Eutrophic Lakes in Montana

18 eutrophic lakes in Montana. High nutrients, frequent algae, reduced clarity.

Eutrophic lakes — TSI 50 to 70 — are nutrient-rich and productive, with persistent summer algal communities that drive visible reductions in clarity. These conditions are not failures; they reflect a lake doing what nutrient-loaded waters do. 18 {{stateName}} lakes register as eutrophic. Only 0% are deep — eutrophy correlates with shallowness, agricultural watersheds, and the kind of nutrient-input history that takes decades to shift.

Eutrophic lakes typically grade C or D on the LakeGrade rubric. The same nutrient richness that defines the trophic class drives the lower grade.

#	Lake	County	Grade	Clarity	Depth	TSI
1	Nla Lake	Flathead	B	19.3 ft	-	68
2	Nla Lake	Beaverhead	B	24.6 ft	-	67
3	Nla Lake	Carbon	B	18.5 ft	-	68
4	Nla Lake	Teton	B	19.8 ft	-	66
5	Mary Ronan Lake West	Lake	B	-	-	52
6	Canyon Ferry Lake	Broadwater	C	8.9 ft	-	50
7	Canyon Ferry Lake	Broadwater	C	4.3 ft	-	54
8	Nla Lake	Park	C	5.6 ft	-	55
9	Nla Lake	Beaverhead	C	2.6 ft	-	55
10	Nla Lake	Powell	C	-	-	59
11	Bighorn Lake	Big Horn	C	-	-	57
12	Flathead Lake Unknown Culvert	Lake	C	-	-	58
13	Boneau Reservoir	Chouteau	D	-	-	64
14	Nla Lake	Glacier	D	1.6 ft	-	64
15	Nla Lake	Sheridan	D	1.9 ft	-	64
16	Nla Lake	Rosebud	D	4.9 ft	-	60
17	Clark Canyon Reservoir South of Island	Beaverhead	D	4.9 ft	-	54
18	Lake Helena Center Deep	Lewis and Clark	D	5.6 ft	-	52

All eutrophic lakes →Minnesota version →Wisconsin version →Illinois version →Michigan version →Iowa version →Ohio version →Pennsylvania version →New York version →Missouri version →Indiana version →North Dakota version →South Dakota version →Florida version →Colorado version →North Carolina version →Vermont version →Texas version →Oregon version →Arkansas version →Oklahoma version →Kansas version →Nebraska version →Nevada version →Arizona version →Idaho version →New Mexico version →Tennessee version →Alabama version →Virginia version →Kentucky version →Utah version →Washington version →Wyoming version →All Montana lakes →

Source: EPA National Aquatic Resource Surveys, 2026.