Implementing Multi-faceted Strategies for Phased TMAL at Cherry Creek Reservoir, Colorado

Cherry Creek Reservoir was constructed in 1950 to protect the Denver area from floods. It soon proved to be popular for recreation, and became the heart of Colorado's first State Park. In the 1980s, phosphorous was identified as a limiting factor for algae growth in the reservoir with the potential for negative impacts to beneficial uses, and the Cherry Creek Basin Water Quality Authority (Authority) was formed as a local management agency. The Cherry Creek Reservoir has a chlorophyll a standard of 15 μ g/L, which was adopted to protect the reservoir from eutrophication. The reservoir also has a phosphorus goal and a total maximum annual phosphorus load (TMAL) established to maintain the chlorophyll a standard, which is being implemented as a phased TMAL. The phased TMAL provides for the implementation of point and nonpoint source requirements and controls while concurrent required investigations are implemented, as stated in the reservoir Control Regulation, "to better define hydrology, phosphorus sources, chemical processes, and relative loads to the watershed and reservoir." The reservoir has met the TMAL in 14 of the past 15 years, but the reservoir has only met the chlorophyll a standard in 4 of the past 15 years. The wastewater treatment facilities in the watershed already achieve a high level of phosphorus treatment (i. e., effluent phosphorus concentrations of 0.05 mg/L), and the Authority has developed and implemented a multi-faceted approach for watershed management strategies. The activities in this watershed are unique in that they have been implemented and monitored for over 15 years, and the Authority and participating entities continue to identify new strategies to address issues associated with the continued urbanization in the watershed. The Authority's phased TMAL strategies provide others with multiple examples of on-the-ground projects, the benefits and obstacles of working collaboratively with multiple land use agencies and special districts, and long-term monitoring results.