Oregon Lake Water Quality

65 of the 659 Oregon lakes in this index carry a recent clarity, phosphorus or algae reading, spread over 35 counties. Those readings come from the Oregon Department of Environmental Quality, alongside federal programs and volunteer monitoring networks. The rest are listed for their size, depth and location, with no recent nutrient or algae sampling behind them. Measured Oregon lakes sit mostly in the middle of the nutrient range, with the highest phosphorus and algae readings 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 measured 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 has the lowest nutrient and algae readings on average. With only 35 counties in the state, the cross-county spread is narrower than the within-county spread for most lakes.