Boneau Reservoir
Chouteau County, Montana136 acresView mapDirections
Nutrients and algae run well above the regional average.
↓ down from D at the last regrade
- Hypereutrophic
- #1 of 1 in the county
- 12 ft max depth
- Limited data
Boneau Reservoir in Chouteau County, Montana has a water quality grade of F (Algae-prone). Phosphorus: 50 µg/L. Chlorophyll-a (algae): 39 µg/L. Surface area: 136 acres.
- Max depth
- ~12 ft est.
- Area
- 136 acres
- Type
- Hypereutrophic
- Phosphorus
- 50 µg/L
- Chlorophyll-a
- 39 µg/L
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Boneau Reservoir is a 136-acre highly nutrient-loaded hypereutrophic lake in Chouteau County, Montana with an estimated average depth of about 12 feet. Water quality is graded F based on 2 sampling years through 2022.
Source: EPA Water Quality Portal sampling records, Montana DEQ, last sampled 2022-09-20. 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 F is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
Water Quality Grade: F (Algae-prone)
No clarity data. Phosphorus 50 µg/L. Chlorophyll-a 39 µg/L. TSI 64. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No recent samples | |
| Phosphorus | 50 µg/L | Good |
| Chlorophyll-a (Algae) | 39 µg/L | Poor |
| Trophic State Index (TSI) | 64 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | ~12 ft est. |
| Surface Area | 136 acres |
Depth marked “est.” is a modeled maximum from GLOBathy (Khazaei et al. 2022, CC0), not a measured survey. Montana has not published bathymetry for Boneau 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 Boneau Reservoir
Reservoir Info (USACE NID)
Boneau Reservoir is a man-made reservoir impounded by the Bonneau Dike (completed 2005), built primarily for irrigation on the Box Elder Creek; earth-type dam, 15 ft tall and 893 ft long.
- Surface area
- 136 ac
- Normal storage
- 0 ac-ft
- Max storage
- 2,571 ac-ft
- Drainage area
- 49.9 sq mi
- Hazard class
- Undetermined
- Owner
- BIA
All listed purposes: Irrigation;Recreation.
Source: USACE National Inventory of Dams, NID ID MT01054 · Operator website · Matched by proximity (0.25 km)
County Ranking
Ranked #1 of 1 lakes in Chouteau County
Nearby Lakes in Chouteau County
Frequently asked questions about Boneau Reservoir
Where is Boneau Reservoir?
- Boneau Reservoir is a lake in Chouteau County, Montana. LakeQuality grades it F for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Boneau Reservoir?
- Boneau Reservoir in Chouteau County, Montana has a water quality grade of F (Algae-prone). Phosphorus: 50 µg/L. Chlorophyll-a (algae): 39 µg/L. Surface area: 136 acres.
Is Boneau Reservoir safe to swim in?
- The F is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
How deep is Boneau Reservoir?
- No published bathymetric survey covers Boneau Reservoir, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 12 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 136 acres.
Explore Boneau Reservoir across LakeQuality
More Montana Lake Rankings
Hypereutrophic Lakes in Montana
Where this data comes from
21 measurements from 3 monitoring stations, 2022. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- USGS Montana Water Science Center
USGS-MT21 samples · 2022
Analytical methods
- Secchi disk method
- P, wu, WCA, persulfate CF color
- Plant pigments, sonication & 445
Monitoring station identifiers (3)
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
Lake size & watershed from HydroLAKES (HydroSHEDS)