MS4 stands for Municipal Separate Storm Sewer System. It is a conveyance or system of conveyances that is:
- Designed or used to collect or convey stormwater
- Owned by a state, city, town, village, or other public entity that discharges to waters of the U.S.
- Not a combined sewer
- Not part of a publicly owned treatment works (sewage treatment plant)
Regulated conveyance systems include roads with drains, municipal streets, catch basins, curbs, gutters, storm drains, piping, channels, ditches, tunnels, and conduits.
MS4 program, commonly known as Stormwater Phase II, is a federally mandated program through the Clean Water Act which requires municipalities to take measures to reduce pollutants in stormwater runoff through MS4s in urbanized areas to protect waterways.