Stormwater or runoff from sources like sprinklers and hoses flows over the ground into the storm drain system. In the Los Angeles area, storm drain systems consist of gutters, storm drains, underground pipes, open channels, culverts, and creeks. Storm drain systems are designed to drain directly to the Pacific Ocean with no treatment.

Stormwater runoff is part of a natural hydrologic process. However, land development and construction activities can significantly alter natural drainage patterns and pollute stormwater runoff. Runoff picks up pollutants as it flows over the ground or paved areas and carries these pollutants into the storm drain system. Common sources of pollutants from construction sites include:

• Sediments from soil erosion
• Construction materials and waste (e.g., paint, solvents, concrete, drywall)
• Landscaping runoff containing fertilizers and pesticides
• Spilled oil, fuel, and other fluids from construction vehicles and heavy equipment