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).
Quick Answers for Union County
Planning a trip? Check special fishing regulations for Thief Valley Reservoir — or check the official Oregon fishing regulations.
All Lakes in Union County
| Rank | Lake | Grade | Clarity | Phosphorus |
|---|---|---|---|---|
| 1 | Thief Valley Reservoir | F | - | - |
Other Counties in Oregon
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.
Where to stay near Union County
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Source: EPA National Aquatic Resource Surveys, 2026.