Liquid water moving across Earth’s surface from rain or melting snow is called runoff. When runoff gathers to form streams or lakes, it is called surface water. Visualize the difference between runoff and surface water by thinking of water flowing off your driveway after a rain storm (runoff) and then gathering together in the gutter (surface water). Runoff covers a much larger area of Earth’s surface than surface water. Some of the water that runs off infiltrates the ground where it may be trapped by the soil, used by plants, or continue underground to form groundwater. The amount of water that infiltrates the ground depends on how fast the runoff is moving and how easily the ground can absorb it. Humans have a profound effect on the processes of runoff and infiltration. When we cut down trees and cover natural vegetation with pavement water runs off more quickly and soaks in less.

Places that previously did not have a flooding problem may develop one as urbanization expands and more rain must run off into streams and washes rather than soak into the ground. Places that are already prone to flooding experience an increase in both the frequency and magnitude of flooding events when towns and cities are built. In fact, much urban flooding is strictly due to the alteration of land surfaces.