Definition of a Municipal Separate Storm Sewer System (MS4) 

The term MS4 does not solely refer to municipally-owned storm sewer systems, but rather is a term with a much broader application that includes, in addition to local jurisdictions: State departments of transportation, public universities, local sewer districts, public hospitals, military bases and prisons. An MS4 is not always just a system of underground pipes; it can include roads with drainage systems, gutters, and ditches. The regulatory definition of an MS4 is:

According to 40 CFR 122.26(b)(8), “municipal separate storm sewer means a conveyance or system of conveyances (including roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches, man-made channels, or storm drains): 

(1) Owned or operated by a State, city, town, borough, county, parish, district, association, or other public body (created by or pursuant to State law)...including special districts under State law such as a sewer district, flood control district or drainage district, or similar entity, or 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 into the waters of the United States.” (Note: “Waters of the United States” refers to surface water only.) 

(2) “Designed or used for collecting or conveying storm water 

(3) Which is not a combined sewer; and  

(4) Which is not part of a Publicly Owned Treatment Works (POTW) as defined at40 CFR 122.2"

According to 40 CFR 122.26(b)(16)iii, small MS4s (population less than 100,000) “includes systems similar to separate storm sewer systems in municipalities, such as systems at military bases, large hospitals or prison complexes, and highways and other thoroughfares. The term does not include separate storm sewers in very discrete areas such as individual buildings.”


The purpose of the Stormwater Management Plan is to implement programs and practices to controll polluted storm water runoff.