In a natural landscape, water from rain and snow melt is either absorbed into the ground or is evaporated.
 

But when land is developed, the hard impervious surfaces of roofs, parking lots, roads, and buildings prevent water from absorbing into the ground.  Water flows off of these surfaces and becomes stormwater.

In developed areas, stormwater drains are installed to capture and redirect water that would otherwise flood roads, buildings and parking lots.  In areas with Municipal Separate Storm Sewer Systems, this runoff is piped and/or directed into rivers, lakes and streams (learn more about MS4s)

However, this means any pollutants on the ground can be swept along with stormwater runoff, impacting water quality and aquatic habitat.