Thief Valley Reservoir
Union County, Oregon807 acresView mapDirections
Oregon DEQ publishes the water-quality measurements for Thief Valley Reservoir. Here is what has been measured.
- Hypereutrophic
- Impaired · Mercury
- Public access
- Limited data
Thief Valley Reservoir in Union County, Oregon has water-quality measurements published by Oregon DEQ, shown here as reported. Chlorophyll-a (algae): 51.2 µg/L. Surface area: 807 acres.
Thief Valley Reservoir is a 807-acre highly nutrient-loaded hypereutrophic lake in Union County, Oregon. 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. Methodology: how these readings are compared.
What Thief Valley Reservoir is good for
From access and monitoring records, not editorial picks.
- Boat launch1 documented ramp
Swimming conditions
Readings onlyLimited data
Based on clarity and algae readings from past sampling — there is no live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
Nutrient and algae readings are 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.
Whether Thief Valley Reservoir is safe to swim in today is a call for the Oregon Department of Environmental Quality and the county health department, who post current advisories. LakeQuality does not issue swimming clearances.
Water quality measurements for Thief Valley Reservoir
These are the measurements Oregon DEQ publishes for Thief Valley Reservoir, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
No clarity data. Chlorophyll-a 51.2 µg/L. TSI 69.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | No reading How deep a black-and-white disk stays visible from the surface. Higher is clearer. | |
| Total phosphorus | No reading The nutrient that most limits algae growth in freshwater. More phosphorus, more algae potential. | |
| Chlorophyll-a (algae) | 51.2µg/L | Poor GoodFairPoor |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 69 | Hypereutrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Very high nutrients, dense algae, poor clarity
Ask about Thief Valley Reservoir
Answers come straight from the public measurements and survey records on this page.
Lake Details
Surface area
807 acres
Public access
Yes
Listed by Oregon DEQ
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
Stays via Stay22
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 readings cover nutrients and algae, not lodging. Cabins and rentals around Thief Valley Reservoir are listed below.
Top-rated places to stay near Thief Valley Reservoir
Prices for Oct 30–Nov 1, 4 adults. Change dates on the map below.
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Fishing Thief Valley Reservoir
Water Quality Trend: Worsening
Based on 6 years of monitoring (2020–2025).
Water clarity
Worsening5 years
Phosphorus
Worsening+37.95 µg/L/yr5 years
Chlorophyll-a
Worsening+14.26 µg/L/yr6 years
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
Designated uses not supporting: Aquatic life, Fish consumption.
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit OR_LK_1705020306_05_100597 · Official waterbody report
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. Its water-quality measurements are published by Oregon DEQ and shown on this page as reported.
What is the water quality of Thief Valley Reservoir?
- Thief Valley Reservoir in Union County, Oregon has water-quality measurements published by Oregon DEQ, shown here as reported. Chlorophyll-a (algae): 51.2 µg/L. Surface area: 807 acres.
Is Thief Valley Reservoir safe to swim in?
- Nutrient and algae readings are 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.
Can I fish at Thief Valley Reservoir?
- Thief Valley Reservoir has 1 documented boat landing in the USGS National Boat Ramps inventory. Seasons, size and bag limits, and whether you need a license are set by Oregon's fish and wildlife agency, so check its current regulations before you go.
Explore Thief Valley Reservoir across LakeQuality
4 thermal springs within 12.9 mi, nearest Kropp Hot Spring
Species activity outlook by water temperature and season
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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)
EPA condition classes from the National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See where the data comes from 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