Littleton, Colorado

Chatfield Reservoir is a flood-control reservoir located ~13 miles southwest of Denver, Colorado. In 2014, the Chatfield Reservoir Reallocation Project (Project) was approved by the U.S. Army Corps of Engineers, which reallocated a portion of the reservoir storage space from the flood pool to other uses. As a result, the normal maximum storage of the reservoir increased from ~27,000 AF to ~48,000 AF. In response to concerns that higher storage - and inundation of an additional 580 acres - could adversely affect water quality in the reservoir, Hydros was contracted by the Chatfield Reservoir Mitigation Company (CRMC) to develop a dynamic water-quality model of the reservoir.

Prior to receiving approval to store water in the reallocated space, Hydros developed and calibrated a water-quality model of Chatfield Reservoir using CE-QUAL-W2, using observed data from 2013 – 2016. The calibrated model was then used to anticipate the combined effects of the newly-inundated area and increased storage on chlorophyll a and total phosphorus concentrations, among other constituents. The model scenario runs suggested that decay of organic matter in the newly-inundated areas may lead to increases in nutrient, organic carbon, and chlorophyll a concentrations in the first two years following inundation, after which time the model suggested a net decrease in summer chlorophyll a, reflecting the benefits of increased reservoir depth.

The Chatfield Reservoir water-quality model, which now simulates from 2013 – 2022, is extended on an annual basis to incorporate the prior year of data into the model simulation. Since the Project received approval to store in the reallocated pool in 2020, the model has also been used to understand whether the Project has negatively affected water quality in Chatfield Reservoir by comparing model simulations with and without the Reallocation Project. As of 2022, the model results suggest the Project has had minimal impact on reservoir water-quality, and the reservoir has been meeting the site-specific chlorophyll a and total phosphorus standards.