Montana is a headwater state: most of the water in our rivers is generated by precipitation in Montana, along with inflows from Wyoming and Canada. Mountain streams are driven by mountain snowmelt, large prairie and valley rivers are driven both by mountain snowmelt and lower elevation rain and snow events, and small prairie streams tend to be ephemeral, only flowing during nearby snowmelt and after rainfall. Streamflows in Montana rivers tend to undergo huge swings throughout the year, as shown by these hydrographs for the Boulder River (a medium-sized mountain river in Sweet Grass County) and the large Yellowstone River in eastern Montana:
High flows associated with mountain snowmelt tend to occur in May and June.
Increased flows from prairie snowmelt can occur earlier, and high flows in prairie streams can occur throughout the summer in response to intense rain events.
These floodwaters spread across the floodplain, the section of adjacent low-lying land. The floodplain slows and stores floodwaters, reducing the damaging energy of flows in the channel. Floodplains are formed by deposition of sediment over long periods of time and play important roles in maintaining healthy plant and animal communities.