Stormwater Overview

The California Small Phase II MS4 Permit Program was developed in response to the growing recognition of stormwater runoff as a major source of water pollution in urbanized areas. In 1972, the federal government passed the Clean Water Act (CWA), establishing the National Pollutant Discharge Elimination System (NPDES), which initially targeted large industrial dischargers and larger municipal storm sewer systems (Phase I). However, it became clear over time that smaller urbanized areas also contributed significantly to stormwater pollution.

In 1999, the U.S. Environmental Protection Agency (EPA) introduced the Phase II Stormwater Rule to address this gap, requiring smaller municipalities, counties, and other public entities to develop programs to reduce stormwater pollution. California adopted and implemented the Phase II MS4 permit program in 2003 to comply with federal requirements, extending stormwater management efforts to smaller communities. This program was a critical step toward ensuring that all urban areas, regardless of size, played a role in protecting water quality and maintaining the health of California’s waterways.