Report details hydrological impact of Kootenai fire

by WILL MOSS - Ravalli Republic

When wildfire burns its way through a forest, it’s often the immediate and obvious effects that garner the most attention: Smoke in the air and homes threatened by flames are at the fore of most people’s concerns.

But for forest managers and landowners, fire’s impacts on the soil, wildlife, water and other resources can be just as important.

In mid-October, the Forest Service conducted a Burned Area Emergency Response (BAER) assessment on the post-burn impacts of the Kootenai Creek fire.

On Monday, they released a hydrology report detailing the effects the burn will have on watersheds.

“It’s all based on the extent of the fire and its severity, how much of the area burned at high intensity and where those spots are located,” said Bitterroot National Forest Hydrologist Ed Snook, a member of the team that flew over the area last month.

Depending on where a fire burns, the angle of the slope and the effects on vegetation, varying degrees of post-fire watershed response are possible.

The responses include an initial flush of ash, overland flow and associated rill and gully erosion on steep slopes, rainfall-derived flash floods and debris flows and an increase in snowmelt water yield due to changes in evapotranspiration.

According to the report, the Brooks Face watershed, which drains to the east between Stevensville and Florence, received the most severe impact with about 41 percent of the area burning with moderate or high severity.

That area had about 1,120 total burned acres with 560 acres burning at moderate or high severity.

“That’s probably where the post-fire watershed response (erosion events) will come closest to affecting people,” Snook said. “There are ranches right at the base of that slope and the effects tend to carry as long as you have slope to push it downhill. They may see some localized erosion and some tongues of sediment and ash deposit on the ranch roads and in their head gate areas.”

Snook said the Forest Service will contact landowners to inform them of what to expect.

The Kootenai-Larson Creek watershed, which held the bulk of the fire, only saw a moderate to high severity burn on about 13 percent of total acres burned.

In that watershed, 5,388 total acres burned with about 2,694 acres burning at moderate to high severity.

According to the report, the probable watershed responses in those areas could put some man-made resources at risk, including forest road crossings in upper Larson Creek, foot and horse trails in Kootenai Creek Canyon and ranch roads or headgates below Brooks Face.

The report estimates the vegetative recovery period for the burned areas will be three to five years.

The burn is not expected to affect residences in the Kootenai Creek valley bottom because the main fire area within Kootenai Creek Canyon is far enough from residences that debris flows or flash flooding within the high severity burn areas would dissipate in the several miles of channels before approaching residences.

Hydrologic models suggest the fire effects may produce minor increases in flow during melt or rain events, and especially during moderate to intense summer thunderstorms.

That may exacerbate water level and bank erosion issues by a small amount where they pre-dated the fire, but it’s not likely to add substantially to flood or erosion risks.

The report surmises that increased erosion may have an effect on the Kootenai Creek Trail, especially where water bars have burned out or fire debris has plugged runout ditches.

In some places, wood incorporated into the trail tread has burned out, altering trail drainage and increasing erosion potential.

While the increased burn severity on the Brooks Face suggests an elevated potential for surface erosion and gully formation, channels draining the face do not closely approach any residences before they fade away into agricultural lands below.

Residents should avoid crossing flooded ranch roads and be aware they may be stranded in some locations for short periods.

The report made recommendations as to what actions could be taken to reduce any adverse effects.

The recommendations include cleaning culvert outlets on Forest Road 740 in upper Larson Creek, repairing damaged drainage structures and tread on trails before the next projected damaging storm and notifying land owners about possible high flows and increased ash and sediment deposit.

Snook said that from an ecological standpoint, the burn presented little out of the ordinary.

“The fire was well within the realms of a natural event,” he said. “It burned in varying intensities in different areas. It will provide habitat for post-fire species and the amount of fuel that it utilized will change the pattern of fuels on the landscape, so that the next fire that comes along will respond differently when it breaks into this area.

“And despite the fact that so many people were inconvenienced by the smoke, it’s really not a bad thing. It’s a good thing for the environment for it to burn in pieces like that.”

The lightning-caused fire began in July and burned a total of about 6,700 acres.

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Reporter Will Moss can be reached at 363-3300 or wmoss@ravallirepublic.com.