Burro Fire likely to have minor increase in erosion, runoff

Tuesday, Aug. 7, 2018 6:50 PM
A stump burns in the Burro Fire on June 17.
A burn scar in Bear Creek Canyon during the Burro Fire.
Severity of soil burn in the Burro Fire. Yellow areas indicate moderate burn severity, in which soil structure is largely unchanged.

Fire recovery specialists predict a minor increase in erosion and runoff resulting from the Burro Fire, which scorched 4,866 acres in Bear Creek Canyon northeast of Dolores.

In July, members of a Burned Area Emergency Response team conducted a flyover to map the land affected by the wildfire and determine where emergency and long-term mitigation measures were needed. Surveys of the burn area also were conducted from the ground.

At the conclusion of its analysis, the BAER team produced a map that labels areas according to the severity of soil burn, a key component to identifying where increased soil erosion, surface water runoff, and debris flows might impact human life and safety, property and critical natural and cultural resources.

The map shows that 38 percent of the area within the fire’s perimeter was unburned, and 45 percent burned at a low level. Areas that are ranked at a low burn level may have little change from pre-fire conditions. Typically, less than 50 percent of the litter is consumed in a low-burn area, and needles, leaves and small subsurface roots are mostly intact.

Only 17 percent of the area sustained a moderate soil burn severity, and no areas were found to be of high soil burn severity, according to a San Juan National Forest press release.

In a moderately burned area of a mixed conifer forest, fire may consume up to 80 percent of the ground cover. Surface litter such as leaves or needles will be charred but recognizable. Gray or black ash may cover much of the soil surface, but soil structure and roots remain largely unchanged.

In specific areas that experienced moderate soil burn severity, there could be concern for runoff from steep slopes and resultant increases in post-fire erosion, flooding and debris flows. Soil in moderately burned areas still absorbs water, but more slowly than before a fire.

By comparison, an area that sustains high soil burn may burn small roots and repel water 6 inches deep into the soil, substantially increasing the threat of significant runoff and debris flows.