Floodplain Management in Missoula County

Brian Maiorano, Floodplain Administrator
Missoula Office of Planning and Grants

  County floodplain regulations are meant to prevent property from being built in harm’s way, prevent damage to property caused by others’ actions, and allow streams and rivers to convey floodwaters naturally. They can also have the added benefits of protecting river health and natural beauty.

Proposed amendments to County floodplain regulations would prohibit new levees and limit new rip rap and other bank armoring.  Rip rap and levees, while providing a short-term localized fix, often increase bank erosion and flood damage to nearby properties.  In addition these structures harm aquatic and terrestrial habitats.

Missoula County discourages bank stabilization projects unless they are part of a larger project that addresses the cause of the instability. Cosmetic bank stabilization projects may increase instability of nearby property and harm stream health by limiting natural stream migration. However, the County recognizes that misguided historic actions have contributed to bank instability and may need to be addressed.  For example, channelization of the upper Clark Fork and Lolo Creek by railroads and highways has caused very unstable banks as the watercourses try to regain length.  Increased sedimentation and loss of riparian vegetation on Ninemile Creek have resulted in a shallow, braided stream with unstable banks and decreased aquatic habitat.  Where bank stabilization efforts seem needed to address these historic problems, the County encourages the use of “softer” bank stabilization techniques that can slow bank erosion, increase stream complexity and allow for long term bank revegetation.

Floodplain regulations can prevent some types of flood damage by ensuring that new bridges do not cause constrictions or limit sediment transport.  Another preventative measure is to prohibit large scale clearing of native streamside vegetation, which can help prevent bank failure during high water.