Map courtesy of the U.S. Forest Service
A map of the soil burn severity that resulted from the Crescent Mountain Fire.

Erosion, flooding, debris flows are potential threats

The Crescent Mountain Fire burned one-third of the Twisp River watershed last summer, increasing risks of erosion, flooding and debris flow in numerous steep drainages, particularly the Scaffold Creek, War Creek, Reynolds Creek and Williams Creek drainages.

A post-fire study found that those are among the most severely burned areas by the Crescent Mountain Fire, which was ignited by lightning on July 29 and continues to smolder in mountains southwest of Twisp.

“Debris flows in these areas are a risk to life and safety for forest visitors and workers, and to property including roads, trails and campgrounds,” according to the Burned Area Emergency Response (BAER) report.

When slopes are burned bare of vegetation during wildfires, and soils are damaged so that they repel rather than absorb water, the potential for post-fire runoff and erosion increases dramatically. Sediments may move downstream and damage roads, trails, campgrounds and houses, and put wildlife and community water supplies at risk. U.S. Forest Service BAER teams — comprised of hydrologists, soil scientists, engineers, biologists, vegetation specialists, archeologists — evaluate the post-fire risks and recommend steps to reduce them. 

The Crescent Mountain Fire perimeter includes 52,610 acres of Forest Service land in the Okanogan-Wenatchee National Forest, according to a summary of the BAER assessment released last week. The fire spread through a combination of slow creeping and short runs, spotting and burnout operations, resulting in a mosaic of fire severity on USFS land — 2,285 acres of high, 18,454 acres of moderate, 24,073 acres of low and 7,715 of unburned,” the report found. A BAER report on the McLeod Fire is still underway.

About 11,491 acres were found to have burned so hot that soil composition changed, making soil “hydrophobic” or water-repellant, and contributing to erosion and runoff.

The Crescent Mountain Fire assessment found 33,737 acres to have a high “soil erosion hazard rating,” with potential to produce up to 3.84 tons of material per acre, and up to 1,534 cubic yards of sediment per square mile.

The BAER analysis concluded that the “probability of debris flows is likely (above 60 percent) to very likely (above 80 percent) for numerous steep side channels within the Scaffold Camp Creek, Oval Creek, War Creek and North Creek sub watersheds and numerous steep drainages directly tributary to the Twisp River. To a lesser extent, debris flows are also likely to very likely in steep side channels within the Williams Creek and South Creek sub watersheds.”

There is a risk to human life and safety and private landowners downstream of the burned area, due to potential impacts of flooding, debris flows, hazard trees and rockfall along trails and roads in the Twisp River drainage, according to the BAER report. “Individuals who may find themselves in portions of the burn area along any of the drainages or roads affected by fire are at risk during storm events,” the report said.

Emergency treatments

The BAER team recommended a number of “emergency treatments” to protect human safety and prevent damage to property and natural resources on Forest Service land.

The report recommended treatments to support recovery of native plants by minimizing proliferation of noxious weeds. The analysis advised early detection and rapid response to prevent invasive weeds from becoming established in fire-damaged areas, including on 13 miles of bulldozer lines created to fight the fire.

Weeds can be introduced on Forest Service land when bulldozers or other equipment pass through weed-infested areas on their way to fight the fires. “Dozer lines below 4,000 feet elevation and roads and trails through the burned areas were judged to be at greatest risk for spreading whitetop, dalmatian toadflax, diffuse knapweed, sulphur cinquefoil and other aggressive invasive weeds,” the report said.

Early detection and rapid response, particularly in the first year following the fire, is key to minimizing the spread of those weeds into previously undisturbed national forest lands, according to the report.

The BAER team also recommended treatments to protect roads and trails within the Crescent Mountain Fire perimeter. There are approximately 51 miles of Forest Service roads within the fire perimeter, and about 22 miles of roads are located on or below moderate- and high-severity burn areas, and therefore at risk of damage.

Road treatments designed to improve drainage to handle higher amounts of runoff were recommended for 14 miles of the road system in the fire perimeter. The treatments included drainage dips to handle runoff, and armored dips at stream crossings to increase overflow capacity. The BAER team also recommended that vulnerable roads be evaluated during or immediately after significant runoff-producing events, to ensure that drainage is functioning well enough to prevent damage.

Trail work

Treatments were recommended on trails where fire destroyed brush, roots and logs that stabilize the trails. Trails that pass through or below areas of moderate or high soil burn severity may need to be stabilized by installing dips and water bars, removing berms and improving stream crossings. Hazards such as rocks and trees should be removed, the BAER report said. The recommended improvements should be designed to last up to three years.

Recommendations also included installing warning signs or imposing temporary closures for areas at particular risk of flooding, debris flow and rockfall. “Based on a high-risk rating, a temporary closure is proposed for the War Creek Trail to protect public safety. Warning signs are proposed to lower the risk for users of other trails within the burned area,” the report said.

A trail bridge over Eagle Creek was partially burned and needs to be assessed before the trail is opened to the public, the report advised.

Two campgrounds — Roads End and Horse Camp — were proposed for temporary closure “based on high risk ratings for debris flow and flood risk.” Warning signs advising of risks of recreating in burned areas were recommended for users of dispersed campsites near the Gilbert ghost town adjacent to North Creek.

The BAER team advised the Forest Service to coordinate with the National Weather Service, the Natural Resources Conservation Service, Okanogan County and Okanogan Conservation District to support those agencies’ efforts at mitigating flood and debris flow risks for communities and private land owners in areas below the burn. Monitoring of treatments for weeds, roads and trails should continue, the report said.

A funding request based on the recommended treatments was submitted to the Pacific Northwest regional forester in Portland. Information on that portion of the BAER report was not yet available.

Information on BAER and reports can be found at centralwashingonfirerecovery.info.