On the second picture a part of the University of Texas campus and downtown Austin as it is today can be seen. As cities grow, natural groundcover changes dramatically. Natural vegetation and undisturbed soil are replaced with concrete, asphalt, brick, and other impermeable surfaces. This means that, when it rains, water is less likely to be absorbed into the ground and, instead, flows directly into river channels. Not only does more water reach the stream channels, but it arrives far more quickly after a storm.
Natural vegetation slows run-off, concrete and asphalt speed the flow.
The result diagrammed below indicates how urbanization speeds run-off and produces higher peaks of flow. The result is a spread of the channel vertically and horizontally to carry the extra flow. Rapid erosion of stream banks and down-cutting of stream beds occurs. When the existing stream beds cannot handle the increased flow, they flood the surrounding urban area, particularly development within adjacent floodplains. Flooding can be a major problem as cities grow and stream channels attempt to keep up with these changes.