Cleanest and Dirtiest Lakes in America 2026

Of 14,370 monitored lakes across the 36 states and 2 Canadian provinces we cover, 3,471 earn an A and 706 earn an F. Here are the cleanest and dirtiest ends of that distribution — graded on Secchi clarity, total phosphorus, and chlorophyll-a from the EPA Water Quality Portal.

Why most “cleanest lakes” roundups get the sample wrong

The roundups that dominate Google for “cleanest lakes in the US” usually start from a short list of large, famous lakes — often fewer than 50 waterbodies with enough recent chemistry to score. That produces a clean PR table. It does not produce a national water-quality picture. LakeQuality grades every monitored lake we can join to Secchi, phosphorus, and chlorophyll-a readings, then ranks on that uniform A–F scale. The result is a very different top and bottom than a 46-lake chemical sample of the biggest waterbodies.

Right now the cleanest graded lake in our dataset is Rainy Lake at about 13.9 feet of clarity (MN). At the other end, O'Leary Lake sits among the lowest overall scores, with phosphorus around 153 µg/L. Both lists below exclude thin one-parameter records so the ranking rests on complete grades.

The 15 cleanest lakes we grade

These lakes hold the highest overall water-quality scores across 38 jurisdictions. Clarity is Secchi depth in feet. Phosphorus is total phosphorus in micrograms per liter — lower is better.

Rank	Lake	State	Grade	Clarity (ft)	Phosphorus (µg/L)
1	Rainy Lake	MN	A	13.9	12
2	Crane Lake	MN	A	9	15
3	Lobster Lake	MN	A	13	21
4	East Fox Lake	MN	A	16.4	10
5	Deep Portage Lake	MN	A	15.4	9
6	Clearwater Lake	MN	A	29	4
7	Clearwater Lake	MN	A	9.8	22
8	Clearwater Lake	MN	A	13	12
9	Clearwater Lake	MN	A	10	16
10	Lower Hay Lake	MN	A	18.5	13
11	Latoka Lake	MN	A	19	11
12	Little Birch Lake	MN	A	13	19
13	North Long Lake	MN	A	15.1	14
14	Bad Medicine Lake	MN	A	25	6
15	Mud Lake	MN	A	12.5	22

For the always-current national table, see the cleanest-lakes ranking and the best clean-water lakes hub. State-by-state picks live in The Cleanest Lakes in Every State We Track.

The 15 dirtiest lakes we grade

“Dirtiest” here means the lowest combined clarity–nutrient–algae grade, not a claim about industrial dumping. Across the dataset, 1,359 lakes grade D and 706 grade F. The bottom fifteen share the usual pattern: shallow basins, heavy watershed nutrient load, and elevated chlorophyll-a.

Rank	Lake	State	Grade	Clarity (ft)	Phosphorus (µg/L)
1	O'Leary Lake	MN	F	2.6	153
2	North Long Lake	MN	F	2.5	102
3	Mud Lake	MN	F	0.9	150
4	Mud Lake	MN	F	3	166
5	Mud Lake	MN	F	1.2	89
6	Mud Lake	MN	F	1.6	126
7	Snelling Lake	MN	F	1.8	143
8	Round Lake	MN	F	4.1	67
9	Pike Lake	MN	F	2.3	124
10	Horseshoe Lake	MN	F	4	120
11	Horseshoe Lake	MN	F	4.8	40
12	Long Lake	MN	F	2.5	61
13	Long Lake	MN	F	5	92
14	Long Lake	MN	F	3.3	62
15	Long Lake	MN	F	1.6	110

The live bottom of the distribution is on the worst water-quality ranking. For causes and restoration options, see Lakes with the Worst Water Quality and Why Lakes Turn Green.

What the distribution actually looks like

A–F grades are not evenly spread. In the current refresh, A and B lakes make up the plurality of graded waterbodies, while D and F together still cover thousands of lakes where recreation is limited by algae and murk. That middle-and-tail pattern is invisible when a roundup only samples the 100 largest lakes by area — most of those giants are neither the cleanest nor the dirtiest waterbodies people actually visit.

A grades: 3,471 lakes — high clarity, low nutrients, low algae.
D grades: 1,359 lakes — poor clarity and/or elevated nutrients.
F grades: 706 lakes — the worst combined scores in the dataset.
Coverage: 14,370 monitored lakes across the 36 states and 2 Canadian provinces (38 jurisdictions).

How to read these lists before you travel

Use the clean list for trip planning and cabin hunting; use the dirty list to understand risk, not as a day-of swim call. Even an A-graded lake can bloom after a hot, calm week, and a low-graded lake can look fine on a windy day after a cold front. Pair the grade with current algae advisories and our swim-advisory explainer. Methodology detail is on the methodology page.

Note on size: the cleanest lake above covers about 129 acres — often far smaller than the household-name lakes that headline PR roundups.

Frequently Asked Questions

Among the 10,087 lakes we grade across 38 jurisdictions, the cleanest are the ones with the highest combined Secchi clarity, lowest phosphorus, and lowest chlorophyll-a. The top of that list changes as new monitoring samples land — the live ranking is always on /ranking/cleanest.

That report scores about 46 of the 100 largest U.S. lakes on a chemical pollution index (dissolved oxygen, turbidity, TDS, and related analytes). We grade thousands of monitored lakes — large and small — on the three signals limnologists use for recreational water quality: clarity, phosphorus, and algae density. Different question, different sample, different ranking.

A D or F grade means long-term clarity is poor and/or nutrients and algae are elevated. That raises the odds of murky water and harmful algal blooms, but day-of safety still depends on current advisories. Never treat a grade as a substitute for posted beach or algae warnings.

National PR lists often start from the largest lakes by surface area. Our ranking starts from every lake with enough monitoring data to grade. A small oligotrophic lake with 25 feet of Secchi clarity will outrank a huge reservoir that is chemically “fine” but only moderately clear.