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.
Structures located in these flood plains are subject to damage. In a natural watershed, flooding can be affected by ice jams , the accumulation of debris at channel constrictions, and even the dam-building activity of beavers. Human activity has profound impacts on flooding. The two major activities which impact flooding are land use change and the building of flood control structures. Hundreds of years ago, the Delaware River Basin was covered by forests.