Big Arm Bay Lake
Lake County, MontanaView mapDirections
Clarity, phosphorus, and algae sit in the clean band for this region.
↓ down from A at the last regrade
- Oligotrophic
- #11 of 18 in the county
- Public access
Big Arm Bay Lake in Lake County, Montana has a water quality grade of B (Good). Phosphorus: 1.5 µg/L.
- Type
- Oligotrophic
- Phosphorus
- 1.5 µg/L
- Trend
- ↓ Declining
- Public access
- Yes
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Big Arm Bay Lake is a cold, clear oligotrophic lake in Lake County, Montana. Water quality is graded B based on 2 sampling years through 2026. 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. Grade methodology: LakeQuality methodology.
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 Safety
Grade onlyLow algae
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 B is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
Water Quality Grade: B (Good)
No clarity data. Phosphorus 1.5 µg/L. TSI 10. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No recent samples | |
| Phosphorus | 1.5 µg/L | Good |
| Chlorophyll-a (Algae) | No recent samples | |
| Trophic State Index (TSI) | 10 | Oligotrophic |
Low nutrients, clear water. The letter is not a swimming clearance.
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
Fishing Big Arm Bay Lake
Water Quality Trend: ↓ Declining
Based on 3 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Phosphorus | ↓ Declining | +0.5 µg/L/yr | 3 |
County Ranking
Ranked #11 of 18 lakes in Lake County
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. LakeQuality grades it B for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Big Arm Bay Lake?
- Big Arm Bay Lake in Lake County, Montana has a water quality grade of B (Good). Phosphorus: 1.5 µg/L.
Is Big Arm Bay Lake safe to swim in?
- The B is nutrients and algae, 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.
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.
Explore Big Arm Bay Lake across LakeQuality
22.3 mi away · recorded at 113°F
Species activity outlook by water temperature and season
Where to stay at Montana's top lakes, each with its grade
Low nutrients, clear water. The letter is not a swimming clearance.
Montana's cleanest and most-polluted lakes, ranked
More Montana Lake Rankings
Other Grade B Lakes in Montana
Oligotrophic Lakes in Montana
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)
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
Named boat-access points from the USGS National Boat Ramps inventory
Public campgrounds from USGS The National Map Structures, within about 3 miles