Marin County, CA Environmental Rules: Stormwater Management (2026)

Chapter 23.18, the County of Marin Stormwater Runoff Pollution Prevention Ordinance, is administered by the public works department (called "the agency" throughout the chapter). Section 23.18.060 prohibits discharging anything other than stormwater to a county storm drain or watercourse unless the discharge is separately authorized by an NPDES permit; § 23.18.061 lists narrow exemptions such as water line flushing, dechlorinated pool discharges and incidental irrigation overspray. Illicit connections to the storm drain system are banned outright and retroactively under § 23.18.080, even connections made lawfully decades ago.

Construction sites trigger their own duties. Section 23.18.093 requires erosion and sediment control best management practices, and any project needing a grading permit under Chapter 23.08, or one the agency decides risks significant erosion or sediment discharge, must file an erosion and sediment control plan meeting County Code § 24.04.625. New development and redevelopment projects that add or replace impervious surface must go further under § 23.18.094: the agency can condition approval on permanent controls, limits on impervious area, and, where the phase II stormwater permit applies, a stormwater control plan submitted under § 24.04.627.

Watercourses running through private land carry a maintenance duty of their own under § 23.18.095: the owner must keep the channel free of trash and debris and may not strip healthy native bank vegetation beyond what maintenance requires. The agency can inspect any premises for BMP compliance under § 23.18.120, request an administrative warrant if entry is refused, and order sampling or monitoring paid for by the property owner if there is reasonable cause to suspect pollution.