Urban runoff is rain and other water flowing down gutters and streets.
It sometimes comes from over-watered lawns, washing cars, and other daily activities. As this water flows over lawns, gardens, sidewalks and streets, it picks up pollutants such as pesticides, pet waste, litter and oil that drips from cars. Runoff is the greatest source of water pollution, contributing from 50 to 60 percent of the pollutant load. One serious effect of stormwater and polluted runoff is the creation of conditions that may promote fish killing organisms such as Pfiesteria . Polluted stormwater is a serious problem for many older coastal cities that have a combined sewer system.