During 2023, the Cherry Creek Basin Water Quality Authority worked with its partners to preserve and protect water quality in Cherry Creek Reservoir (Reservoir). This work includes an extensive reservoir and watershed monitoring program, stream reclamation projects, operation and maintenance of Pollutant Reduction Facilities, operation of the Reservoir Destratification System, special studies and modeling efforts, public education and outreach, and other efforts. Local governments and entities operating wastewater reclamation facilities use advanced treatment technology to maintain total phosphorus in treated effluent at 30-day average concentrations below 0.05 mg/L. Municipal stormwater managers implemented stormwater management programs in accordance with Regulation 72 requirements to minimize the adverse effects of stormwater runoff on streams and the Reservoir.

Highlights of our 2023 activities are briefly summarized below.

How is the Reservoir?

Cherry Creek State Park has a record number of visitors every year, and the Reservoir continues to provide space where people enjoy recreating and connecting. Colorado Parks and Wildlife biologists reported that the walleye fishery is doing well.

Seasonal phosphorus concentrations in the Reservoir were much higher than they have been relative to the average over the past 20 years, likely due to large storm events that occurred in May and June. Despite on-going efforts to reduce nutrient loading to the Reservoir, the Reservoir did not attain its chlorophyll-a standard of 18 ug/L during 2023. The average chlorophyll-a concentration was 20.9 ug/L for the growing season of July through September. The Reservoir has exceeded the standard for four of the past five years; however, the average chlorophyll-a concentrations were the lowest they’ve been in the last four years. Additionally, an undesirable blue-green algae bloom in late July was responsible for closure to human contact due to the detection of toxins. The toxin was only detected for a few days and the bloom dissipated in two weeks. The Reservoir attained the Regulation 38 water quality standards for temperature, pH, and dissolved oxygen.

Some of the factors affecting conditions in the Reservoir are challenging or impossible to control and result in year-to-year variations in water quality and environmental conditions.

Some of these factors include weather conditions such as temperature, wind, and precipitation patterns. Other factors include natural sources of phosphorus loading from the watershed and alluvial groundwater and the release of stored nutrients from Reservoir sediments (internal loading). CCBWQA operates a Reservoir Destratification System from April through October to help mitigate some of these influences.

How is the watershed?

The 386 square mile watershed has experienced significant growth since Control Regulation 72 was implemented.   Baseline loading of phosphorus from wastewater reclamation facilities is well controlled, with these point sources contributing less than three percent of the phosphorus load to the Reservoir.

CCBWQA monitors phosphorus and nitrogen concentrations in Cherry Creek and Cottonwood Creek since they are the two main inflows to the Reservoir.

Key findings from 2023 include:

• Median total phosphorus concentrations in baseflows of both Cherry Creek and Cottonwood Creek were lower than long term medians. However the median phosphorus concentration was much higher than the historical median during the storms sampled in WY 2023.
• Significant erosion occurred in response to major storm events, contributing to higher phosphorus concentrations in the creeks.
• Phosphorus concentrations in Cottonwood Creek are ~ 70% lower than concentrations in Cherry Creek.
• Total nitrogen concentrations in both Cherry Creek and Cottonwood Creek were higher than the long term historical median.
• Median nitrogen concentrations in Cherry Creek were 50% lower than concentrations in Cottonwood Creek in base flows but only 7% less during storms.
• The CCBWQA pollutant reduction facilities (PRFs) on Cottonwood Creek are effectively reducing phosphorus and suspended solids during storm flows.
• Median nutrient concentrations are lower downstream of the stream reclamation project on McMurdo Gulch

What did we do to preserve water quality?

CCBWQA and its partners continue to implement pollutant reduction facilities and stormwater control measures, construct stream reclamation projects and conduct other activities to reduce nutrient loading throughout the watershed. Key 2023 capital improvement projects in the watershed include:

• Happy Canyon Creek upstream of I-25: Approximately 3000 linear feet of stream reclamation was conducted from February through November 2023. The project promoted natural infiltration by raising the incised channel and included engineered bed and bank protection.  Project partners included Douglas County, Mile High Flood District, and the City of Lone Tree.
• Dove Creek Phase 1 from Otero Avenue to Chambers Road: In 2023, Phase 1 of the Dove Creek stream restoration project began. The project included step pool structures for grade control, bank protection (void-filled riprap, soil lifts, and vegetation), and grading to create overbanks providing a wider stream corridor which stabilizes the stream and reduces erosion potential.   The Southeast Metro Stormwater Authority (SEMSWA) is a project partner.
• Wetlands Harvesting Project: In 2023, CCBWQA completed year three of a six-year pilot project to cut and dispose of wetland vegetation to reduce phosphorus and nitrogen from being carried to Cherry Creek Reservoir after the plants decay.

What we learned from studies and special projects conducted in 2023.

During 2023, CCBWQA conducted several special studies including:

Stormwater Best Management Practice (BMP) Effectiveness Study: CCBWQA continued a study to synthesize the most current information on the expected effectiveness of stormwater BMPs (also known as stormwater control measures). This study will be completed in 2024.

Receiving Pervious Area Study: CCBWQA is partnering with SEMSWA and the Mile High Flood District to develop a more quantitative understanding of volume reduction benefits of receiving pervious areas such as grass buffers, grass swales and other landscape areas. Reducing runoff volumes through green infrastructure can reduce pollutant loads and channel erosion.

What are our plans for 2024?

CCBWQA will continue its routine activities along with some new activities in 2024. Highlights include:

• Continue the extensive long-term monitoring program that includes: weather and stream flow conditions; water quality in the Reservoir, groundwater, Cherry Creek and Cottonwood Creek, and other tributaries; Pollutant Reduction Facility performance; and phytoplankton and zooplankton dynamics in the Reservoir.
• Use long-term Reservoir data and the Reservoir model to develop appropriate site-specific nutrient standards for the Reservoir.
• Continue a major update to the 2012 Watershed Plan, including joint TAC and Board workshops and subcommittee participation. The updated Watershed Plan includes a significant effort to integrate geospatial data from multiple partners into the CCBWQA’s Data Portal.
• Link selected modeling scenarios from the watershed model to the Reservoir model to help prioritize future nutrient-reduction strategies.
• Invest over $3.9 million in stream reclamation projects in the watershed. These include projects on Cherry Creek, McMurdo Gulch, Happy Canyon, Lone Tree Creek, Dove Creek, and Piney Creek and the Reservoir shoreline.
• Complete extension of a drainage master plan on Lone Tree Creek, Windmill Creek and Cottonwood Creek from the Park boundary to the Reservoir, collaborating with SEMSWA and Mile High Flood District.
• Operate the Reservoir Destratification System from April through October.
• Continue the wetland harvesting pilot project for the Pollutant Reduction Facility on Cottonwood Creek and evaluate if we can quantify benefit through water quality analysis.Continue to host the Cherry Creek Stewardship Partners annual watershed conference in the fall of 2024.
• Communicate and share information and data with sister watersheds on the front range, including Bear Creek, Chatfield, and Barr-Milton Watershed Authorities.
• Work with the USACE to evaluate if there is a water quality benefit of change of storage timing in the Reservoir