Oklahoma Lake Water Quality

24 of the 319 Oklahoma lakes in this index carry an A–F water-quality grade, spread over 71 counties. Those grades rest on measurements from the Oklahoma Department of Environmental Quality, 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. Only about 0% of Oklahoma's graded lakes pull an A. The smaller, agricultural-watershed lakes that dominate the graded set tend to carry persistent nutrient loads, and 92% of them fall in the D or F brackets.

Oologah Lake is the largest lake in this index at about 29,729 acres, one of 83 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. Eutrophic conditions dominate the graded set: nutrient-loaded, productive, with algal pressure that shows up most years in late summer. These lakes tend to be shallow, in agricultural watersheds, and over-warmed by the time July ends. The cleanest county on average is Grayson (grade C); the most-stressed is Pottawatomie. The gap between the top and bottom counties is large enough that a single figure for Oklahoma lake water quality hides real local variation.