Montana DNRC launches project to update Missoula-area floodplain maps

According to Missoula Mayor John Engen’s office, there are 4,857 residential units, with 10,200 residents, living within 150 feet of flowing water in the city of Missoula. In the past 10 years, more than 300 residential units, with nearly 640 residents, have moved into areas adjacent to streams and rivers. (Missoula County Sheriff’s Office)

With a $2.6 million grant from the Federal Emergency Management Agency, the Montana Department of Natural Resources and Conservation has launched a multi-year effort to update floodplain maps and assess levees along the Clark Fork and Bitterroot rivers and Rock Creek.

“It’s a pretty large project and will take several years to complete,” said Tiffany Lyden, a mapping outreach specialist for the DNRC. “We’re kicking off the project with presentations to explain the process, requirements and timeline.”

The DNRC plans to have draft maps ready for public review and comment by early 2022, and complete the project by 2023, Lyden told Missoula City Council members.

A collaborative effort between FEMA, the DNRC, the city of Missoula and Missoula and Granite counties, the process includes surveying and measuring topography; marking locations of culverts, bridges and road crossings; determining how much water would flow through the area during floods; calculating where water would likely go during a flood, and then compiling it all into maps.

“Simply put, an area shown to be underwater during a flood is considered the regulatory floodplain,” said Nadene Wadsworth, a floodplain outreach specialist for the DNRC. “The information and maps are used by emergency responders, the insurance industry, community planners, developers and mortgage lenders.”

As part of the effort, the DRNC will also be reviewing, certifying and accrediting levees, known to FEMA as “flood risk reduction systems,” and defined as “a manmade structure, usually an earthen embankment, designed and constructed in accordance with sound engineering practices to contain, control or divert the flow of water so as to provide protection from temporary flooding.”

The floodplain mapping and levee review are required by the National Flood Insurance Program, created by Congress as part of the National Flood Insurance Act of 1968, to provide communities with incentives to reduce flood risks and related emergency response costs. 

The program offers government flood insurance to property owners in participating communities to insure against losses from flooding for existing buildings. In return, participating communities are required to adopt land use and control measures, with effective enforcement provisions, to reduce flood risk and damage by restricting development in areas prone to flooding.

Although part of the Clark Fork River map was updated after the removal of the Milltown Dam in 2008, most of the mapping is based on “pretty old data,” Lyden said. “There’s been no comprehensive studies done since the 1980s. We need to bring it up to current times. Not only do rivers change, but we also have better, improved methods and technology to gather and compile data.”

According to Missoula Mayor John Engen’s office, there are 4,857 residential units, with 10,200 residents, living within 150 feet of flowing water in the city of Missoula. In the past 10 years, more than 300 residential units, with nearly 640 residents, have moved into areas adjacent to streams and rivers.

“As our city grows, we are seeing increases in requests for development close to ditches, creeks and rivers,” Engen said. “Floodplain studies to replace our existing, outdated floodplain maps are critical to foster smart development and encourage growth in a manner that maintains the integrity of our waterways and riparian areas while protecting the safety of life and property located along the water’s edge.”  

Contact reporter Dave Stalling at: dstalling@missoulacurrent.com