In Montana, a floodplain is a low-lying area of land that's prone to flooding from a nearby waterway: 

Definition -  A floodplain is the area that would be covered by floodwater from a 100-year flood event, which has a 1% chance of happening in any given year. 

Formation - Floodplains are made up of nutrient-rich river sediments and are formed when rivers overflow their banks after heavy rains or snowmelt. 

Risk - Floodplains are prone to repeat flooding, which can be damaging to communities. 

Mapping - The Montana floodplain mapping program works with FEMA and local communities to identify flood risks. Floodplain maps are updated regularly to account for changes in flood risk. 

Building - To avoid flooding, buildings and permanent structures should be built outside of the 100-year floodplain. 

Permits - A floodplain permit is required for any development or activity within a floodplain.