When natural areas such as meadows, fields, and woodlands are developed with houses and stores, roads and parking lots, there are fewer places where rain can sink directly into the ground. When rain falls on the hard surfaces of buildings and pavement, it simply runs off, most often flowing downhill and picking up speed, volume, and sometimes pollutants as it goes.
Runoff can cause a number of problems, including overburdening the storm sewer systems, creating flooding, and degrading the quality of the water.

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