Urban runoff

urban runoff illustration
Credit: Illustration by Cecily Mireles

After rainstorms — particularly the “first flush” of California’s wet season — pollutants wash into streams and other surface waters. Stormwater and other urban runoff can deposit trash, oil, pesticides, fertilizers, sewage and sediment into streams that provide drinking water. 

The largest source of runoff in California is its more than 50,000 miles of highways. Caltrans is required by the state Water Resources Control Board to control stormwater runoff to the “maximum extent practicable.” As a result, Caltrans monitors runoff and takes steps to prevent contaminants from flowing off roads during rainstorms, such as preventing erosion and removing debris.

The state water board also regulates construction sites and industries through permits that require companies to develop plans to prevent runoff.

Nevertheless, anything that is dumped into a street or spread onto a lawn or garden can wind up in lakes, rivers and streams.

Stormwater runoff flows from roofs, yards and roads into storm drains.
Credit: Caltrans