Stormwater is any precipitation that collects in a natural or constructed storage or transport system following a storm event.
For example, during construction of a new building or neighborhood, sites are often cleared and the soil is firmly compacted, which prevents rainfall or snowfall from soaking into the soil. As a result, the rainfall streams along the surface of the ground. This is stormwater runoff.
After construction activities, impervious areas such as roads, rooftops, parking areas, and sidewalks prevent infiltration of moisture from rain and snowfall, thus increasing natural stormwater runoff. This runoff can be too much for the existing natural drainage systems to handle.
As a result, natural drainage systems are often altered to rapidly collect runoff and convey it away (using curb and gutter, enclosed storm sewers, and lined channels). The stormwater runoff is then discharged to downstream waters such as streams, reservoirs, and lakes.