Boneau Reservoir
Chouteau County, Montana136 acresView mapDirections
Montana DEQ publishes the water-quality measurements for Boneau Reservoir. Here is what has been measured.
- Hypereutrophic
- 12 ft max depth
- Limited data
Boneau Reservoir in Chouteau County, Montana has water-quality measurements published by Montana DEQ, shown here as reported. Phosphorus: 50 µg/L. Chlorophyll-a (algae): 39 µg/L. Surface area: 136 acres.
Boneau Reservoir is a 136-acre highly nutrient-loaded hypereutrophic lake in Chouteau County, Montana with an estimated average depth of about 12 feet.
Source: EPA Water Quality Portal sampling records, Montana DEQ, last sampled 2022-09-20. Methodology: how these readings are compared.
Boneau Reservoir covers about 140 acres in Chouteau County. For a place on the water, look through cabin and lake-house rentals near Boneau Reservoir on Vrbo. For a hotel or motel room, Booking.com and Expedia both search around the lake.
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. Check local advisories before you swim.
Whether Boneau Reservoir is safe to swim in today is a call for the Montana Department of Environmental Quality and the county health department, who post current advisories. LakeQuality does not issue swimming clearances.
Water quality measurements for Boneau Reservoir
These are the measurements Montana DEQ publishes for Boneau 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. Phosphorus 50 µg/L. Chlorophyll-a 39 µg/L. TSI 64.
| 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 | 50µg/L | Good GoodFairPoor |
| Chlorophyll-a (algae) | 39µg/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. | 64 | Hypereutrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Very high nutrients, dense algae, poor clarity
Ask about Boneau Reservoir
Answers come straight from the public measurements and survey records on this page.
Lake Details
Max 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
Watershed (drainage) area
50 sq mi
HydroLAKES / HydroSHEDS
Shoreline
1.1 mi
HydroLAKES / HydroSHEDS
Elevation
3,153 ft
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.
Stays via Stay22
Stays near Boneau Reservoir
Compare lakeside cabins, hotels, and vacation rentals near Boneau Reservoir in Chouteau County, Montana — 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 Boneau Reservoir are listed below.
Top-rated places to stay near Boneau Reservoir
Prices for Oct 30–Nov 1, 4 adults. Change dates on the map below.
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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)
Nearby Lakes in Chouteau County
Frequently asked questions about Boneau Reservoir
Where is Boneau Reservoir?
- Boneau Reservoir is a lake in Chouteau County, Montana. Its water-quality measurements are published by Montana DEQ and shown on this page as reported.
What is the water quality of Boneau Reservoir?
- Boneau Reservoir in Chouteau County, Montana has water-quality measurements published by Montana DEQ, shown here as reported. Phosphorus: 50 µg/L. Chlorophyll-a (algae): 39 µg/L. Surface area: 136 acres.
Is Boneau Reservoir safe to swim in?
- Nutrient and algae readings are 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.
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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)
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
Lake size & watershed from HydroLAKES (HydroSHEDS)