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What is Stormwater?

What is stormwater?

Stormwater is rain or melting snow that flows over land or impervious surfaces, such as paved streets, parking lots, and building rooftops, and does not soak into the ground.  In more natural areas, such as forests and wetlands, water is quickly able to soak into the ground, where it is stored and filtered.  As urban areas have less vegetation and more impervious surfaces, less rain is able to infiltrate into the ground, and more runoff is generated. 

How does stormwater pollute our waterbodies?

In Contra Costa County we get an average of 21 inches of precipitation each year.  When it rains in our urban or developed areas, this stormwater picks up pollutants like trash, chemicals, oils, and dirt/sediment.  The stormwater is conveyed directly to nearby rivers, streams or the Bay without treatment, potentially harming wildlife and the health of these waterbodies. 

Close up of storm drain inletPopulation growth and the development of urban/urbanized areas are key contributors to the amount of pollutants in runoff as well as the volume and rate of runoff from impervious surfaces. Together, they can cause changes in hydrology and water quality that result in habitat modification and loss, increased flooding, decreased aquatic biological diversity, and increased sedimentation and erosion. When it rains or there is irrigation, water runs off and eventually makes its way to a river, lake, or the ocean. While there is some dilution of these pollutants before entering waterbodies, polluted runoff results in large enough quantities of pollutants to impair receiving waters. 

Who regulates and manages stormwater?

The Clean Water Act regulates stormwater discharges from municipal separate storm sewer systems (MS4s).  Each of the 21 local municipal agencies in Contra Costa County owing and operating an MS4 are named as Permittees in the Municipal Regional Stormwater Permit (MRP) issued by the California Regional Water Quality Control Board for the San Francisco Bay Region (RWQCB).  Through Best Management Practices (BMPs) and preventing anything except stormwater from entering the MS4, these municipalities work to protect our creeks, wetlands, and the Bay/Delta.

The benefits of effective stormwater runoff management can include:  protection of wetlands and aquatic ecosystems, improved quality of receiving waterbodies, conservation of water resources, protection of public health, and flood control.