Flooding occurs in both natural and developed watersheds.
When the rate of rainfall or snowmelt exceeds the rate of infiltration to the ground, the excess water, called runoff, moves across the ground surface toward the lowest section of the watershed. As the surface runoff enters stream channels, stream levels increase. If the rate of runoff is high enough, water in the stream overflows the banks and flooding occurs. This area of over-bank flow is called the flood plain. All natural watersheds have flood plains.