Missoula County Government: Channel Migration Zone Study Highlights Potential River Migration Concerns

Press release from the Missoula County Government:

07/14/2021 12:33 PM

The Missoula Valley Water Quality District and Missoula Conservation District recently received findings from a Channel Migration Zone study that highlight potential river channel migration hazards in developing areas along the Clark Fork and Bitterroot rivers.    

During the floods of 2018, the Clark Fork River channel moved in many locations in the county, taking out several mobile homes and eroding levees and berms. This prompted the districts to come together to look at how the river channels have changed overtime, and how they are predicted to move in the future. The study used aerial photos and annual maps of the riverbanks and found that in some areas of the county, the Clark Fork and Bitterroot rivers have migrated between 1,000 to 2,000 feet. 

“The extent of movement of the river is highly dependent on the geology and the flood history; 2018 was close to a 25-year flood on the Clark Fork below Missoula,” said Elena Evans, Water Quality District hydrogeologist. “As we see the frequency of flooding increase, we are likely to see more migration of these two rivers.” 

An important component of the study is a look into the future and the potential impacts river migration will have on current structures or proposed developments. This study looks at areas with erodible soils, calculates migration rates in feet per year and then projects out 100 years to identify areas that may see the river migrate or develop a completely new path (avulsion). 

There is an important distinction between floodplains and channel migration zones. Floodplains are predictions or models of areas that would be under water during a flood. Channel migration zones are areas where a home or structure could be washed away. The study will help residents be aware of the hazards that exist in residential and developing areas along the rivers. An interactive webmap that shows how channel migration zones overlay and differ from floodplain mapping can be found online at https://gis.missoulacounty.us/mcchd/cmz/.   

“The Missoula Conservation District’s number one goal is to protect water quality, stream corridors and riparian habitat,” Resource Conservationist Radley Watkins said. “When we are reviewing proposed projects, we need to be confident that these projects aren’t going to be taken out by the river in the next 20 years or change the river in such a way that downstream properties are affected.”  

Channel migration zone mapping is a tool used to assist land managers, property owners, agency personnel and other stakeholders in making sound land use decisions along rivers. It has been conducted on more than a dozen rivers around the state. The first study of channel migration on the Clark Fork was done in 2009. This most-recent version uses updated methods and new aerial photos to bring the mapping up to date. 

The full report and video presentation by Applied Geomorphology and DTM Mapping Inc. of Bozeman can be found on the Missoula Valley Water Quality District’s website, missoula.co/waterquality. 

For more information about the study, contact Evans eevans@missoulacounty.us or call the Water Quality District at 406-258-4890.

This press release was produced by the Missoula County Government. The views expressed here are the author’s own.