Phase I Municipal Stormwater Permit
We develop and administer Clean Water Act National Pollutant Discharge Elimination System (NPDES) municipal stormwater permits in Washington. The Phase I Municipal Stormwater Permit requires local governments to manage and control stormwater runoff so that it does not pollute downstream waters. This permit regulates stormwater discharges in the most highly populated areas in our state.
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 current permit is effective Aug. 1, 2019, and expires on July 31, 2024.
Overview of permit timelines
Permit timelines for new, continuing, and secondary permittees will be available in the coming weeks. These timelines will provide an overview of major deadlines for implementing permit requirements.
Annual reports for the calendar year are due by March 31 the following year.