Colorado Lake Water Quality

54 of the 733 Colorado lakes in this index carry an A–F water-quality grade, spread over 62 counties. Those grades rest on measurements from the Water Quality Control Division of the Colorado Department of Public Health and Environment, alongside federal programs and volunteer monitoring networks. The rest are listed for their size, depth and location, without enough recent nutrient or algae sampling behind them to support a letter. About 17% of graded Colorado lakes earn an A. The bulk sit in the middle B to C range; 44% fall into D or F, almost all of those in counties where row-crop agriculture dominates the watershed.

John Martin Reservoir is the largest lake in this index at about 11,590 acres, one of 79 here that clear 500 acres. The rest run smaller, and small and shallow is where nutrient loading shows up first, because there is less water to dilute it and the bottom sediment stays within reach of the sun. Mesotrophic is the dominant trophic class among the graded lakes, a workable middle ground between the pristine oligotrophic deep lakes and the algal-pressured eutrophic shallow ones. The cleanest county on average is Pueblo (grade A); the most-stressed is Logan. The gap between the top and bottom counties is large enough that a single figure for Colorado lake water quality hides real local variation.