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

In unincorporated Colusa County, new agricultural development that requires an administrative permit under the F-A (Foothill Agriculture) and E-A (Exclusive Agriculture) zone standards must meet a set of performance standards spelled out in the Colusa County Zoning Code, and drainage is one of them. 20(B)(e) requires that any permitted or administratively-permitted use in the agricultural zones produce 'no net increase in off-site drainage flows, including peak flows during a storm event,' and requires that water quality measures be implemented to reduce stormwater pollutants. The rule sits alongside five other performance thresholds in the same subsection: a 60 dB noise ceiling at the property line of non-agricultural, non-industrial neighbors; a cap of 100 car/light-truck trips or 70 heavy-truck/bus trips per day; proof of adequate on-site water and sewer capacity; compliance with the County's Flood Damage Prevention chapter (Chapter 33); and a Chief Building Official sign-off on fire protection measures.

20(B) does not simply reject it: the code elevates the application to a Minor Use Permit, which brings discretionary Planning Commission or Zoning Administrator review, added findings, and public notice before the drainage design can be approved. Colusa County's Community Development Department administers this review and can condition approval on retention basins, detention facilities, or other on-site controls that hold peak runoff to pre-project levels. 20(B)(c) for the Natural Resources zones (River Frontage, Flood Management, Forest Residential, Forest Management and Recreation, Resource Management), so it functions as the county's baseline drainage performance standard outside the incorporated cities of Colusa and Williams.