The Trophic State Index (TSI) is a measure of the biological productivity of a water body. The most common TSI uses three water quality parameters to determine the trophic state: Total Phosphorus, Secchi Depth (a measure of water transparency), and Chlorophyll a.

There are four main levels of biological productivity: Oligotrophic (Low), Mesotrophic (Moderate), Eutrophic (High), and Hypereutrophic (Excessive). Higher TSI numbers are associated with increased probabilities of encountering nuisance conditions, such as algal scums.

Based on the historical Carlson TSI from 2002-present, Cherry Creek Reservoir has been considered eutrophic for Secchi depth and chl α, and ranges between eutrophic and hypereutrophic based on Total Phosphorus concentrations.