Cleanest Lake in Montana: Tally Lake (2026)

The single cleanest lake in Montana is Tally Lake, ranked first on its measured clarity and nutrients in the 2026 sampling record. Cleanliness here is measured by Secchi depth (clarity), total phosphorus, and chlorophyll-a — all sourced from EPA Water Quality Portal and state agency sampling, shown as reported. The full top 22 ranked list appears below.

Ranking the cleanest Montana lakes means picking the basins with the lowest nutrient and algae readings, then the clearer water among lakes that tie. Tally Lake leads the list, with Whitefish Lake close behind.

The top of the cleanest-lakes list is dominated by deep, glacial, cold-water basins — the same basins that carry the lowest nutrient readings overall. The pattern is not coincidence: depth and watershed land-use are the dominant inputs to water-clarity and nutrient-loading both.

Rank	Lake	County	Clarity	Phosphorus	Chlorophyll-a	Area
1	Tally Lake	Flathead	15.2 ft	4.7 µg/L	0.8 µg/L	-
2	Whitefish Lake	Flathead	22 ft	2.8 µg/L	0.9 µg/L	-
3	Swan Lake North End	Lake	—	3.1 µg/L	1.1 µg/L	-
4	Little Bitterroot Lake Center	Flathead	—	4 µg/L	1 µg/L	-
5	Inez Lake	Missoula	—	7.1 µg/L	1.8 µg/L	-
6	Alva Lake	Missoula	—	4.7 µg/L	0.7 µg/L	-
7	Cooney Reservoir	Carbon	12.5 ft	20 µg/L	2.6 µg/L	1,078 acres
8	East Bay Lake	Lake	—	3 µg/L	—	-
9	Big Arm Bay Lake	Lake	—	1.5 µg/L	—	-
10	Dickey Lake	Lincoln	—	11.3 µg/L	2.2 µg/L	-
11	Mcgilvray Lake	Flathead	—	5.9 µg/L	1.7 µg/L	-
12	Hanson-Doyle Lake	Flathead	—	7.3 µg/L	2.3 µg/L	-
13	Fish Lake	Lincoln	—	8.1 µg/L	2.3 µg/L	-
14	Bailey Lake	Flathead	—	6.9 µg/L	2 µg/L	-
15	Big Sky Lake	Missoula	—	10 µg/L	2.1 µg/L	-
16	Loon Lake	Lake	—	6.7 µg/L	2.2 µg/L	-
17	Mary Ronan Lake West	Lake	—	33.6 µg/L	4.7 µg/L	-
18	Foys Lake	Flathead	—	15.8 µg/L	4.1 µg/L	-
19	Rogers Lake	Flathead	—	11.6 µg/L	2.6 µg/L	-
20	Jette Lake	Lake	—	16.6 µg/L	4.7 µg/L	-
21	Mary Ronan Lake East	Lake	—	15.8 µg/L	3.5 µg/L	-
22	Lake Helena Outlet	Lewis and Clark	—	115.5 µg/L	—	-

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All-states list →Clearest lakes in Montana (by Secchi clarity) →Minnesota version →Wisconsin version →Illinois version →Best swimming lakes in Montana →All Montana lakes →

Clearest lakes in Montana by water clarity

“Clearest” ranks by Secchi depth — how far you can see into the water — which can differ from the combined cleanliness score above. The clearest water in Montana is Big Therriault Lake at 37.3 ft of Secchi visibility.

1. Big Therriault Lake — 37.3 ft, Lincoln County
2. Ashley Lake — 33.1 ft, Flathead County
3. Holland Lake — 30.5 ft, Missoula County
4. Clearwater Lake Deep — 28.6 ft, Missoula County
5. Lindbergh Lake — 26.5 ft, Missoula County

See the full clearest-lakes ranking for Montana →

Frequently Asked Questions

What is the cleanest lake in Montana?

Tally Lake ranks first on measured clarity and nutrients in the 2026 Montana sampling record. Each lake's nutrient and algae readings — total phosphorus, total nitrogen, chlorophyll-a, and turbidity — are shown beside the EPA National Lakes Assessment 2022 condition class for that lake's own ecoregion, so a reading is compared against the lakes it would naturally resemble. LakeQuality publishes no letter grade. Cleanliness here means a low nutrient load and little algae, measured directly from EPA and state DNR samples; where two lakes tie on nutrients and algae, the deeper Secchi reading ranks first.

Where in Montana has the cleanest water?

Montana's clearest waters are concentrated in deeper, lower-nutrient glacial basins with forested or low-agriculture watersheds. Among the current top-ranked lakes above, counties like Flathead, Lake, and Missoula produce the highest-scoring lakes.

What is the clearest lake in Montana?

Big Therriault Lake is the clearest lake in Montana by water clarity, with a Secchi depth of 37.3 feet — the deepest transparency in our 2026 dataset. "Clearest" ranks by Secchi depth (how far you can see into the water) specifically, while "cleanest" combines clarity with phosphorus and chlorophyll-a. See our full clearest-lakes ranking for Montana for the top 25 by Secchi clarity.

Are these lakes safe to swim in?

Not on these readings alone. Clarity and nutrient readings are not a swimming clearance: they do not measure E. coli, cyanotoxins, or a posted beach closure, and a lake with low chlorophyll-a can still bloom. Always check current posted advisories before swimming: water quality changes seasonally, especially in late summer when blue-green algae bloom risk peaks.

How is this ranking calculated?

Lakes are ranked on phosphorus, chlorophyll-a and clarity as measured by EPA and state agency sampling, read against EPA National Lakes Assessment 2022 condition classes for that lake’s ecoregion, not a Montana-only 0–100 average. Water clarity is shown beside the nutrient readings. Lakes with limited sampling do not headline the table.

What are the top 5 cleanest lakes in the US?

There is no single national ranking of cleanest lakes — each state agency uses different metrics. Crater Lake (Oregon) is widely cited as the clearest US lake, with Secchi depths over 30 m. Lake Tahoe and Lake Superior are also top-tier nationally. Within Montana, the 22 lakes ranked above have the lowest nutrient and algae readings in our 2026 dataset.

How we rank cleanest lakes

Cleanliness here means three things working together:

Secchi clarity — how deep you can see into the water (in feet). Greater depth = clearer water = lower suspended sediment and algae.
Total phosphorus — the nutrient that fuels algae. Lower phosphorus levels mean less algae growth potential.
Chlorophyll-a — the green pigment in algae. Direct measurement of how much algae is currently in the water.

Each signal is normalized against Montana state norms and combined into a single 0–100 score. The lakes above are sorted by that combined score.Full methodology →

More on clean water

Lake Water Quality Explained: What the Readings Mean

How nutrient and algae readings are read against EPA ecoregion condition classes, what each class means in the water, and why a clean reading is not a clearance.

Cleanest Small Lakes in America 2026

The cleanest U.S. lakes under 500 acres, ranked on measured nutrients, algae, and water clarity — with the size cutoff applied to the lake, not the sample.

How to Choose a Clean Lake for a Cabin or Weekend Trip

A practical checklist for picking a lake with good water quality — clarity, phosphorus, algae history, depth, and the data to check before you commit.

Where to stay near Montana's cleanest lakes

Compare cabins, resorts, and rentals around Tally Lake and the other lakes in this ranking, with live availability and prices for your trip. Each pin on the map is one of the ranked lakes above.

Top-rated places to stay near Tally Lake

Prices for Oct 30–Nov 1, 4 adults. Change dates on the map below.

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Source: EPA National Aquatic Resource Surveys, 2026.