Stormwater runoff is the water that drains from a land surface after a rain event. Because runoff after heavy rainfall can cause flood damage and contribute to water pollution, stormwater is carefully managed by local governments to minimize these negative impacts. The federal Clean Water Act regulates stormwater discharges through the National Pollutant Discharge Elimination System (NPDES). The NPDES program is designed to reduce pollutant sources that discharge to surface waters of the state. Surface waters of the state include lakes, rivers, tributaries, ponds, bays, springs, creeks, sloughs, estuaries, oceans, and other watercourses.

The NPDES stormwater program regulates stormwater discharges from three potential sources: municipal separate storm sewer systems (MS4s), construction activities, and industrial activities. A Municipal Separate Storm Sewer System (MS4) is a publicly-owned conveyance or system of conveyances that includes structures such as ditches, curb inlets, catch basins, underground pipes, and stormwater ponds. The MS4 captures and directs stormwater to surface waters of the state, and is separate from a conveyance system that collects and directs municipal sewage to treatment facilities.

More information on stormwater and stormwater management may be found on the Orange County Stormwater Education page and the Orange County Environmental Protection Division home page.