A municipal separate storm sewer system, or MS4, is a conveyance or system of conveyances (roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches, man-made channels, storm drains, etc.) that is also:

  • Owned or operated by a public entity (which can include cities, townships, counties, military bases, hospitals, prison complexes, highway departments, universities, etc.) having jurisdiction over disposal of sewage, industrial wastes, stormwater or other wastes. This includes special districts under state law, such as a sewer district; flood control district, drainage districts or similar entity; an Indian tribe or an authorized Indian tribal organization; or a designated and approved management agency under section 208 of the Clean Water Act that discharges to waters of the United States.
  • Designed or used for collecting or conveying stormwater
  • Not a combined sewer
  • Not part of a publicly-owned treatment works

The following document is a list containing Missouri's Regulated Municipal Separate Storm Sewer Systems (MS4s).

Automatically Designated MS4s

MS4s become automatically designated when they are located within an urban area with a population of 50,000 or more people (40 CFR 122.32(a)(1).

MS4 Urbanized Area Clarification

Missouri's Nine Urbanized Area Reference Maps 2010 - U.S. Census Bureau


Minimum Control Measures (MCMs) are required focus areas in MS4 permits. These are individual areas, however they can overlap or work together to make a successful Stormwater Management Program (SWMP).

Six MCMs Required for Phase II Regulated Communities

Resources for Developing MCMs


Facility Name Sort descending Permit # County
Fort Leonard Wood MO-R040088 Pulaski Fort Leonard Wood, MO Municipal Stormwater Management Plan, 2021-2026
Greene County MO-R040014 Greene Greene County MS4
Metropolitan St. Louis Sewer District (MSD) 1 MO-R040005 St. Louis MSD MS4
O'Fallon MO-R040039 St. Charles City of O'Fallon MS4
St. Charles County MO-R040058 St. Charles St. Charles County MS4



In response to the 1987 Amendments to the Clean Water Act, the Environmental Protection Agency (EPA) developed Phase 1 - Overview of the Storm Program of the National Pollutant Discharge Elimination System (NPDES). Phase I Stormwater regulations address runoff in medium and large communities determined by population size in 1990, identified 11 industrial categories required to obtain permits (with some exemptions), and addressed statewide land disturbance on five acres or greater. Missouri MS4s regulated under Phase I are Kansas City, Independence and Springfield.

EPA published the national Stormwater Phase II Final Rule Fact Sheet Series in the Federal Register Dec. 8, 1999. Phase II regulations expanded the program to include communities as determined by population size in 1999 (and subsequent decennial census counts) and land disturbance on one acre or greater. 

  • Certain Communities were exempt under the Phase I Stormwater regulations in 1990, and became subject to Phase II regulations. Regulated MS4s were required to obtain a NPDES permit by March 10, 2003, and have their stormwater management program (SWMP) in place within five years from date of initial permit issuance. Approximately 160 MS4s are designated and currently regulated under the Master General Permit for Phase II MS4s.
  • Industrial Classifications Modified There are still 11 categories of industrial classifications covering 30 sectors of industrial activity that are required to obtain stormwater discharge permits. However, the North American Industry Classification System classifies establishments by their main type of activity. There is also now an option for industrial no exposure certifications in lieu of an operating permit. Municipally operated industries exempted by the Intermodal Surface Transportation Efficiency Act of 1991, are no longer exempt under Phase II. Visit Stormwater Permits for more information.
  • Land Disturbance Permits are now required for disturbing one or more acres. Permits must be in place before land disturbance begins at the construction site. This requirement applies statewide. Visit Construction Land Disturbance Permits for more information.