The water that accumulates in large amounts in urban areas because of impervious surfaces like pavement, rooftops, and roads (Forests, grasslands, and wetlands allow precipitation to filter into the ground). Usually channeled away through storm drains and storm sewer systems into local streams.

Runoff in urban streams contributes to streambed erosion, fluctuations in water level, higher water temperatures, increases in sediment loads. Both urban streams and downstream water sources see increases in pollutants (grease, oil, litter, garden pesticides, road salts, and heavy metals) from urban runoff.