Two canoes crossing open water between forested points in the Boundary Waters

Updated August 2026

Biggest Lake in Montana: Fort Peck Reservoir (246,000 acres)

Fort Peck Reservoir is Montana's largest lake per the Montana Fish, Wildlife & Parks. Below: 50 Montana lakes ranked by surface area.

The largest lake in Montana is Fort Peck Reservoir at 246,000 acres , in McCone, Valley, Garfield, Phillips, Fergus, Petroleum County, according to the Montana Fish, Wildlife & Parks.

Fort Peck Reservoir isn't in our water-quality index — we grade lakes with enough EPA Water Quality Portal samples to score, which leaves out many large reservoirs and border waters. The ranking below covers the 50 Montana lakes we do grade, led by Lake Koocanusa.

Source: Montana Fish, Wildlife & Parks (2023 (FWP Statewide Fisheries Management Plan 2023-2026, section 2.31)). The table below ranks the Montana lakes in our own index by surface area.

Lakes with area data

46,500

Largest (acres)

2,008

Median of top 50

5/50

Reservoirs (vs natural)

Reading Montana's size profile

Montana's largest lake by surface area is Lake Koocanusa in Lincoln County, spanning 46,500 acres. Most of Montana's biggest waters are natural basins left by glaciers, with stable shorelines and deep main basins.

Surface area is a poor proxy for water quality on its own — a large lake can hold a clean, deep core and a nutrient-loaded bay at the same time. Each lake below links to its full report card with grade, clarity, depth, and EPA impairment status. Area figures come from state DNR surveys and the USACE National Inventory of Dams; grades come from EPA and state monitoring data.

The 50 biggest lakes in Montana by surface area

#	Lake	County	Surface area	Max depth	Grade
1	Lake Koocanusa	Lincoln	46,500 acres	-	NR
2	Hungry Horse Reservoir	Flathead	23,596 acres	-	NR
3	Lake Elwell	Liberty	13,072 acres	-	NR
4	Hebgen Lake	Gallatin	12,407 acres	-	NR
5	Medicine Lake	Sheridan	9,017 acres	-	NR
6	Noxon Reservoir	Sanders	7,851 acres	-	NR
7	Clark Canyon Reservoir	Beaverhead	6,606 acres	-	C
8	Lake Frances	Pondera	5,252 acres	-	NR
9	Fresno Reservoir	Hill	5,007 acres	-	NR
10	Holter Reservoir	Lewis and Clark	4,550 acres	-	NR
11	Lima Reservoir	Beaverhead	4,373 acres	-	NR
12	Saint Mary Lake	Glacier	3,964 acres	-	NR
13	Ennis Lake	Madison	3,758 acres	-	NR
14	Lake Bowdoin	Phillips	3,659 acres	-	NR
15	Freezeout Lake	Teton	3,013 acres	-	NR
16	Upper Red Rock Lake	Beaverhead	2,947 acres	-	NR
17	Georgetown Lake	Deer Lodge	2,858 acres	-	NR
18	Ashley Lake	Flathead	2,852 acres	-	NR
19	Bynum Reservoir	Teton	2,819 acres	-	NR
20	Cabinet Gorge Reservoir	Sanders	2,789 acres	-	NR
21	Benton Lake	Cascade	2,700 acres	-	NR
22	Big Lake	Stillwater	2,583 acres	-	NR
23	Lower Red Rock Lake	Beaverhead	2,218 acres	-	NR
24	Lower Saint Mary Lake	Glacier	2,179 acres	-	NR
25	Tongue River Reservoir	Big Horn	2,158 acres	-	NR
26	Deadmans Basin Reservoir	Wheatland	2,008 acres	-	NR
27	Wild Horse Lake	Petroleum	1,929 acres	-	NR
28	Bowman Lake	Flathead	1,737 acres	-	NR
29	Pablo Reservoir	Lake	1,637 acres	-	NR
30	McGregor Lake	Flathead	1,525 acres	-	NR
31	Kingsbury Lake	Chouteau	1,464 acres	-	NR
32	Duck Lake	Glacier	1,449 acres	-	NR
33	Lake Sherburne	Glacier	1,399 acres	-	NR
34	Gibson Reservoir	Teton	1,281 acres	-	NR
35	Homestead Lake	Sheridan	1,262 acres	-	NR
36	Dry Lake Unit	Phillips	1,224 acres	-	NR
37	Ninepipe Reservoir	Lake	1,214 acres	-	NR
38	Big Lake	Chouteau	1,213 acres	-	NR
39	Bull Lake	Lincoln	1,202 acres	-	NR
40	Aloe Lake	Toole	1,180 acres	-	NR
41	Logging Lake	Flathead	1,127 acres	-	NR
42	Weigand Reservoir	Phillips	1,097 acres	-	NR
43	Cooney Reservoir	Carbon	1,078 acres	-	B
44	Alkali Lake	Pondera	1,012 acres	-	NR
45	Martinsdale Reservoir	Wheatland	989 acres	-	NR
46	Upper Waterton Lake	Glacier	988 acres	-	NR
47	Big Salmon Lake	Flathead	973 acres	-	NR
48	Ruby River Reservoir	Madison	949 acres	-	NR
49	Lake Como	Ravalli	912 acres	-	NR
50	Quartz Lake	Flathead	879 acres	-	NR

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Frequently Asked Questions

What is the largest lake in Montana?

Fort Peck Reservoir, at 246,000 acres, in McCone, Valley, Garfield, Phillips, Fergus, Petroleum County. Source: Montana Fish, Wildlife & Parks, 2023 (FWP Statewide Fisheries Management Plan 2023-2026, section 2.31).

How is lake surface area measured?

Surface area comes from state DNR lake surveys and, for reservoirs, US Army Corps of Engineers (USACE) National Inventory of Dams records. Area is reported in acres at normal pool level. Reservoirs rise and fall with management and drought, so their listed area is the long-term normal rather than a single-day reading.

Are Montana's largest lakes natural or reservoirs?

Of the top 50 Montana lakes by area, 5 sit on a dammed river (reservoirs) and 45 are natural basins. Glacial states (MN, WI, MI, NY) skew natural; plains and southern states (MO, IA, ND, OH) skew toward reservoirs built for water supply, flood control, and recreation.

Do larger lakes have better water quality?

Not reliably. Quality varies more within a big lake than between lakes — a large lake can have a clean, deep main basin and a murky, algae-prone bay fed by a tributary, so one grade can mask wide swings. Depth, watershed land use, and how fast the lake flushes matter far more than raw surface area.

How big is the typical Montana lake in this ranking?

Across the top 50 Montana lakes by surface area, the median is 2,008 acres (Deadmans Basin Reservoir). This list is biased toward the biggest waters; the typical Montana lake is far smaller.

Where to stay near Montana's largest lakes

Planning a trip? Jump to live cabin, resort, and hotel availability near Montana's biggest lakes.

Deepest lakes in Montana →Cleanest lakes in Montana →Largest lakes (all states) →All Montana lakes →