Denver, Colorado

Cherry Creek Basin Water Quality Authority requested that Hydros develop a model of Cherry Creek Reservoir for several related purposes:

• To better understand the causes of chlorophyll a exceedances and cyanobacteria blooms;

• To determine the effectiveness of the existing destratification system and test system modification options; and

• To support system management planning.

A two-dimensional hydrodynamic and water-quality model of Cherry Creek Reservoir was developed and calibrated with CE-QUAL-W2 to simulate temperature, dissolved oxygen, nutrients, algae, and chlorophyll a, as well as the effects of the destratification system. The model included inflow loading, internal loading, and in-reservoir water-quality dynamics. A custom, linked bubble plume model was also developed to support further detailed evaluation of the existing destratification system and the utility of possible modifications to the system.

The model produced important findings to support reservoir management decision making, including:

• Algal growth in Cherry Creek Reservoir is driven by a combination of high inflow loading, high internal loading, and internal nutrient cycling related to the shallow reservoir bathymetry;

• Cyanobacteria blooms occur in the reservoir when conditions are warm and calm, allowing weak stratification and development of nitrogen limitation at the surface, giving nitrogen-fixing bacteria an advantage; and

• The existing destratification system is undersized so it is not effective at increasing dissolved oxygen at the bottom of the reservoir except during spring when the thermal resistance to mixing is lower. Vertical mixing from the destratification system would need to be roughly tripled to adequately increase oxygen at the bottom of the reservoir.