When rainwater or melted snow does not soak into the ground, but instead flows across streets, parking lots, lawns, or other surfaces, it is called 'storm water'. In areas with hard pavement and roofs that prevent the water from naturally soaking into the ground, even a small rain can cause a fair amount of storm water.
Storm water, if not planned for and handled correctly, can cause issues such as flooding of roads, buildings and streams, erosion of stream banks, turbidity (muddiness) and habitat loss in the stream due to the eroded soil in the water.
As storm water runoff flows over the land and impervious surfaces (paved streets, parking lots, curb gutters), it accumulates debris, oils, grease, leaves, grass clippings, pet waste, chemicals, sediments, and other pollutants that can adversely affect water quality if the runoff is discharged into a waterway without being treated.
Most storm water runoff is not treated for water quality before it reaches a ditch, stream, or river.