Erosion from Tahoe fire may hurt lake's health

Last week's fire occurred in a key watershed responsible for a quarter of all the pollutants entering the lake.

By Ben Arnoldy | Staff writer of The Christian Science Monitor

from the July 5, 2007 edition

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South Lake Tahoe, Calif. - With the Angora wildfire contained, officials are now racing to stave off damage to the famous, cobalt-blue Lake Tahoe.

In just over a week, the fire burned 3,100 acres, forced the evacuation of 3,500 people, and cost $11.3 million to fight. Property losses from more than 325 homes and structures could tack another $150 million onto the tab.

Long after the embers fade, the fire is expect to impact the clarity and health of North America's largest alpine lake. Its stewards are scrambling to prevent runoff from the burn area, an effort that could be complicated by community frustration with past antierosion regulations.

"Lake Tahoe is revered for its cobalt blue, clear water," says Charles Goldman, a lake researcher at the University of California at Davis. "It's one of the clearest large lakes in the world, even with the transparency loss [in recent years]."

The lake has lost a third of its transparency since Dr. Goldman began monitoring it in 1959. Visitors can still see 22 meters down, but that figure shrinks about one foot each year. Human impacts are to blame: auto exhaust, smoke, road dust, and runoff from nearby development.

That runoff may be accelerated by the fire's destruction of the vegetation that holds soils in place, bringing fine particles into the lake. And nitrogen and phosphorous from the fire threaten to turn the lake green by spurring algae blooms.

These effects aren't merely aesthetic. In the long run, such impacts could diminish the deep-water oxygen needed for the lake's trout, says Goldman.

"It's kind of like the canary in the coal mine: We've used water clarity as a symbol of whether the whole lake ecosystem is getting better," says Michael Donahoe, conservation co-chair with the Tahoe Area Sierra Club.

A greater threat, however, is erosion. The burn area covers 10 percent of the Upper Truckee River watershed, and 25 percent of all water and pollutants entering the lake come from that river, says John Reuter with the Tahoe Environmental Research Center at UC Davis.

The forest service, state officials, and local planners are working on erosion mitigation strategies. Even during the firefighting effort, trenches known as water bars were dug to manage runoff. Next steps include planting trees, such as willows, to hold together steep, burned-out slopes. Another possibility, says Dr. Reuter, is diverting the flow of Angora Creek through a nearby meadow that could act as a natural water filter.

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