U.S. Water News Online
RENO, Nev. -- Lake Tahoe's clarity last year hovered around the same range it has been the past five years after showing a marked decline in previous decades into the late 1990s, researchers said.
The lake's waters were clear to an average depth of 72.4 feet in 2005, a slight change from the average depth of 73.6 feet that a white plate called a "Secchi disk" could be seen the year before, scientists at the University of California-Davis said in an annual report.
When measurements began in 1968, the disk was visible at an average depth of 102.4 feet. With few exceptions, the clarity level steadily declined on average through the 1970s into the mid-1980s.
The worst reading was recorded in 1997 -- 64 feet -- the year then-President Clinton and Vice President Al Gore convened an environmental summit at the lake to address growing concerns about pollutants and soil particles in the waters.
U.S. Interior Secretary Dirk Kempthorne and others will reconvene the summit for an update a restoration efforts at the lake, the second deepest lake in the United States at 1,645 feet, behind Oregon's Crater Lake at 1,949 feet.
Scientists believe the fine particles of soil and nutrients that fuel algae growth are causing the loss of clarity. The pollutants enter the lake through erosion, runoff and atmospheric deposition.
The clarity is directly affected by the scattering of light by fine particles and by the absorption of light by algae, the experts said.
John Reuter, associate director of the UC Davis Tahoe Environmental Research Center, said the clarity varies from year to year because precipitation varies. He said that makes it difficult to use data from any single year or even a small number of years to draw conclusions about whether the lake is improving overall or getting murkier.
He said restoration efforts led by local, state and federal land managers in the Tahoe Basin are focused on the long term.
"While this year's clarity number is encouraging, the annual measurements remind us how crucial it is to stay the course in our efforts to restore Lake Tahoe and to preserve it for future generations," said Julie Regan, spokeswoman for the Tahoe Regional Planning Agency.
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