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Phase I Municipal Stormwater Permit

The Phase I Municipal Stormwater Permit requires local governments to manage and control polluted stormwater runoff so that it does not pollute downstream waters. This permit regulates stormwater discharges in the most highly populated areas in our state.

Current status of of the permits

The current permit expire on July 31, 2018. After careful consideration of many factors, we have decided to extend the current (2013-2018) Western Washington Permits for one year. The Permits will reissue on July 1, 2019, and become effective on August 1, 2019. We are in the process of re-issuing both Western Washington Phase I and Phase II Municipal Stormwater permits.

We regulate water that discharges from municipal separate storm sewer systems (MS4s) through municipal stormwater permits. These permits protect the quality of Washington's rivers, streams, and other water bodies. 

The Phase I Municipal Stormwater Permit regulates systems owned or operated by:

  • Clark, King, Pierce, and Snohomish counties.
  • Seattle and Tacoma.
  • Incorporated cities with a population of over 100,000* people.
  • Unincorporated counties with populations of more than 250,000* people.
*Populations are based on the 1990 U.S. census.

The permit also applies to MS4s owned by public entities located in a Phase I city or county including the ports of Seattle and Tacoma.

The Phase I Municipal Stormwater Permit

Supporting documents

Municipal stormwater permit guidance

For guidance about applying, changing, or implementing and complying with your municipal stormwater permit, see municipal stormwater guidance. Stormwater manuals provide stormwater permit implementation and management guidance. For more general stormwater permit guidance, see the stormwater guidance page.

Frequently asked questions

Have a question? Check out our frequently asked questions page

Stormwater monitoring

Are our permits successful at preventing or reducing pollution from stormwater runoff? Check out the Stormwater Action Monitoring (SAM) program and our other results on the Stormwater monitoring page.

Keep in touch with us

Join our municipal stormwater permit listserv to receive updates about this permit.