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Mudslide

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.

The chocolate-brown water flowing down the Clark Fork Wednesday could be from a washout caused by recent storms in the Yellowjacket Creek drainage over Monture Trail 27.

Kate Jerman, a spokesperson for the Lolo National Forest, said they’re not sure this is where the sediment is coming from, and had a crew heading up Wednesday to assess the damage. But Tuesday evening a Forest Service trail crew that had been working in the area reported the washout in the Yellowjacket Creek drainage.

Yellowjacket Creek flows into Monture Creek, which feeds the Blackfoot River, then drains into the Clark Fork River.

“We do not know the extent of damage to the trail at this point until the crews come back in,” Jerman wrote in an email.

The sediment flow originated in the area burned by the Rice Ridge fire in 2017. A debris flow in the burned area in July, which sent a sediment plume into the Blackfoot River and temporarily damaged Dunham Creek Road, also turned the Blackfoot into a chocolate-looking river. 

Unlike this plume, the one in July diluted once it reached the Clark Fork River.

After the July sediment release, Patrick Uthe, a fisheries biologist with Montana Fish, Wildlife and Parks, noted that the glacial sediments from the mudslide are "super-fine," and they were watching for long-term impacts, 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.

The flip side, he added, are long-term benefits from fire-related disturbances. The sediments are nutrient-rich, which can increase the insect production, which is a food base for the bull trout.

Uthe was in the Monture Creek area Wednesday, and noted the Blackfoot "definitely was chocolate milk right now."

"It's hard to say where it's all coming from but certainly Monture Creek was pumping out all sorts of colored water. That's the big culprit in my opinion," Uthe said.

Uthe is doing FWP's annual bull trout spawning survey around Sept. 21, and will look at their spawning beds, or redds, in the gravel in Monture Creek and other tributaries.

"Looking at Monture Creek right now, we wouldn't be able to see a redd if it was present," Uthe said. "Unfortunately, a big sediment pulse like this when they're spawning will further exacerbate the situation; envision them digging a nest in the gravel, then a blanket of sediment covers it up, eventually smothering the eggs.

"It's definitely concerning. We will hope for the best but this type of event can have a negative effect."

He theorized that after the last mudslide, stream flows subsided and left sediments along the creek banks. After the rain Sunday and Monday, the National Weather Service stream gauge on the Blackfoot River near Bonner showed an increase in flows from 627 cubic feet per second (cfs) to about 1,000 cfs, or 2.1 feet to 2.6 feet from Sept. 7 to Sept. 10. The gauge shows flows are starting to subside.   

The National Weather Service in Missoula reported that the Swan Lake area experienced about 3.28 inches of rain during a 24-hour period between Sunday and Monday afternoons. During that time frame, Seeley Lake experienced 1.85 inches of rainfall.

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