Chapter 5: Sensitive Areas in this Special Place - Floodplains

In the Gallatin, where few dams exist to alter natural flows, spring runoff typically results in flooding of valley streams and rivers. Floods have regularly occurred in the Bozeman area since 1893, and major flood events have occurred at increasing intervals to the present. In the Gallatin, particularly along the main and East Gallatin rivers, floods occur nearly every year in some areas.

The floodplain is the low-lying area adjacent to a stream or river where water spreads out when it leaves its banks. Floodplain soils are often poorly drained, due to the nearness of the water table to the surface. Floodplains result from the continuous process of deposition of material and later cutting away of the surface material over time by the river’s meandering. The floodplain may be not much wider than the river’s channel where banks are steep. It may extend for many hundreds of feet from the ordinary high water line in low-lying areas. Because of the deposited sediments and high water table, floodplains contain a high diversity of plants and animals.

Flooding is a natural process. Floodplains dissipate the energy of spring torrents, reducing flood damage downstream and providing recharge areas for adjacent streams. However, runoff from a watershed can be greatly influenced by the kinds of plants and soils in the watershed. Vegetated stream banks reduce the likelihood of flooding. Where overgrazing, timbering, or homesite development removes vegetation, flood events occur more often and with greater severity. Healthy floodplains are those where the activities occurring within the floodplain are benefited by or can adapt to occasional flooding, such as agriculture, recreation, and wildlife.

Floodplain management involves reducing the risk of damage to property from flooding, as well as maintaining the natural functions that floodplains provide. To this end, federal, state and local governments regulate certain activities that occur in “floodplains” (areas susceptible of being inundated by flood waters):

Federal: the Federal Emergency Management Agency (FEMA) administers the National Flood Insurance Program (NFIP). The NFIP is the only source of flood insurance in the country.

State: Montana regulates floodplains and floodways, including limitations on the following uses within the floodplain:

• requiring structures built in the floodplain to be constructed on fill to bring the lowest floor 2 feet above the 100-year flood elevation;
• and prohibiting within the “floodway” (an area that must be kept free of encroachment if a structure built there would increase the flood flow height by more than half a foot, MCA 76-5-401 et seq.):
• residential construction;
• structures that will cause water to be diverted, obstructed, or reduce the carrying capacity of the floodway, or cause erosion;
• construction or storage of objects subject to flotation or movement during floods.

County: Gallatin County Subdivision Regulations prohibit residential subdivision within the floodplain.

City: Bozeman Zoning Ordinance and Bozeman Area Subdivision Regulations restrict development in the floodplain without a permit.

Floodplain delineations are the province of the Water Division, Montana Department of Natural Resources and Conservation.

What is a 100-Year Flood?

A 100-year flood is one that can be expected to occur once every 100 years (one percent of the time), based on watershed records. However, just because a 100-year flood occurred last year does not mean that another 100-year flood couldn't occur next year. The likelihood is related to the magnitude of the flood, not the time that has passed between the 100-year flood events.

Flooding is a natural process that occurs frequently in the Gallatin.

Information

(including maps of delineated floodplains)

Gallatin County Floodplain Coordinator, Gallatin County Planning Office
582-3130

Bozeman Floodplain Administrator, Bozeman City Engineer
582-2380

Gallatin County Planning Office - Gallatin County Subdivision Regulations
582-3130

Bozeman Planning and Zoning Office -Bozeman Zoning Ordinance and Bozeman Area Subdivision Regulations
582-2360

Resources

Flood insurance is available in Gallatin County through the NFIP. Contact your commercial insurance broker for more information.

Permitting

Permits to build in designated floodplains or floodways must be obtained through the local Floodplain Administrator
582-3130; 582-2380

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