Grade F Lakes, Algae-prone

706 lakes (5% of lakes we track)

Grade F is the algae-prone end of the nutrient scale for that ecoregion. 706 lakes (5% of lakes we track) end up here. Cabins and rentals still sit on these lakes; skip the water if you see scum. Insufficient evidence is UNRATED, not an F.

A B C D F

706

Grade F lakes

States in this list

MN 100

Lake	County	State	Clarity	Depth	Fish
O'Leary Lake	Dakota	MN	2.6 ft	33.5 ft	7
North Long Lake	Ramsey	MN	2.5 ft	-	0
Mud Lake	Hennepin	MN	0.9 ft	-	0
Mud Lake	Stearns	MN	3 ft	20 ft	16
Mud Lake	Washington	MN	1.2 ft	-	0
Mud Lake	Martin	MN	1.6 ft	19 ft	17
Snelling Lake	Hennepin	MN	1.8 ft	9.5 ft	10
Round Lake	Itasca	MN	4.1 ft	24 ft	17
Pike Lake	Ramsey	MN	2.3 ft	16 ft	13
Horseshoe Lake	Stearns	MN	4 ft	57 ft	25
Horseshoe Lake	Washington	MN	4.8 ft	11.3 ft	8
Long Lake	Stearns	MN	2.5 ft	35 ft	23
Long Lake	Kandiyohi	MN	5 ft	16 ft	16
Long Lake	Hennepin	MN	3.3 ft	33 ft	14
Long Lake	Dakota	MN	1.6 ft	-	0
Big Swan Lake	Meeker	MN	2.6 ft	32 ft	23
Unnamed Lake	Meeker	MN	0.7 ft	15 ft	10
Unnamed Lake	Polk	MN	2 ft	-	0
Unnamed Lake	Hennepin	MN	-	-	0
Jonathan Lake	Carver	MN	1.6 ft	-	3
Peltier Lake	Anoka	MN	2.5 ft	18 ft	16
Tetonka Lake	Le Sueur	MN	3 ft	35 ft	19
Clear Lake	Meeker	MN	3 ft	18 ft	13
Clear Lake	Brown	MN	0.8 ft	8 ft	12
Clear Lake	Waseca	MN	2.5 ft	34 ft	12
Clear Lake	Jackson	MN	1.5 ft	9 ft	15
Clear Lake	Le Sueur	MN	1.2 ft	18 ft	11
Clear Lake	Martin	MN	1 ft	7 ft	12
Marion Lake	McLeod	MN	-	15.2 ft	13
Eagle Lake	Hennepin	MN	3 ft	34 ft	17
Eagle Lake	Carver	MN	2 ft	14 ft	8
Colby Lake	Washington	MN	3.8 ft	11 ft	8
Star Lake	Meeker	MN	1 ft	15 ft	10
Fish Lake	Wright	MN	3 ft	38 ft	17
Fish Lake	Becker	MN	1 ft	-	0
Washington Lake	Le Sueur	MN	4.5 ft	51 ft	18
Sand Lake	Washington	MN	2.5 ft	18 ft	0
East Solomon Lake	Kandiyohi	MN	2.6 ft	14 ft	15
Norway Lake	Kandiyohi	MN	2.4 ft	42 ft	15
Wheeler North Lake	Kandiyohi	MN	1 ft	-	0
East Auburn Lake	Carver	MN	5 ft	-	0
Roberds Lake	Rice	MN	2.5 ft	43 ft	18
Grace Lake	Carver	MN	2.6 ft	22 ft	11
Charlotte Lake	Stevens	MN	0.9 ft	20 ft	10
Bass Lake	Hennepin	MN	4 ft	31 ft	8
Island Lake	Mahnomen	MN	4.5 ft	43 ft	11
Cedar Lake	Rice	MN	2 ft	42 ft	18
Cedar Lake	Scott	MN	2 ft	13 ft	10
Emily Lake	Le Sueur	MN	2 ft	37 ft	14
Emily Lake	Chisago	MN	1.6 ft	-	0
Rice Lake	Hennepin	MN	2.3 ft	11.5 ft	16
Rice Lake	Stearns	MN	3 ft	41 ft	21
Wassermann Lake	Carver	MN	2.3 ft	-	0
Thompson Lake	Dakota	MN	3.3 ft	8 ft	0
Pokegama Lake	Pine	MN	2 ft	25 ft	23
Sandy Lake	Anoka	MN	2.1 ft	-	0
George Lake	Martin	MN	2 ft	11 ft	15
George Lake	Blue Earth	MN	2.2 ft	28 ft	9
Jefferson Lake	Le Sueur	MN	2 ft	-	0
