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Keep Clear

Andrew Pridgen
Sun News Service
Emma Garrard/Sierra Sun
ALL |

Lake Tahoe may see a spike in clarity if dry weather continues through the summer months, said scientists and local agency officials who monitor the lake’s health.

Runoff from streams, which researchers have pinpointed as the major cause of the decrease in the lake’s clarity, should be among the lowest in a decade if the short and dry winter should stay status quo, researchers said.

Currently the Tahoe Basin is at 45 percent of its water content in snowpack and only 65 percent of its average accumulated precipitation, according to Snotel, a snowpack monitoring service.

“Most of the variation in annual clarity is dependent on stream flow and how much snow and rain there was in the winter,” said University of California, Davis researcher Geoff Schadlow.

Unless April storms mirror the “Miracle March” activity of the last two years, or the area is bombarded by summer thunderstorms, summer 2008 may reveal some of the deepest clarity depths in years.

“Of course it’s all very speculative at this point,” said TRPA Communications Chief Julie Regan. “Clarity is all very much runoff related. We would expect this to be a better (clarity) year than the year’s previous, but it’s too hard to say at this point.

“A lot (of clarity can be lost) due to summer thundershowers.”

Both TRPA officials and researchers from UC Davis warned that a one-year-surge in clarity, or a severe drop, does not necessarily mean the lake is trending in one direction or another.

“One of our (least-clear) years was 1997 and that was because of the New Year’s flood and later storms,” Regan said. “The last two years have been wet with late-winter storms and that may show up ” if you ask researchers they’ll tell you that we’re looking at 10-20 year trends, not just a single year.”

According to the most recently released number by UC Davis scientists, the waters of Lake Tahoe were clear to an average depth of 72.4 feet in 2005. This keeps the clarity measurement in the range where it has been for the past five years -and where it was for other multiyear periods in the 1990s, Davis researchers said, reiterating that one year does not tell the whole story. Last year’s lake clarity numbers won’t be released until July or August.

“Lake Tahoe clarity varies from year to year because precipitation varies,” said John Reuter, associate director of the UC Davis Tahoe Environmental Research Center. “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.”

Researchers believe the runoff of fine particles and nutrients fuels algae growth which results in the loss of clarity in Lake Tahoe. The particles and nutrients enter the lake through erosion, runoff and atmospheric deposition.

Clarity is directly affected by the scattering of light by fine particles and by the absorption of light by algae.


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