Arapahoe County, CO Environmental Rules: Stormwater Management (2026)

Arapahoe County and its cities operate under Clean Water Act Section 402 Municipal Separate Storm Sewer System (MS4) permits issued by CDPHE Water Quality Control Division. The Mile High Flood District (formerly Urban Drainage and Flood Control District) publishes the Urban Storm Drainage Criteria Manual (USDCM) which governs stormwater design across the Denver metro. New development and redevelopment over 1 acre require stormwater management plans with water quality capture volume (WQCV) facilities—rain gardens, bioretention, permeable pavers, extended detention basins. Aurora City Code §138-180 enforces MS4 requirements. Centennial participates through the Centennial Stormwater Utility. The Cherry Creek Reservoir Watershed Protection Zone (5 CCR 1002-72) imposes additional phosphorus load limits to protect reservoir water quality—any development in the watershed must meet stringent stormwater best management practices (BMPs). Illicit discharge to storm drains (paint, motor oil, soapy wash water, pet waste) is prohibited under the Colorado Discharge Permit System (CDPS). Construction sites over 1 acre require a Stormwater Construction Permit from CDPHE. Property owners with private stormwater facilities (detention ponds, water quality structures) must maintain them per approved operation and maintenance plans.