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. This maximized the infiltration of rainfall and slowed the movement of runoff. As the land was cleared for agriculture, infiltration rates were reduced and runoff rates increased. The increase in runoff rates widened flood plains and stream channels in many of the basin's watersheds.