Thief Valley Reservoir is a productive, nutrient-rich eutrophic lake in Union County, Oregon. Water quality is graded F based on 1 sampling years through 2025. Compared to the 1 other monitored lakes in Union County, Thief Valley Reservoir ranks #2 for water quality.
Source: EPA Water Quality Portal sampling records, Oregon DNR, last sampled 2025-10-29. Grade methodology: LakeGrade methodology.
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Swimming Safety
Avoid swimming, very poor water quality, potential algae toxins
Water Quality Grade: F, Very Poor
No clarity data. Chlorophyll-a: 51.2 µg/L. Trophic State Index: 69.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No data | |
| Phosphorus | No data | |
| Chlorophyll-a (Algae) | 51.2 µg/L | F |
| Trophic State Index (TSI) | 69 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Where to stay near Thief Valley Reservoir
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Fishing Thief Valley Reservoir
Location
County Ranking
Ranked #2 of 2 lakes in Union County
Cleaner Lakes Within 30 Miles
Thief Valley Reservoir holds Grade F. One nearby lake has a higher grade.
Nearby Lakes in Union County
Explore Thief Valley Reservoir across LakeGrade
More Oregon Lake Rankings
Other Grade F Lakes in Oregon
Eutrophic Lakes in Oregon
More records, surveys & data sources
Data Sources
Water quality data from the EPA Water Quality Portal
Grading methodology based on Metropolitan Council standards
Most recent sample: 2025-10-29
Monitoring stations: 1