 67.3 feet
— Source: UC Davis

A concerted effort has been under way since 1997, when then President Bill Clinton visited Lake Tahoe for the first environmental summit, to restore the lake’s clarity. That summit led to the environmental improvement program being created and the Lake Tahoe Restoration Act providing money for the program. In the past 15 years more than $1 billion has been spent on various projects in the basin with the primary focus of reducing sediment that clouds the waters of Tahoe.

While monitoring is in place at many of the projects to test what is reaching the lake, it is not possible at this time to point to one thing that is responsible for the improvements.

Researchers say they need more data on stormwater to make more definitive conclusions about why the numbers are changing. Another troubling thing to scientists is the summer clarity numbers continue to decline and they don’t know why.

The Secchi disc has been used since 1968, when the apparatus could be seen to an average depth of 102.4 feet.