Stormwater is water that falls from the sky as rain or snow. Most of it seeps into the ground. However, if the ground is saturated, frozen or covered with impermeable surfaces (paved parking lots, roads or rooftops), then the water flows over these surfaces, often picking up pollutants and causing what is known as stormwater runoff. The velocity, temperature and content of this stormwater runoff can all have effects on the land, plants and animals. Ultimately, polluted stormwater runoff from throughout the state is the primary cause of the decline of the Chesapeake Bay.