A 50-foot larch, approximately 3 feet in diameter, was discovered at the top of a debris flow that damaged Durham Creek Road in three places and sent a sediment plume into the Blackfoot River in July 2018. The debris flow was as high as 15 feet in some places.

Debris flows in the area burned by the 2017 Rice Ridge fire ended up sending a sediment plume into the Blackfoot River this week, making fishing difficult temporarily and possibly causing problems for bull trout.

The debris flow occurred last week, after a microburst of rain let loose a mix of mud, rock and trees up to 15 feet tall in places, noted Kate Jerman, a spokesperson for the Lolo National Forest. The Rice Ridge fire burned particularly hot in the area, and the mix of hydrophobic soils, lack of vegetation, dead trees and heavy rainfall created the right recipe for the flows.

Dunham Creek Road was damaged in three places and temporarily trapped a group that was trying to get out. They called for help via a satellite phone, and were rescued the next day. Contractors worked throughout the weekend to remove the rest of the debris, unplug and install a new culvert to enhance water flows under the road, and perform additional cleanup.

Dunham Creek Road is a popular access route to the Lodgepole Creek Trailhead and Bob Marshall Wilderness Area.

“Bull Creek Forestry did excellent work … to stabilize the road where it was impacted,” Jerman said. “There was a remarkable amount of movement; they found mud marks on trees 15 feet tall. In other areas they found debris piled up; woody materials and rocks were 15 feet high. So there was a substantial amount of water movement from that storm last week.”

Patrick Uthe, a fisheries biologist with Montana Fish, Wildlife and Parks, said the debris flows deposited a large sediment plume into the area’s creeks, which eventually made their way into the Blackfoot River below Monture Creek last Wednesday and to Bonner by Thursday. The thick, brown sediment plume diluted once it hit the Clark Fork.

Uthe said people fishing the Blackfoot told him the plume looked like chocolate milk.

“The glacial sediments are super-fine,” he said. “We’ve had sediment-rich runoff in that area before, but this was more significant because of the landslide and large debris flow. And it’s highly possible it will happen again. If we have a thunderstorm or significant rainfall, it will mobilize that sediment and cause the tributaries to become turbid again and flow into the Blackfoot. So for the foreseeable future there’s likely to be short-term increases in sediment.”

He added that it’s hard to say if any future sediment plumes will be as large as this one.

Montana Fish, Wildlife and Parks is watching for long-term impacts from the sedimentation, especially on bull trout populations, which are listed as threatened under the Endangered Species Act. Bull trout’s peak spawning occurs in early- to mid-September, and the sediment fallout can act as a blanket, covering their eggs and smothering them.

“But the flip side is with fire-related disturbances there are some long-term benefits,” he added. “There can be increased productivity from the nutrient-rich sediments going into the stream. That will increase the insect production, which is a food base for those fish.

“So it’s kind of a double-edged sword. Those fish have adapted in this system.”

He said the biggest problem from the plume probably was for anglers, who can get caught off guard and were alarmed by the sediment plume. The area where it occurred is one of the highest-use fisheries on the Blackfoot River.

“The biggest disturbance is to river users themselves, who find it inconvenient and make it hard to see the obstacles in the river,” Uthe said. “But those inconveniences are definitely normal and expected in these types of landscapes. People need to be aware and know when there’s a lot of sediment to adjust their plans and get above Monture Creek if there’s discoloration in the water.”

Quinn Carver, the Seeley Lake District ranger, added that people who venture into the backcountry, especially in areas that have experienced large fires, need to be prepared for debris flows.

“We would like to commend the folks who called in via satellite phone and remained calm and patient as we worked quickly to address the issue,” Carver said in a news release. “This is a good reminder to all who travel in the backcountry, especially through burned areas, to know before you go, have extra food, water, shelter and a means of communication for unpredictable situations.”

The Lolo Forest plans to plant trees on about 140 acres south of Dunham Creek and south of the immediate Spruce Creek drainage in the general vicinity of where the slides occurred, as part of the Burned Area Emergency Response. In 2018, 26 acres were planted just to the south of the area in Nome Creek, Jerman said.

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