Channel migration concerns prompt changes to Target Range project

Martin Kidston

(Missoula Current) Citing concerns over channel migration, Missoula County on Thursday sent a proposed Target Range subdivision back to the drawing board after the developers said they'd be willing to consider a greater setback from the river to protect future homes.

While the proposed Creighton subdivision isn't large at seven lots, it rekindled conversations over the Clark Fork River's timeless migration and how, or even if, the county should address it through a levy or other measures.

As it is, the river has flooded nearly every spring in recent years, washing over homes and properties built in the area in prior decades. One nearby property has lost an estimated 10,000 square feet of land between 2014 and 2022 due river erosion, according to the county.

“The big issue with this (Creighton) subdivision is channel migration of the Clark Fork River,” said county planner Tim Worley. “The County Water Quality District in 2021 found that the river portion of the (Creighton) property lies within the historic migration zone.”

The Creighton subdivision is proposed for Third Street and Clements Road several hundred feet from the river and within or near the channel migration zone. But a geotechnical analysis found that soils harden and grow more compact above a terrace south of the river, where most of the development is proposed.

Initially, a 200-foot river setback had been proposed but even that left officials concerned. Commissioners are now seeking a 250-foot setback. That would preserve the migration zone and allow development on a portion of the property.

“We have to imagine what if that (river migration) does happen. If that does happen, how do we mitigate risk by design right now, so we don't build in anticipation of mitigation,” said Commissioner Josh Slotnick. “If we say we need $10 million in tax money to protect a house, people will ask why we allowed them to build the house anyway. We are not accountable for those (older) houses. We'd be accountable for these (new) houses.”

Channel Migration and Geology

A 2021 study conducted for the county mapped a channel migration zone for roughly 69 miles of the Clark Fork River and 22 miles of the Bitterroot River. The study broke the Clark Fork down into 10 reaches and recommended various setbacks for each area.

In the area near the proposed Creighton subdivision, a Geotech analysis took a deeper look at the area's soils and concluded that a recommended 580-foot buffer from the river would be the “conservative yet prudent” approach.

But such a wide buffer would essentially make the Creighton property worthless, as much of it lies within that 580-foot buffer. Some have described such a wide buffer as a “taking” of property. But another report said that given the closest active stream bank, a 200-foot buffer would suffice to protect homes.

Brian Throckmorton with 406 Engineering said the project followed that 200-foot setback when designed. He added that it's easy to cherry pick details from the channel migration study to make an argument against the project.

“We're talking the same document. We went through and used what's required in that document to prove our mitigation, which is setback. It's the best information from science we have right now,” he said.

However, when pressed by commissioners on Thursday, the development team said it could accommodate a 250-foot setback. With a verbal agreement regarding setback in place, the county plans to consider the project again in October.

“I think we can do 250-feet for no build,” said Throckmorton. “We've already deleted a lot in recognition of the (migration zone). We can put a 250-foot no-build zone and be fairly sound. It's a safety factor on the data we looked at already.”