Oregon Lake Water Quality

55 of the 659 Oregon lakes in this index carry an A–F water-quality grade, spread over 35 counties. Those grades rest on measurements from the Oregon 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. About 18% of graded Oregon lakes earn an A. The bulk sit in the middle B to C range; 64% fall into D or F, almost all of those in counties where row-crop agriculture dominates the watershed.

By surface area the largest lake here is Fremont Bridge Lake, at about 85,000 acres, and 93 of the lakes in this index cover more than 500 acres. Size is worth knowing before you go: a large deep basin buffers nutrient inputs by sheer volume, while a shallow one warms quickly and can turn over its whole water column in a single summer storm. Most graded Oregon lakes sit in the mesotrophic band: moderate productivity, decent clarity, capable of supporting both warm-water and cool-water fisheries. This is the band where well-managed working lakes tend to land. Wallowa ranks highest on average county grade (A), and Umatilla sits lowest. With only 35 counties in the state, the cross-county spread is narrower than the within-county spread for most lakes.