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Missoula levees set for accreditation while Army Corps finishes mapping floodplain

Missoula has four levies certified by the Army Corps, including two federal levies on the Clark Fork River. Both were built around 1966 and are located on the north bank, protecting much of the downtown district and West Broadway corridor. (Martin Kidston/Missoula Current file)

As the water rises for another spring, the City of Missoula and the Army Corps of Engineers are in the final stage of re-certifying the city’s system of levees and completing new flood mapping.

Tracy Campbell, superintended of the city’s stormwater system, said the new floodplain mapping is expected “very soon,” and the levees look poised to retain their accreditation.

“Because the river is being remapped, we have to get our Clark Fork levees re-certified so they can retain accreditation status,” Campbell said. “The Corp is preparing their risk assessment and will recommend accreditation. We’re really lucky in that sense.”

Missoula has four levees certified by the Army Corps, including two federal levees on the Clark Fork River. Both were built around 1966 and are located on the north bank, protecting much of the downtown district and West Broadway corridor.

The other two are considered local levees and include both Grant and Pattee creeks. Loss of accreditation to any levy could cost the city millions of dollars in repairs and have large implications on homeowner insurance policies.

“There are significant affordable housing developments behind these levees,” said City Council member Heidi West. “If we lost that certification, a lot of people would be subject to flood insurance, which is incredibly expensive. It would make all those affordable units unaffordable. The levees are a huge community value.”

Some areas of the Missoula Valley remain subject to seasonal flooding, including the Orchard Homes area. It’s unknown how the new floodplain mapping will impact other parts of the city, though Campbell said it shouldn’t impact Missoula’s existing Clark Fork levies.

“That part of the river isn’t looking to change significantly as far as base flood elevations or anything like that,” Campbell said. “Where we’re going to see the most change is south of the river where we don’t have levees and probably won’t have levees. We’ll be looking at mitigation opportunities and floodplain restoration potentially.”

Many homes in the Clark Fork River floodplain west of Missoula flood nearly every spring. The Army Corps of Engineers is wrapping up its new floodplain mapping. (Missoula County Sheriff’s Office file)

Maintaining the levees requires upkeep and, recently, the city removed most of the trees along the downtown levee. While the Army Corps likes to see the levees void of vegetation, exceptions can be provided when they have environmental benefits.

The river is home to bull trout, which is on the list of threatened and endangered species. Campbell said the city removed large-diameter trees along the downtown levee and is working on a vegetation plan that includes willow and brush.

“Our long-term goal for our levees is to remove all non-native vegetation. We don’t want invasive species for number of reasons, and we don’t want cottonwoods,” she said. “Some of the non-native vegetation we see on the levees like Siberian elms, Norway maples and cottonwoods – those really large diameter trees can cause a lot of bank instability when they fall. We want to transition the slopes to more of a native shrub cover.”

One of the planting efforts on the downtown levee last summer got off to a strong start but was followed by high heat. Most of the vegetation didn’t survive, and planting over riprap isn’t easy due to the lack of organic matter, Campbell said.

Shading the riprap from the sun keeps the water cooler and cuts down on reflective heat. Campbell said the Army Corps is rebuilding one stretch of levee that has eroded, and the city plans to help plant the banks.

“Part of that, they’re going to cover the riprap with a foot a topsoil and seed it. We’ll have a soft bank, which we’re excited to see,” Campbell said. “I hope that does work. It potentially could be a model for restoring the rest of the levees. We need more organic matter for shrubs and vegetation to establish in that riprap.”