A small MS4 (Municipal Separate Storm Sewer System) is any MS4 not already covered by the Phase I program as a medium or large MS4. 

The 1999 Phase II regulation requires small MS4s in U.S. Census Bureau defined urbanized areas, as well as MS4s designated by the permitting authority, to obtain NPDES permit coverage for their stormwater discharges. Phase II also includes non-traditional MS4s such as public universities, departments of transportation, hospitals and prisons.

Most of the 6,695 Phase II MS4s are covered by statewide General Permits, however some states use individual permits. There are three Watershed Permits that cover 3 Phase I and 40 Phase II MS4s.

The Phase II Rule automatically covers on a nationwide basis all small MS4s located in Urbanized Areas (UAs) as defined by the Bureau of the Census (unless waived by the NPDES permitting authority), and on a case by case basis those small MS4s located outside of the UAs that the NPDES permitting authority designates. 

The City of Clarksburg is designated as a "small MS4."