Upper Orono Lake	Sherburne	MN	3.5 ft	-	0
Silver Lake	Ramsey	MN	3.6 ft	47 ft	12
Big Woods Lake	Carver	MN	1.5 ft	-	3
Goose Lake	Ramsey	MN	0.7 ft	6 ft	0
Goose Lake	Carver	MN	1.2 ft	10 ft	12
Goose Lake	Goodhue	MN	0.4 ft	5.7 ft	0
Goose Lake	Hennepin	MN	0.8 ft	8 ft	2
Goose Lake	Washington	MN	1.5 ft	-	0
Goose Lake	Washington	MN	3 ft	25 ft	7
Typo Lake	Anoka	MN	1 ft	6 ft	13
Rock Lake	Wright	MN	2.6 ft	37 ft	15
Powderhorn Lake	Hennepin	MN	1.5 ft	24 ft	6
Hall Lake	Martin	MN	3 ft	27 ft	19
Como Lake	Ramsey	MN	2.5 ft	15.5 ft	11
Swan Lake	McLeod	MN	2.6 ft	10 ft	14
Fountain Lake	Wright	MN	1 ft	10 ft	5
Crystal Lake	Hennepin	MN	3.5 ft	39 ft	12
Crystal Lake	Blue Earth	MN	1.5 ft	8 ft	12
Ann Lake	Wright	MN	4 ft	18.5 ft	16
Ann Lake	Kanabec	MN	2 ft	17 ft	13
Benton Lake	Carver	MN	1 ft	-	0
Foster Lake	Wright	MN	1.6 ft	10 ft	0
Knife Lake	Kanabec	MN	2 ft	15 ft	19
Diamond Lake	Hennepin	MN	1.7 ft	8 ft	0
Mink Lake	Wright	MN	3 ft	39 ft	8
Ardmore Lake	Hennepin	MN	1.3 ft	20 ft	0
Red Rock Lake	Douglas	MN	3.6 ft	22 ft	13
Stowe Lake	Douglas	MN	2.6 ft	14 ft	14
Oak Lake	Polk	MN	3 ft	-	0
Oak Lake	Becker	MN	3 ft	-	0
Thomas Lake	Hennepin	MN	3.4 ft	-	0
Elk Lake	Sherburne	MN	2 ft	9 ft	17
Elk Lake	Sherburne	MN	2.5 ft	15 ft	15
Francis Lake	Isanti	MN	2 ft	7 ft	14
Rebecca Lake	Dakota	MN	2.6 ft	15 ft	8
Elbow Lake	St. Louis	MN	3.6 ft	22 ft	8
Rush Lake	Chisago	MN	3 ft	24 ft	16
Rush Lake	Sherburne	MN	2 ft	11 ft	18
Sisseton Lake	Martin	MN	2.5 ft	19 ft	20
Schultz Lake	Kandiyohi	MN	1.3 ft	-	0
Alice Lake	Otter Tail	MN	0.8 ft	-	0

Showing 100 of 706 Grade F lakes.

Frequently Asked Questions

How many lakes have a grade of F?

There are 706 Grade F (Algae-prone) lakes across the 49 states and 2 Canadian provinces LakeQuality covers. That is 5% of the 14,370 lakes we track. Lakes without enough recent nutrient or algae samples are published as Not Rated — they are not in this count and they are not an F.

What does a Grade F mean?

Grade F means nutrients and algae are in the poorest EPA condition class for that ecoregion. Check current advisories and skip the water if you see scum or a paint-like bloom. Insufficient evidence is UNRATED, never an F. The letter measures nutrients and algae, not whether the lake is worth a cabin trip.

Featured Grade F Lakes

O'Leary Lake

Dakota County, MN · 2.6 ft clarity

North Long Lake

Ramsey County, MN · 2.5 ft clarity

Mud Lake

Hennepin County, MN · 0.9 ft clarity

Mud Lake

Stearns County, MN · 3 ft clarity

Mud Lake

Washington County, MN · 1.2 ft clarity

Mud Lake

Martin County, MN · 1.6 ft clarity

Source: EPA National Aquatic Resource Surveys, 2026.