Big Therriault Lake
Lincoln County, MontanaView mapDirections
Not Rated: one monitoring station are on record, but they measured water clarity only. LakeQuality’s A–F method grades total phosphorus, total nitrogen, chlorophyll-a, and turbidity; clarity is shown, not scored.
- Limited data
Big Therriault Lake in Lincoln County, Montana is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 37.3 ft visibility.
- Clarity
- 37.3 ft
- Trend
- → Stable
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Stays near Big Therriault Lake
Compare lakeside cabins, hotels, and vacation rentals near Big Therriault Lake in Lincoln 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.
Top-rated places to stay near Big Therriault Lake
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Watch Big Therriault Lake’s letter
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Why Big Therriault Lake has no grade
Big Therriault Lake has monitoring on record from one station, but only water clarity was measured. LakeQuality’s A–F letter uses total phosphorus, total nitrogen, chlorophyll-a, and turbidity against EPA National Lakes Assessment condition classes. With none of those grading measurements on record, Big Therriault Lake is Not Rated — not an F. Its 37.3-foot median from 6 readings collected 2020–2023 is still shown as clarity, but clarity alone cannot earn a letter.
Not Rated is EPA Integrated Reporting Category 3 — insufficient information. Everything below that has been measured is still reported on this page.
Sampling is run by Montana DEQ and volunteer programs, and published through the EPA Water Quality Portal. Read the full grading methodology.
Big Therriault Lake is a lake in Lincoln County, Montana.
Source: EPA Water Quality Portal sampling records, Montana DEQ, last sampled 2023-07-18. Grade methodology: LakeQuality methodology.
What Big Therriault Lake is good for
From access and monitoring records, not editorial picks.
- Clear water37.3 ft of visibility
- Camping3 public campgrounds nearby
Swimming Safety
Not ratedLimited data
The letter is not a swimming clearance. Check local advisories before you swim.
Water quality measurements for Big Therriault Lake
These readings exist but are not enough for an A–F grade — see why.
Crystal clear, you can see 37.3 ft down. TSI 25.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 37.3 ft | Not graded |
| Phosphorus | No recent samples | |
| Chlorophyll-a (Algae) | No recent samples | |
| Trophic State Index (TSI) | 25 | Oligotrophic |
Low nutrients, clear water. The letter is not a swimming clearance.
Recreation & Access
Public campgrounds near Big Therriault Lake
- Big Therriault Lake0.4 km
- Little Therriault Lake1.5 km
- Horse Camp1.7 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 Therriault Lake
Water Quality Trend: → Stable
Based on 11 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | → Stable | -0.133 m/yr | 3 |
| Phosphorus | → Stable | +0.01 µg/L/yr | 10 |
| Chlorophyll-a | → Stable | 0 µg/L/yr | 11 |
Graded lakes near Big Therriault Lake
Big Therriault Lake has no grade, but these lakes within a short drive do.
Nearby Lakes in Lincoln County
Frequently asked questions about Big Therriault Lake
Where is Big Therriault Lake?
- Big Therriault Lake is a lake in Lincoln County, Montana. LakeQuality has not rated this lake (UNRATED) — the only measurement available is water clarity, which is not a water-quality indicator on its own — EPA's National Lakes Assessment does not grade clarity.
What is the water quality grade for Big Therriault Lake?
- Big Therriault Lake in Lincoln County, Montana is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 37.3 ft visibility.
Is Big Therriault Lake safe to swim in?
- The letter is not a swimming clearance. Check local advisories before you swim.
Are there campgrounds on Big Therriault Lake?
- Big Therriault Lake has 3 public campgrounds from USGS The National Map within 5 km, including Big Therriault Lake, Little Therriault Lake, Horse Camp. 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 Therriault Lake across LakeQuality
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Where this data comes from
6 measurements from 1 monitoring station, 2020–2023. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- North American Lake Management Society
NALMS6 samples · 2020–2023
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
Public campgrounds from USGS The National Map Structures, within about 3 miles