Federal experts said Tuesday that the Rim fire did severe damage to soil on 7percent of its vast acreage soil that could erode into water supplies if not treated.
The 7percent figure is fairly low compared with other blazes, and considering how deep into the dry season the Rim fire happened, the Burned Area Emergency Response team said in a brief report on its early findings.
Almost all of the fire, which was reported at 256,528 acres as of Tuesday, has burned in the Tuolumne River watershed. It supplies the Modesto and Turlock irrigation districts and a San Francisco-owned system serving much of the Bay Area.
The report said 37percent of the Rim area had moderate soil damage, meaning that much of the structure stayed intact and could help buffer the rainstorms that might cause erosion in fall and later. The other 56percent of the land within the fire perimeter had light damage or did not burn at all.
Tuolumne Irrigation District spokesman Herb Smart said the district is in frequent contact with the team as it studies the effects on the watershed this year and beyond.
Yes, 7percent of the fire area has high risk for erosion, he said by e-mail, but even the 37percent that received a moderate soil burn severity can pose problems for TID depending on the timing, magnitude and duration of future precipitation.
Large storms could bring silt and debris into Don Pedro Reservoir, where the TID and Modesto Irrigation District store water for farmers. The MID also treats water for domestic use in the Modesto area.
The fire has destroyed 11 homes, extensive timber stands and several recreation sites in the Stanislaus National Forest and Yosemite National Park. It remained at 84percent containment Tuesday, but its spread has slowed greatly since its Aug.17 start. Authorities say it was sparked by a hunters campfire.
The response team a partnership of federal agencies made the soil conditions a priority because of the looming storm season. The recovery also will involve salvage logging in parts of the national forest, replanting timber stands, and repairs to meadows, campgrounds and other sites.
Team member Randy Westmoreland, a soil scientist for the U.S. Forest Service, said the worst damage tends to be in places where the fire burned the forest canopy, the ground cover, and perhaps even roots and organic matter in the subsurface.
In the moderate areas, he said, the canopy might have burned but the soil retains organic matter that could shield it from erosion, as well as vegetation that could quickly sprout.
(The fire) was moving across the land pretty fast, so we did not get a lot of heat in the ground in all of the areas, Westmoreland said.
The light soil damage includes land that burned in recent decades, with brush or young timber growing on it.
One other thing in the water agencies favor: Most of the Tuolumne watershed lies outside the Rim fire perimeter. It stretches to the crest of the Sierra Nevada, taking in forest, rocky areas and glaciers far from the flames.
Over the next few weeks, land managers will consider measures against erosion, such as erecting physical barriers and seeding with grasses. The team plans to present a more detailed report next week to Forest Supervisor Susan Skalski, who will pass it on the the Forest Services national office.
Despite the low rate of intense burning, this is the third-largest fire in the states recorded history, and it has taken a tremendous toll, including mature timber and some growth that followed earlier fires.
The intensity of it and the enormity of it takes your breath away, said Mike Albrecht, a forester and president of Sierra Resource Management, a Jamestown-based company that does logging and other forest work.
He advocates for forest thinning, which produces logs for sawmills while leaving the remaining trees more resistant to fire. Thinned stands helped reduce the Rim fires intensity, he said.