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LakeQuality
F

Union County Lake Quality

Oregon, 2 lakes, average grade F (Very Poor)

Only 2 lakes in Union County carry an active grade. The county-level summary is therefore representative of those specific lakes, not necessarily of every water body in the county. The county average is F — the most-stressed bracket in our rubric. Most monitored lakes here show persistent nutrient loading and algal pressure.

The county's lakes lean shallow, which makes the grades sensitive to wind, sediment-resuspension, and any nutrient inputs from the watershed. The cleanest lake on the books is Thief Valley Reservoir (F); the most-stressed is Thief Valley Reservoir (F).

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All Lakes in Union County

RankLakeGradeClarityPhosphorus
1Thief Valley ReservoirF--

See how Union County stacks up in the national county water-quality rankings, or browse the full directory of graded lakes across all 27 states.

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Source: EPA National Aquatic Resource Surveys, 2026.