Cherry Creek Reservoir exceeded chlorophyll-a standard in 2025
The basin authority identified multiple nutrient sources and continued projects to reduce phosphorus loading, including stormwater and stream work.
Cherry Creek Reservoir’s 2025 growing-season average for chlorophyll-a was 23.1 micrograms per liter, exceeding the state standard of 18 micrograms per liter, according to a Douglas County report.
The finding came from the Cherry Creek Basin Water Quality Authority’s 2025 annual water-quality report, which was presented to the Colorado Water Quality Control Commission on Aug. 11. Chatfield Reservoir remained compliant, and Cherry Creek met standards for temperature, pH and dissolved oxygen, the county report says.
The available records do not identify one cause for the exceedance. The Authority’s 2025 report cites phosphorus from natural and alluvial groundwater, nutrients stored in reservoir sediments, weather variability and runoff from developed areas. It also says precipitation was below average in 2025, particularly from May through July.
Wastewater treatment plants were not identified as the primary source. Wastewater point sources contributed less than 3% of the reservoir’s phosphorus load, and all basin wastewater facilities met their 2025 phosphorus and total-inorganic-nitrogen discharge limits, according to the Authority’s point-source-controls summary.
A separate Authority stormwater-control study estimated that developed areas contributed nearly half of baseline phosphorus loading in its model. That is a source estimate, not a finding that developed-area runoff alone caused the 2025 exceedance.
Planned responses
The Authority and its partners are continuing projects intended to reduce nutrient loading, including operating a reservoir destratification system, building or retrofitting stormwater-control facilities, stabilizing and reclaiming streams, harvesting wetlands and monitoring water quality.
The Authority’s 2025 report estimated that several projects could reduce or immobilize phosphorus. It estimated annual phosphorus reductions or immobilization of 45 pounds for the East Shade Shelter Stabilization project, 5.1 to 11.4 pounds for a Southeast Metro Stormwater Authority pond retrofit, and up to 73 pounds and 41 pounds for stream-stabilization projects at Scott Road and Dransfeldt, respectively.
For 2026, the Authority planned to invest $2.8 million in stream-reclamation projects and complete a feasibility study of in-reservoir strategies to reduce internal phosphorus loading. Those plans do not establish that the 2025 standard has been met.
Who sets the standard?
The Colorado Water Quality Control Commission sets surface-water classifications and standards through state regulations. The 18-micrograms-per-liter chlorophyll-a criterion is contained in Regulation 38, which covers the South Platte basin and other Colorado basins.
The Cherry Creek-specific control framework is in Regulation 72. The Cherry Creek Basin Water Quality Authority has a monitoring and management role, while the Colorado Department of Public Health and Environment’s Water Quality Control Division administers permits and compliance programs. CDPHE describes water-quality standards as the legal basis for controlling pollutants entering state waters in its standards guidance.