Runoff occurs when there is more water than land can absorb

Runoff occurs when there is more water than land can absorb. The excess liquid flows across the surface of the land and into nearby creeks, streams, or ponds. Runoff can come from both natural processes and human activity.

The most familiar type of natural runoff is snowmelt. Mountains that cannot absorb water from heavy snowfalls produce runoff that turns into streams, rivers, and lakes. Glaciers, snow, and rain all contribute to this natural runoff.

Runoff also occurs naturally as soil is eroded and carried to various bodies of water. Even toxic chemicals enter waterways through natural processes, such as volcanic eruptions. Toxic gases released by volcanoes eventually return to the water or soil as precipitation.

Runoff from human activity comes from two places: point sources and nonpoint sources. Point source pollution is any source that empties directly into a waterway. This might include a pipe from specific sewage treatment plant, factory, or even a home. Regulations determine what type of runoff, and how much, industries are allowed to release. These regulations vary by region, state, and nation.

Nonpoint source pollution is any source where runoff does not go directly into a waterway. Nonpoint sources of runoff can be large urban, suburban, or rural areas. In these areas, rainwater and irrigation wash chemicals into local streams. Runoff from nonpoint sources includes lawn fertilizer, car exhaust, and even spilled gasoline from a car. Farms are a huge nonpoint source of runoff, as rainwater and irrigation drain fertilizers and pesticides into bodies of water.

Impervious surfaces, or surfaces that can't absorb water, increase runoff. Roads, sidewalks, and parking lots are impervious surfaces. Materials as diverse as car-washing soaps, litter, and spilled gas from a gas station all become runoff.