Big Arm Bay Lake
Lake County, MontanaView mapDirections
Montana DEQ publishes the water-quality measurements for Big Arm Bay Lake. Here is what has been measured.
- Oligotrophic
- Public access
Big Arm Bay Lake in Lake County, Montana has water-quality measurements published by Montana DEQ, shown here as reported. Phosphorus: 1.5 µg/L.
Big Arm Bay Lake is a cold, clear oligotrophic lake in Lake County, Montana. Overall water quality is declining over recent sampling years. Limited fish-species data available. Public access is available with 2 public boat landings. Compared to the 17 other monitored lakes in Lake County, Big Arm Bay Lake ranks #11 for water quality.
Source: EPA Water Quality Portal sampling records, Montana DEQ, last sampled 2026-06-30. Methodology: how these readings are compared.
What Big Arm Bay Lake is good for
From access and monitoring records, not editorial picks.
- Boat launch2 documented ramps
- Camping1 public campground nearby
Swimming conditions
Readings onlyLow algae
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 Big Arm Bay Lake 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 Big Arm Bay Lake
These are the measurements Montana DEQ publishes for Big Arm Bay Lake, 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 1.5 µg/L. TSI 10.
| 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 | 1.5µg/L | Good GoodFairPoor |
| Chlorophyll-a (algae) | No reading The green pigment in algae — a direct measure of how much algae is in the water. | |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 10 | Oligotrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Low nutrients, clear water. No reading is a swimming clearance.
Ask about Big Arm Bay Lake
Answers come straight from the public measurements and survey records on this page.
Recreation & Access
- ElmoPublic
- WalstadPublic
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Public campgrounds near Big Arm Bay Lake
- Big Arm Unit Campground1.2 km
Nearest public campgrounds in the USGS National Map inventory, within about 3 miles. An empty list means USGS records no campground that close, which is not the same as camping being unavailable. Where the source gives an official reservation link, it is used; the cabin and hotel search does not cover campsites. More camping lakes
Stays via Stay22
Stays near Big Arm Bay Lake
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Top-rated places to stay near Big Arm Bay Lake
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Fishing Big Arm Bay Lake
Water quality readings over time
3 seasons of samples on record (2023–2025). Calling a direction takes at least 4 years of the same measurement.
Phosphorus
Worsening+0.5 µg/L/yr3 years
Nearby Lakes in Lake County
Frequently asked questions about Big Arm Bay Lake
Where is Big Arm Bay Lake?
- Big Arm Bay Lake is a lake in Lake County, Montana. Its water-quality measurements are published by Montana DEQ and shown on this page as reported.
What is the water quality of Big Arm Bay Lake?
- Big Arm Bay Lake in Lake County, Montana has water-quality measurements published by Montana DEQ, shown here as reported. Phosphorus: 1.5 µg/L.
Is Big Arm Bay Lake safe to swim in?
- Nutrient and algae readings are not a swimming clearance. Check local advisories before you swim.
Are there boat ramps on Big Arm Bay Lake?
- Big Arm Bay Lake has 2 documented boat landings in the USGS National Boat Ramps inventory. Named access points include Elmo, Walstad. Public access is listed as yes.
Can I fish at Big Arm Bay Lake?
- Big Arm Bay Lake has 2 documented boat landings in the USGS National Boat Ramps inventory. Seasons, size and bag limits, and whether you need a license are set by Montana's fish and wildlife agency, so check its current regulations before you go.
Are there campgrounds on Big Arm Bay Lake?
- Big Arm Bay Lake has 1 public campground from USGS The National Map within 5 km, including Big Arm Unit Campground. This is a location join, not a booking. Cabins and hotels are listed separately; campsite reservations use the official link when the source provides one.
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Where this data comes from
41 measurements from 1 monitoring station, 2023–2026. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Confederated Salish and Kootenai Tribes
CSKTRIBE41 samples · 2023–2026
Analytical methods
- Phosphorus by Colorimetry
- 4500 N C ~ Persufate Method for Total Nitrogen
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
Named boat-access points from the USGS National Boat Ramps inventory
Public campgrounds from USGS The National Map Structures, within about 3 miles