What is a Municipal Separate Storm Sewer System?
A municipal separate storm sewer system, or MS4 for short, is a collection of systems like ditches, roadside gutters, retention basins, underground pipes, and green streets designed to gather stormwater from built-up areas and discharge it into local streams and rivers without treatment. It is called a "separate" system because it is not connected to the sanitary sewer system, which transports wastewater to the wastewater treatment plant.
In Portland, the MS4 equates to roughly 20% of the area within City limits or around 15,000 acres. The remaining 80% of Portland’s area is served by other means of conveying and discharging stormwater runoff. Other drainage areas not covered by the MS4 permit include:
- Combined Sewer Areas
- Groundwater Discharge Areas
- Non-Point Source Runoff Areas
- Private Drainage and Other Non-City MS4s
What is an MS4 Permit?
Under the Clean Water Act, the U.S. Environmental Protection Agency (EPA) developed the MS4 permit through its National Pollutant Discharge Elimination System (NPDES). The intent of MS4 permits is for municipalities to implement measures that reduce pollution in stormwater runoff.
The Oregon Department of Environmental Quality (DEQ) is the state agency that issues MS4 Phase I and Phase II General Permits. The City of Portland and its co-permittee, the Port of Portland, are issued a Phase I MS4, which covers areas with populations greater than 100,000.
View or download the full permit:
Stormwater Management Plan
The Stormwater Management Plan (SWMP) is the City’s comprehensive plan to prevent harmful pollution from entering Portland’s rivers and streams and is an MS4 permit requirement. Environmental Services leads the administration of and compliance with the City’s MS4 permit, but multiple City bureaus are responsible for implementing the requirements.
View or download the Stormwater Management Pan:
Why Is the Plan Important?
When it rains, stormwater washes over roofs, streets, and other hard surfaces. Stormwater can carry oil, pesticides, metals, chemicals, dirt, and other pollutants into storm sewers, which discharge into our local rivers and streams. To prevent this harmful pollution, the SWMP describes how the City will reduce the discharge of pollutants from its storm sewer system and addresses these program areas:
- Public Education and Outreach
- Public Involvement and Participation
- Illicit Discharge Detection and Elimination
- Construction Site Runoff
- Post-Construction Site Runoff for New and Re-Development
- Pollution Prevention and Good Housekeeping for Municipal Operations
- Industrial and Commercial Facilities
The Monitoring Plan describes the City’s monitoring objectives, strategy, and procedures for collecting and analyzing stormwater and surface water samples.
Annual Compliance Reports
Annual compliance reports are submitted to DEQ to fulfill MS4 permit reporting requirements. The reports provide information about activities that have been accomplished in accordance with the SWMP.
Total Maximum Daily Loads
DEQ has established total maximum daily loads (TMDLs) for local rivers, which specify the maximum amount of pollutants that various entities can contribute to the Willamette River and tributaries that do not meet water quality standards. The maximum amount of the pollutant is then divided up and “allocated” to the various dischargers to that waterway. These dischargers are referred to as designated management agencies (DMAs) and are required to develop a TMDL Implementation Plan.
The City has developed a TMDL Implementation Plan that identifies key management strategies to reduce TMDL pollutants from nonpoint sources and improve water quality.
View or download the TMDL Implementation Plan:
The City submits an annual TMDL status report to DEQ to report on the implementation of the strategies.
View or download the TMDL annual status report:
Spill or Pollution Hotline
The Spill or Pollution Hotline can be contacted 24 hours a day, seven days a week, to report pollution or spills.
Educational Materials and Resources
Environmental Services has various online educational materials and other resources to help keep our rivers and streams clean and healthy.
- Preventing Pollution
- Protecting Rivers and Streams
- Managing Stormwater
- Invasive Plants
- Community Cleanups
- What You Can (and Can’t) Flush
For more information about erosion control inspections, floodplain maps, and erosion prevention best management practices, view the Erosion Control webpage.
Pollution Prevention Best Management Practices
For more information about requirements and ways you can prevent pollution, visit the Best Management Practices to Prevent Pollution.
The City's Role and Authority
The MS4 permit requires permittees to adopt, update, and maintain adequate legal authority to control and enforce pollutant discharges. Portland City Code 10.10, 17.38 and 17.39 meet those requirements.
- Portland City Code 10.10 provides requirements for development and construction related activities in order to control the creation of sediment and to prevent the occurrence of erosion at the source during construction and development.
- Portland City Code 17.38 is intended to provide for the effective management of stormwater, groundwater, and drainage, and to protect and improve water quality in the city of Portland.
- Portland City Code 17.39 provides Environmental Services the authority to ensure the City storm sewer and drainage systems are operated in a manner that protects public health and the environment.