Loon Lake
Lake County, MontanaView mapDirections
Nutrients and algae run above the regional average.
↓ down from A at the last regrade
- Mesotrophic
- #15 of 18 in the county
- 14 ft max depth
- Public access
Loon Lake in Lake County, Montana has a water quality grade of D (Nutrient-rich). Phosphorus: 6.7 µg/L. Chlorophyll-a (algae): 2.2 µg/L.
- Max depth
- ~14 ft est.
- Type
- Mesotrophic
- Phosphorus
- 6.7 µg/L
- Chlorophyll-a
- 2.2 µg/L
- Trend
- ↓ Declining
- Public access
- Yes
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Stays near Loon Lake
Compare lakeside cabins, hotels, and vacation rentals near Loon Lake in Lake 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 grade measures nutrients and algae, not lodging — cabins and rentals around Loon Lake are listed below.
Top-rated places to stay near Loon Lake
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Loon Lake is a mesotrophic lake in Lake County, Montana with an estimated average depth of about 14 feet. Water quality is graded D based on 3 sampling years through 2023. 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, Loon Lake ranks #15 for water quality.
Source: EPA Water Quality Portal sampling records, Montana DEQ, last sampled 2023-08-02. Grade methodology: LakeQuality methodology.
What Loon Lake is good for
From access and monitoring records, not editorial picks.
- Boat launch2 documented ramps
Swimming Safety
Grade onlyElevated nutrients
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. Check local advisories before you swim.
Water Quality Grade: D (Nutrient-rich)
No clarity data. Phosphorus 6.7 µg/L. Chlorophyll-a 2.2 µg/L. TSI 35. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No recent samples | |
| Phosphorus | 6.7 µg/L | Good |
| Chlorophyll-a (Algae) | 2.2 µg/L | Fair |
| Trophic State Index (TSI) | 35 | Mesotrophic |
Moderate nutrients, good water quality
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | ~14 ft est. |
| Average Depth | ~11 ft est. |
| Public Access | Yes |
Depth marked “est.” is a modeled maximum from GLOBathy (Khazaei et al. 2022, CC0), not a measured survey. Montana has not published bathymetry for Loon Lake.
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.
Recreation & Access
- Horseshoe Lake/FerndalePublic
- Loon Lake/FerndalePublic
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Fishing Loon Lake
Water Quality Trend: ↓ Declining
Based on 11 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Phosphorus | ↓ Declining | +1.39 µg/L/yr | 10 |
| Chlorophyll-a | ↓ Declining | +0.57 µg/L/yr | 11 |
County Ranking
Ranked #15 of 18 lakes in Lake County
Nearby Lakes With Higher Grades
Loon Lake holds Grade D. 2 nearby lakes hold higher nutrient-and-algae grades — useful for a swim day, not a reason to skip lodging here.
See full comparison →Nearby Lakes in Lake County
Frequently asked questions about Loon Lake
Where is Loon Lake?
- Loon Lake is a lake in Lake County, Montana. LakeQuality grades it D for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Loon Lake?
- Loon Lake in Lake County, Montana has a water quality grade of D (Nutrient-rich). Phosphorus: 6.7 µg/L. Chlorophyll-a (algae): 2.2 µg/L.
Is Loon Lake safe to swim in?
- The D is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
How deep is Loon Lake?
- No published bathymetric survey covers Loon Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 14 feet, but that is a modelled estimate derived from surrounding terrain rather than a measurement on the water, so treat it as approximate.
Are there boat ramps on Loon Lake?
- Loon Lake has 2 documented boat landings in the USGS National Boat Ramps inventory. Named access points include Horseshoe Lake/Ferndale, Loon Lake/Ferndale. Public access is listed as yes.
Explore Loon Lake across LakeQuality
More Montana Lake Rankings
Other Grade D Lakes in Montana
Mesotrophic Lakes in Montana
Where this data comes from
24 measurements from 1 monitoring station, 2012–2023. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Montana Watershed
MTWTRSHD_WQX24 samples · 2012–2023
Analytical methods
- Phosphorus by Colorimetry
- 10200 H ~ Chlorophyll a-b-c Determination
- 445.0 ~ EPA; Chlorophyll and Pheophytin in Algae by Fluorescence
- 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
Lake size & watershed from HydroLAKES (HydroSHEDS)
Named boat-access points from the USGS National Boat Ramps inventory