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.

Reducing Runoff

Runoff is a major source of water pollution. As the water runs along a surface, it picks up litter, petroleum, chemicals, fertilizers, and other toxic substances. From California to New Jersey, beaches in the U.S. are regularly closed after heavy rainfall because of runoff that includes sewage and medical waste.

These chemical pollutants can harm not just a beach, but an entire ecosystem. Tiny microbes, such as plankton or algae, absorb pollutants in the runoff. Fish or shellfish consume the microbes or absorb the pollutants directly. Animals such as birds consume the fish, increasing the level of pollutants in their own bodies. This process in which the concentration of a substance increases as it passes up the food chain is called biomagnification.

Biomagnification means organisms high on the food chain, including people, have a higher concentration of pollutants in their bodies than organisms such as seagrass or algae. As people eat foods such as oysters, they may be ingesting runoff from farms, sewage treatment plants, and city streets.

Runoff is an economic threat, as well as an environmental one. Agribusiness loses millions of dollars to runoff every year. In the process of erosion, runoff can carry away the fertile layer of topsoil. Farmers rely on topsoil to grow crops. Tons of topsoil are lost to runoff every year.

People can limit runoff pollution in many ways. Farmers and gardeners can reduce the amount of fertilizer they use.

Urban areas can reduce the number of impervious surfaces. Soil acts as a natural sponge, filtering and absorbing many harmful chemicals.

Communities can plant native vegetation. Shrubs and other plants prevent erosion and runoff from going into waterways.