Sprawl is threatening almost every stream in the country

That problem is pavement, and the way it has changed the ancient relationship between streams and rain. For most of human history, rain fell onto meadows, fields and forests, and sank slowly into the ground. In fact, most of the rain was intercepted by plants and tree leaves before it ever hit ground, and evaporated back into the air, eventually returning as rain again. (This is still the case in undeveloped areas -- a forest after heavy rainfall is a remarkably dry place.) The small amount of rain that did reach the ground sank slowly down through layers of soil and rock until it reached the underground water table. From there it flowed laterally and downhill, still underground, toward streams, where it seeped into the streambeds and recharged the waterways from below. During a heavy storm, some rainwater might flow downhill on the ground's surface, but that was the exception, not the rule.

Pavement has changed all that. Now, every time it rains, water that once would have been slowly absorbed into meadow or forest floor courses off roads and parking lots and roofs and into curb gutters and storm drains, which funnel it directly to the local creek, at a speed and a volume that, before development, a stream saw only during spectacular storms, the kind that occur once or twice a century. These storm-water surges, as they are called, work as giant routers, scouring out streambeds and banks, flushing away the dirt around the roots of trees along the stream banks, and washing away the small creatures that cling to stream rocks. Under this regular, relentless scouring, stream life is swept away, and the stream becomes little more than a biologically dead sluiceway.

http://www.washingtonpost.com/wp-dyn/content/article/2005/11/22/AR2005112202165.html