Summer rain brings threat of erosion

With firefighters gaining the upper hand on a five-day-old wildfire, authorities turned Friday to the task of preventing soil and ash from washing into Lake Tahoe's famously blue waters before summer thunderstorms set in.

Even as they kept extinguishing hotspots, fire crews pivoted their attention to the work of reversing damage wrought by the fire, as well as their own efforts to combat it. A team of 25 biologists was scheduled to help them thwart erosion, heal bulldozed fire line scars and encourage new plant growth.

"I personally can't think of a more sensitive area that's burned in the western United States in a long time," said Matt Mathes, a U.S. Forest Service spokesman. "It's an international tourist destination and it is a relatively pristine body of water."

Speed was of the essence, soil and water scientists agreed, with perhaps only three or four weeks to go before summer rains arrive. Soil from denuded hillsides muddies the lake, while the nitrogen and phosphorous in ashes makes green algae grow on its sparkling surface.

Scientists said the so-called Angora Fire was the worst wildfire here in a century. It also burned in a particularly sensitive area: a watershed that provides a quarter of the water that runs into the lake. About 10 percent of the watershed - some 3,100 acres - was destroyed.

In some areas, scientists say, the fire burned so intensely it "mineralized" the soil, destroying the nutrients necessary to sustain plant life. It also may have created a layer of soil about 5 centimeters deep that is impervious to water, which could keep moisture from soaking in.

That means workers may have to amend the soil before anything can grow back, and to prevent erosion, lay down mesh mats, mulch and logs.

Wally Miller, a University of Nevada soil and hydrology professor, said the last major fire to hit the Tahoe area, the much smaller Gondola fire of 2002, created the ill effects that scientists fear will result from the current conflagration.

The fire mineralized the soil in areas and created a water repellent layer. A heavy rainfall three weeks after the fire caused serious erosion on the slopes.

If soil and ash flow into the lake from streams and creeks in the newly burned area, algae could harm plants growing beneath the surface where trout spawn. It would not, however, turn the lake from blue to green.

"It will still be blue, but not a deep shade of blue," said John Reuter, associate director of the University of California, Davis Tahoe Research Group. "It could be green in pockets."

The conditions that led to the catastrophic fire have been brewing for more than a century, said Zach Hymanson, executive director of the Tahoe Science Consortium.

Clear cutting during the last part of the 19th century made the forest grow back twice as dense. Then, as people settled in the region, they put out small fires that would have rid the forest of dead wood.

After drought in the 1980s and '90s weakened many trees that then fell victim to beetles., the forest became a tinder waiting to ignite.

"It's a result of our past sins of the last 100 years," Hymanson said.

• Associated Press Writer Scott Lindlaw contributed to this story.