GROVELAND, Calif. — As autumn turns to winter and rain falls over the charred landscape left behind by the Rim fire, forest rangers and emergency planners have a new worry: water.
Over 90 percent of the blaze burned in the Tuolumne River watershed, where more than 2,600 miles of streams cut through steep, now-burned slopes of the Sierra Nevada. Those mountains are primed for flooding and debris flows in a big storm.
The 410-square-mile blaze — California’s third-largest on record — ignited on Aug. 17 in the Stanislaus National Forest and burned into the northwest part of Yosemite National Park. More than two months later, the fire is fully contained, but some of the most serious hazards are just now presenting themselves.
Trails and roads are at risk of washing away, cutting off access to world-class white-water rapids. Burned trees and debris will almost certainly be flushed downstream, fouling irrigation water supplies.
San Francisco officials are closely monitoring hydroelectric facilities, soil conditions and water quality in and around Hetch Hetchy Reservoir, where the fire crept around the edges of the city’s drinking water supply and made some slopes more prone to erosion.
The U.S. Forest Service has rushed to prepare culverts, stabilize roads and trails, and put mulch and straw bales over burned soil to keep it from sliding away in heavy rain. Rangers have closed roads and campgrounds and posted signs to warn of falling rocks and trees.
‘Not over when the fire’s out’
“The emergency’s not over when the fire’s out,” said Jason Carkeet, utility analyst for the Turlock Irrigation District. His agency has purchased extra booms to capture logs and woody debris that the Tuolumne River is likely to dump into 26-mile-long Don Pedro Reservoir, which stores water to irrigate more than 200,000 acres of Central Valley farmland.
After two dry years, officials would welcome rain and snow, but they shudder at the thought of a storm that drops too much at once. Scientists predict that 15 minutes to an hour of intense rainfall — the type of storm that happens about every 10 years — would be enough to unleash a slurry of boulders, fine mud and brush.
Normally, rain bounces off trees and brush, slowly percolating through the soil. But after a fire, the earth sits unprotected and, if severely burned, can even repel water. With fewer twigs, leaves and vegetation to slow down the water, it picks up speed and flows over the soil in sheets.
“You get 3 inches of rain in 15 minutes, and things can happen,” said Jerome DeGraff, a geologist with the U.S. Forest Service who helped draft a post-fire assessment of the burned area. The report found that 44 percent of the soil burned at high and moderate severity, a predictor of how susceptible the slopes are to slides, erosion and runoff.
Most catastrophic would be a rain-on-snow storm, in which showers fall on thin snowpack, melt it and send monster runoff downstream.
That’s what happened in January 1997, when a huge storm dropped rain over snow and sent floodwaters and mounds of debris barreling down the Tuolumne River into Don Pedro Reservoir, clogging its marinas.
Though the area could use a wet winter to fill the reservoir, “we don’t want gully-washers,” said Carol Russell, director of the Don Pedro Recreation Agency. “If we could just have a little bit of rain all winter long, we would be happy. Of course, California doesn’t really do that.”
Among the routes most at risk of being obliterated is Lumsden Road, a steep, narrow dirt road that drops down a 2,000-foot canyon to the banks of the Tuolumne River and provides access to the area’s most popular rafting and kayaking run.
Steve Welch, general manager of Groveland-based American River Touring Association, a nonprofit that charters white-water trips, dispatched employees down that road in August with inflatable rafts to ferry firefighters across the river as the blaze raged.
“That’s our lifeline,” Welch said. “It’s how everyone gets to the launch point. It is now the most tenuous thing that we have, looking forward.”
Before any significant rainfall, portions of roads and trails this fall have already been covered with dirt that has slid down. Trees, roots and underbrush that once held soil in place were burned away.
“Forget rain, it’s already showing high rates of erosion,” said Jeffrey Mount, founding director of the UC Davis Center for Watershed Sciences, who has studied, rafted and hiked along the Tuolumne for more than three decades. Once debris is set loose by rain, it could take years to work through the watershed’s channels, he said.
“If you have a dry winter, you’re better off because you allow some slope-stabilizing vegetation to get in,” Mount said. “The re-colonization of the slopes takes place amazingly fast after that.”
Tests have shown that in many areas, the soil was not as scorched as initially feared.
That includes the steep, granite valley around Hetch Hetchy Reservoir, a priority during the firefighting effort because it stores 85 percent of the drinking water supply for 2.6 million people in the Bay Area, including San Francisco and Hayward. Though some of the Rim fire burned so hot it could take away the soil’s ability to absorb water, scientists for the Hetch Hetchy Regional Water System do not believe areas near the reservoir were scorched enough to be prone to major slides.
The fire’s toll is more evident downstream, where hundreds of wooden utility poles were destroyed by the flames and are being replaced by helicopter. In more severely burned areas, public utility officials worry that debris flows could take out key roads used to access hydroelectric facilities that power San Francisco’s airport, streetlights and city buildings.