Clark Fork, Bitterroot River mapping project to study channel migration

Areas of the Clark Fork River were clogged with ice just a few short weeks ago. As the river begins its spring flow, Missoula County will begin a study of the river’s channel migration. (Martin Kidston/Missoula Current)

With last year’s flooding still alive in conversation and this year’s runoff yet to come, work to map the migrating channels of the Clark Fork and Bitterroot rivers will soon begin.

Missoula County this week approved a $17,000 contract with Access Geographic to begin mapping the course of the two rivers through the county. The project is expected to begin in April and lead to a larger channel migration study.

“The first step in developing that study is collecting aerial photography, the scope of which is the entire Bitterroot and Clark Fork through Missoula County,” said Travis Ross of the Missoula City-County Health Department. “After it’s all tiled together and referenced properly, they’ll contract out the channel migration study.”

Last spring, heavy rain and rapid snowmelt from a record winter snowpack pushed the Clark Fork River to levels not seen in 110 years. When the water finally receded, the river channel was noticeably changed.

Between Clinton and Turah, the river abandoned a side channel. In the middle of Missoula, the gravel bars around Brennan’s Wave were rearranged. Tons of sediment was deposited around Orchard Homes and the Tower Street conservation area.

“The intent of this is to know where our channels are moving,” Ross said. “It would have been helpful in the Target Range and Orchard Homes areas as we’re moving mobile homes back to know where the migration zone is predicted to be.”

The Missoula Conservation District and its legacy grant will provide $15,000 toward the cost of updating the channel migration study.

The work will take place in conjunction with efforts at the city.

“The city is actually collecting aerial photography of the wastewater treatment plant service area, which is larger than the city limits,” Ross said. “We’ll be able to get double the resolution since they were flying high-resolution imagery. We’ll both benefit from the images.”

Ross said the imagery will be given to the GIS department and placed into the property information system for public access.

The Army Corps of Engineers also is conducting a separate “inundation analysis” of the river corridor. Ross said the river migration study could complement that of the Corps.

“County floodplain regulations allow use of additional information besides floodplain maps for determining whether a project is in the floodplain,” he said. “This is additional information that could be considered.”