Thief Valley Reservoir
Union County, Oregon807 acresView mapDirections
Nutrients and algae run well above the regional average.
- Hypereutrophic
- Impaired · Mercury
- #1 of 1 in the county
- Public access
- Limited data
Thief Valley Reservoir in Union County, Oregon has a water quality grade of F (Algae-prone). Chlorophyll-a (algae): 51.2 µg/L. Surface area: 807 acres.
- Area
- 807 acres
- Type
- Hypereutrophic
- Chlorophyll-a
- 51.2 µg/L
- Trend
- ↓ Declining
- Public access
- Yes
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Stays near Thief Valley Reservoir
Compare lakeside cabins, hotels, and vacation rentals near Thief Valley Reservoir in Union County, Oregon — live availability and prices for your trip. Switch the map to Satellite to see the lake and how close each stay is to the water. The water-quality grade measures nutrients and algae, not lodging — cabins and rentals around Thief Valley Reservoir are listed below.
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Watch Thief Valley Reservoir’s letter
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Thief Valley Reservoir is a 807-acre highly nutrient-loaded hypereutrophic lake in Union County, Oregon. Water quality is graded F based on 1 sampling years through 2025. Overall water quality is declining over recent sampling years. Limited fish-species data available. Public access is available with 1 public boat landing.
Source: EPA Water Quality Portal sampling records, Oregon DEQ, last sampled 2025-10-29. Grade methodology: LakeQuality methodology.
What Thief Valley Reservoir is good for
From access and monitoring records, not editorial picks.
- Boat launch1 documented ramp
Swimming Safety
Grade onlyLimited data
Based on clarity and algae grades from past sampling — we do not hold live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
The F is nutrients and algae, not a swimming clearance. Thief Valley Reservoir is listed impaired under Clean Water Act §303(d) (IR 5) for mercury, temperature. Check local advisories before you swim.
Water Quality Grade: F (Algae-prone)
No clarity data. Chlorophyll-a 51.2 µg/L. TSI 69. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No recent samples | |
| Phosphorus | No recent samples | |
| Chlorophyll-a (Algae) | 51.2 µg/L | Poor |
| Trophic State Index (TSI) | 69 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 807 acres |
| Public Access | Yes |
No bathymetric survey has been published for Thief Valley Reservoir, so its depth is not documented here.
Recreation & Access
- Thief ValleyPublic
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Fishing Thief Valley Reservoir
Water Quality Trend: ↓ Declining
Based on 6 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -0.144 m/yr | 5 |
| Phosphorus | ↓ Declining | +37.95 µg/L/yr | 5 |
| Chlorophyll-a | ↓ Declining | +14.26 µg/L/yr | 6 |
Reservoir Info (USACE NID)
Thief Valley Reservoir is a man-made reservoir impounded by the Thief Valley Dam (completed 1932), built primarily for irrigation on the Powder River; buttress-type dam, 73 ft tall and 390 ft long.
- Surface area
- 807 ac
- Max storage
- 13,307 ac-ft
- Drainage area
- 914.3 sq mi
- Hazard class
- High
- Owner
- BUREAU OF RECLAMATION
Source: USACE National Inventory of Dams, NID ID OR00592 · Operator website
EPA Impairment Status
Thief Valley Reservoir is officially listed as impaired under Clean Water Act §303(d) in the 2022 EPA reporting cycle (IR category 5).
Causes of impairment
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit OR_LK_1705020306_05_100597 · Official waterbody report
County Ranking
Ranked #1 of 1 lakes in Union County
Nearby Lakes in Union County
Frequently asked questions about Thief Valley Reservoir
Where is Thief Valley Reservoir?
- Thief Valley Reservoir is a lake in Union County, Oregon. LakeQuality grades it F for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Thief Valley Reservoir?
- Thief Valley Reservoir in Union County, Oregon has a water quality grade of F (Algae-prone). Chlorophyll-a (algae): 51.2 µg/L. Surface area: 807 acres.
Is Thief Valley Reservoir safe to swim in?
- The F is nutrients and algae, not a swimming clearance. Thief Valley Reservoir is listed impaired under Clean Water Act §303(d) (IR 5) for mercury, temperature. Check local advisories before you swim.
Are there boat ramps on Thief Valley Reservoir?
- Thief Valley Reservoir has 1 documented boat landing in the USGS National Boat Ramps inventory. Named access points include Thief Valley. Public access is listed as yes.
Explore Thief Valley Reservoir across LakeQuality
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Species activity outlook by water temperature and season
Where to stay at Oregon's top lakes, each with its grade
Very high nutrients, dense algae, poor clarity
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Where this data comes from
6 measurements from 1 monitoring station, 2024–2025. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Department of Environmental Quality - State of Oregon
OREGONDEQ6 samples · 2024–2025
Analytical methods
- 10200 H ~ Chlorophyll a-b-c Determination
Monitoring station identifiers (1)
Grade thresholds from the EPA National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See how grades are calculated and data sources.
Other records on this page
Impairment status from EPA ATTAINS 303(d) database
Named boat-access points from the USGS National Boat Ramps inventory