Stanley Reservoir
Mountrail County, North Dakota253 acresView mapDirections
Nutrients and algae run above the regional average.
- Hypereutrophic
- Impaired · Nutrients (phosphorus, nitrogen)
- #3 of 5 in the county
- 8 ft max depth
- Public access
- Limited data
Stanley Reservoir in Mountrail County, North Dakota has a water quality grade of D (Nutrient-rich). Water clarity: 1.1 ft visibility. Phosphorus: 395 µg/L. Chlorophyll-a (algae): 19.5 µg/L. Surface area: 253 acres.
- Clarity
- 1.1 ft
- Max depth
- ~8 ft est.
- Area
- 253 acres
- Type
- Hypereutrophic
- Phosphorus
- 395 µg/L
- Chlorophyll-a
- 19.5 µg/L
- Public access
- Yes
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Stanley Reservoir is a 253-acre highly nutrient-loaded hypereutrophic lake in Mountrail County, North Dakota with an estimated average depth of about 8 feet. Water quality is graded D based on 1 sampling years through 2024. Limited fish-species data available. Public access is available. Compared to the 4 other monitored lakes in Mountrail County, Stanley Reservoir ranks #3 for water quality.
Source: EPA Water Quality Portal sampling records, NDDEQ, last sampled 2024-10-03. Grade methodology: LakeQuality methodology.
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 D is nutrients and algae, not a swimming clearance. Stanley Reservoir is listed impaired under Clean Water Act §303(d) (IR 5) for nutrients (phosphorus, nitrogen). Check local advisories before you swim.
Water Quality Grade: D (Nutrient-rich)
Very murky, less than 1.1 ft of visibility. Phosphorus 395 µg/L. Chlorophyll-a 19.5 µg/L. TSI 75. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1.1 ft | Not graded |
| Phosphorus | 395 µg/L | Not graded |
| Chlorophyll-a (Algae) | 19.5 µg/L | Not graded |
| Trophic State Index (TSI) | 75 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | ~8 ft est. |
| Surface Area | 253 acres |
| Public Access | Yes |
Depth marked “est.” is a modeled maximum from GLOBathy (Khazaei et al. 2022, CC0), not a measured survey. North Dakota has not published bathymetry for Stanley Reservoir.
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth here is modelled, not surveyed — HydroLAKES v1.0 estimates lake depth from surrounding terrain rather than from soundings. Treat it as an approximation, and prefer a state bathymetric survey where one exists.
Fishing Stanley Reservoir
Reservoir Info (USACE NID)
Stanley Reservoir is a man-made reservoir impounded by the Stanley Dam (completed 1968), built primarily for water supply on the Little Knife River; earth-type dam, 21 ft tall and 860 ft long.
- Surface area
- 253 ac
- Normal storage
- 1,550 ac-ft
- Max storage
- 3,720 ac-ft
- Drainage area
- 22 sq mi
- Hazard class
- Low
- Owner
- City of Stanley
Source: USACE National Inventory of Dams, NID ID ND00192 · Operator website
EPA Impairment Status
Stanley Reservoir is officially listed as impaired under Clean Water Act §303(d) in the 2022 EPA reporting cycle (IR category 5).
Causes of impairment
Source: EPA ATTAINS assessment unit ND-10110101-009-L_00 · Official waterbody report
County Ranking
Ranked #3 of 5 lakes in Mountrail County
Nearby Lakes in Mountrail County
Frequently asked questions about Stanley Reservoir
Where is Stanley Reservoir?
- Stanley Reservoir is a lake in Mountrail County, North Dakota. LakeQuality grades it D for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Stanley Reservoir?
- Stanley Reservoir in Mountrail County, North Dakota has a water quality grade of D (Nutrient-rich). Water clarity: 1.1 ft visibility. Phosphorus: 395 µg/L. Chlorophyll-a (algae): 19.5 µg/L. Surface area: 253 acres.
Is Stanley Reservoir safe to swim in?
- The D is nutrients and algae, not a swimming clearance. Stanley Reservoir is listed impaired under Clean Water Act §303(d) (IR 5) for nutrients (phosphorus, nitrogen). Check local advisories before you swim.
How deep is Stanley Reservoir?
- No published bathymetric survey covers Stanley Reservoir, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 8 feet, but that is a modelled estimate derived from surrounding terrain rather than a measurement on the water, so treat it as approximate. The lake covers 253 acres.
Explore Stanley Reservoir across LakeQuality
Depth, room, public access, and the state wake-sport rules
Species activity outlook by water temperature and season
Where to stay at North Dakota's top lakes, each with its grade
Very high nutrients, dense algae, poor clarity
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Where this data comes from
24 measurements from 1 monitoring station, 2024. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- North Dakota Department Of Environmental Quality
21NDHDWQ_WQX24 samples · 2024
Analytical methods
- Phosphorus by Colorimetry
- 447.0 ~ EPA; Chlorophyll a and b in phytoplankton by HPLC/UV
- 4500 NO3 E ~ Nitrate in Water- Cadmium Reduction
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
Lake size & watershed from HydroLAKES (HydroSHEDS)
Depth, fish, and access data from North Dakota Game & Fish