Hypereutrophic Lakes in Montana

22 hypereutrophic lakes in Montana. Very high nutrients, dense algae, poor clarity.

Hypereutrophic lakes (TSI above 70) are at the upper end of the productivity scale: visible algal blooms most weeks of summer, clarity often under 3 feet, and meaningful risk of harmful-algal-bloom toxin events. 22 hypereutrophic lakes are on the {{stateName}} books. Almost none of them are deep — 0% break 50 feet. Shallow basin plus heavy nutrient input is the consistent recipe.

Hypereutrophic lakes overwhelmingly grade D or F — the trophic class and the grade are measuring closely-related dimensions of the same underlying nutrient regime.

#	Lake	County	Grade	Clarity	Depth	TSI
1	Nla Lake	Meagher	C	13.3 ft	-	75
2	Nla Lake	Sheridan	C	-	-	115
3	Nla Lake	Flathead	C	-	-	84
4	Williamson Reservoir	Chouteau	C	-	-	70
5	Nla Lake	Valley	D	2.1 ft	-	89
6	Nla Lake	Hill	D	4.2 ft	-	99
7	Nla Lake	Wheatland	D	2.2 ft	-	93
8	Nla Lake	Big Horn	D	4.9 ft	-	93
9	Nla Lake	Phillips	D	2.3 ft	-	74
10	Nla Lake	Phillips	D	2.1 ft	-	96
11	Nla Lake	Toole	F	2.6 ft	-	81
12	Nla Lake	Sheridan	F	0.6 ft	-	124
13	Nla Lake	Chouteau	F	1.1 ft	-	115
14	Nla Lake	Hill	F	0.8 ft	-	117
15	Nla Lake	Valley	F	1 ft	-	81
16	Nla Lake	Blaine	F	0.5 ft	-	90
17	Nla Lake	Blaine	F	1.2 ft	-	82
18	Nla Lake	Phillips	F	0.9 ft	-	80
19	Nla Lake	Liberty	F	-	-	132
20	Kipp Lake	Glacier	F	-	-	84
21	Lake Helena Outlet	Lewis and Clark	F	-	-	73
22	Big Sky_deep Lake	Missoula	F	-	-	79

All hypereutrophic 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.