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.