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Rivers in Montana are wild and do not always stay within their banks, nor should we expect them to.  During the spring melt, or after intense rains, Montana’s rivers periodically spill over their banks into the adjacent land.  These lands are known as the floodplain.  To avoid getting flooded, locate buildings and permanent structures well outside the 100-year floodplain.  The 100-year floodplain is the land that has a 1% change of flooding in any given year.

Floodplain diagram


  1. Before you build, find the 100-year floodplain and check local regulations
  2. Build outside the floodplain to protect your investments from flooding
  3. Rather than happening 1 year out of 100, 100-year floods have a 1-in-100 chance of happening in any given year.

How can I find out where the floodplain is?

  • FEMA provides official floodplain maps
  • Your city or county planning department should have floodplains maps and recommendations
  • If you need help finding floodplain maps in your area, talk to your local conservation district
Floodplain map of the Yellowstone River
Example of 100-year floodplain along the Yellowstone River.


To avoid the risk of flooding, build outside of the floodplain.

Choosing to build outside of the floodplain will reduce the need to build expensive and environmentally damaging flood control structures. While it may be inconvenient, flooding is an important process for maintaining river health. Floodplains spread out and store water during high flows. This reduces damaging erosion and further flooding downstream. Floodwaters exchange nutrients, sediments, and organisms between the channel and the floodplain, creating unique habitats that are critical for the survival and reproduction of many species.

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