Many events that occur in a watershed affect water quality.
Storm Water Runoff
In a natural watershed, rain falls on grassy or vegetated areas. Grass allows the soil to soak up most of the rain that falls. In an urban watershed, rain falls on hard surfaces such as streets, driveways, and rooftops. The rainwater runs quickly over these hard surfaces and is not soaked up. This moving water is called storm water runoff. Storm water runoff is a result of rainfall and melting snow. As this water flows to storm sewers, it picks up pollutants.
Pollutants in storm water runoff include oil and antifreeze from streets and driveways, leaves and grass clippings from gutters and streets, animal wastes from lawns and gutters, and fertilizers and pesticides from lawns and gardens. The loss of vegetated areas due to urbanization and development dramatically increases the volume of storm water runoff. Urbanization also increases the amount of pollutants available to be picked up by storm water.
Storm water runoff is carried directly to lakes and streams through storm sewers. Storm sewers are designed to prevent flooding, but also provide direct routes for pollutants to lakes and streams. By controlling the pollutants in storm water runoff, we can control the amount of pollution entering our lakes and streams.
Phosphorus is one of the most troublesome pollutants in storm water runoff. Phosphorus comes from many sources, and it is the primary cause of water quality problems in our lakes and streams. Everything that is or was living contains phosphorus. It is in leaves. It is in lawn clippings. It is in animal wastes. It is an ingredient in most lawn fertilizers. It is even attached to soil.
When leaves, lawn clippings, animal wastes, fertilizers, and soil are picked up by storm water runoff and are carried directly to our local lakes and streams, they provide the lakes with excess phosphorus. This excess phosphorus causes increased algae growth. Algae are small green plants that live in lakes and streams. Increased algae growth is observed as green algae blooms or scums on lakes. Too much algae is harmful to a lake system. It blocks sunlight and prevents other plants from growing. When it dies and decays, it also takes much needed oxygen away from fish. Limiting phosphorus reduces algae blooms.