Upper Colorado River Basin, Colorado

Grand Lake, Shadow Mountain Reservoir, and Granby Reservoir (the Three Lakes system) are an integral part of the Colorado-Big Thompson (C-BT)– the largest trans-mountain diversion project in Colorado.  Grand Lake is the deepest natural lake in Colorado, as well as the largest by volume. Shadow Mountain and Granby Reservoirs were constructed as part of the C-BT Project in the 1940s. Shadow Mountain Reservoir has a relatively shallow maximum depth of about 37 feet compared to Grand Lake and Granby Reservoir, which have maximum depths of 265 feet and 221 feet, respectively. Flow patterns vary throughout the year depending on system operations.

Since the 1950s, the Three Lakes have experienced several issues including water clarity (specifically in Grand Lake), sediment, algal growth, dissolved oxygen, water temperature, and nuisance macrophyte growth. Due to the interconnected and complex nature of the Three Lakes, an integrated, mechanistic (processed-based) model was developed to capture how operations, hydrology, watershed conditions, bathymetry, and meteorology impact the varied observed responses.

Hydros staff developed a 2-dimensional hydrodynamic water-quality model of the Three Lakes system using CE-QUAL-W2. The model was calibrated over a ten-year period and meets all calibration targets. The model captures in-lake responses as well as responses in the flow between each water body.  Clarity results indicate that there is a 90% confidence that the model will predict the average Grand Lake clarity season Secchi depth (metric used for assessment) to within 0.56 meters (1.8 feet). The model has been used for a variety of purposes including seasonal forecasting and long-term planning.