- Lake Superior is the nation’s cleanest, thanks to its oxygen-rich (10.45 mg/L), clear (0.46 NTU turbidity) and low-mineral water (44.04 mg/L Total Dissolved Solids)
- Lake Okeechobee is the dirtiest lake in the US today, with visibly murky water indicated by its 34.36 NTU turbidity levels and lead contamination at 0.67 µg/L
- Out of 100 largest lakes in the US, 46 had up-to-date chemical composition data, concluding that they are generally very clean and safe to fish and swim in
Lake Superior deserves its name even more by virtue of its second-to-noncrystal-clear water. The title for America’s dirtiest large lake, on the other hand, goes to Florida’s Lake Okeechobee.
The study, conducted by fishing forecast platform Fishbox, analyzed all the available chemical data from the National Water Quality Monitoring Council (NWQMC) for 100 of America’s largest lakes, sampled from January 1st, 2020, until July 15th, 2025.
The researchers evaluated eight of the most measured characteristics that can suggest a lake’s cleanliness level: dissolved oxygen, ammonia, lead, phosphorus, sulfate, total dissolved solids, turbidity and pH. They found 46 lakes to have at least three of these pollution factors measured within the indicated period. The purpose of the study is primarily to assess the impact on the fish population and how safe and pleasant these lakes are to swim in.
Top 10 Cleanest Lakes in America
| # | Lake | Location | Dissolved Oxygen (mg/L) | Ammonia (mg/L) | Lead (µg/L) | Phosphorus (mg/L) | Sulfate (mg/L) | Total Dissolved Solids (mg/L) | Turbidity (NTU) | Difference from pH7 (pH) | Pollution Score /10 |
| 1 | LakeSuperior | Michigan-Minnesota-Wisconsin-Ontario | 10.45 | 0.01 | 0.01 | 3.65 | 44.04 | 0.46 | 0.81 | 0 | |
| 2 | LakeChelan | Washington | 9.44 | 3.60 | 30.20 | 2.00 | 0.01 | 0.14 | |||
| 3 | LakeHartwell | Georgia-South Carolina | 8.55 | 1.90 | 0.51 | 1.73 | |||||
| 4 | Lake of the Ozarks | Missouri | 7.50 | 0.01 | 1.66 | 2.30 | 0.50 | 1.85 | |||
| 5 | Lake Pend Oreille | Idaho | 0.02 | 0.01 | 5.41 | 2.09 | |||||
| 6 | Lake Winnibigoshish | Minnesota | 9.38 | 0.02 | 1.30 | 1.21 | 2.20 | ||||
| 7 | KentuckyLake | Kentucky-Tennessee | 6.21 | 84.00 | 3.63 | 0.36 | 2.20 | ||||
| 8 | LakeNorman | North Carolina | 7.14 | 0.03 | 2.34 | 0.23 | 2.33 | ||||
| 9 | LakeMead | Arizona-Nevada | 4.93 | 0.03 | 3.44 | 0.05 | 2.45 | ||||
| 10 | FlatheadLake | Montana | 9.68 | 136.33 | 1.32 | 2.60 |
Lake Superior is the cleanest lake in America. It has the second-lowest levels of turbidity (0.46 NTU), after Lake Michigan (0.37 NTU), and Total Dissolved Solids (TDS), measuring 44.04 mg/L, just behind Lake Chelan (30.2 mg/L). Lake Superior is also very rich in oxygen (10.45 mg/L), with nearly undetectable algae-friendly phosphorus levels.
Washington’s Lake Chelan is the second cleanest large lake in the US. Its pH is closest to a neutral level, at 7.01, meaning it is optimal for freshwater aquatic life, as well as for humans swimming in Lake Chelan. It has the lowest TDS of all the large lakes included in the study, measuring just 30.2 mg/L. TDS refers to the totality of minerals within water, such as sulfates, chlorides, calcium, magnesium, potassium, bicarbonates and sodium, and indicates the general quality of water.
Lake Hartwell, bordering Georgia and South Carolina, is the third cleanest lake in America. It has incredibly clear water, reflected in its turbidity level of just 1.9 NTU. Two readings of dissolved oxygen last taken on October 31st, 2024, averaged to a healthy 8.55 mg/L.
The next cleanest large lakes in America are Lake of the Ozarks (Missouri), Lake Pend Oreille (Idaho), Lake Winnibigoshish (Minnesota), Kentucky Lake (Kentucky-Tennessee), Lake Norman (North Carolina), Lake Mead (Arizona-Nevada) and Flathead Lake (Montana).
Top 10 Dirtiest Lakes in America
| # | Lake | Location | Dissolved Oxygen (mg/L) | Ammonia (mg/L) | Lead (µg/L) | Phosphorus (mg/L) | Sulfate (mg/L) | Total Dissolved Solids (mg/L) | Turbidity (NTU) | Difference from pH7 (pH) | Pollution Score /10 |
| 1 | Lake Okeechobee | Florida | 6.69 | 0.04 | 0.67 | 0.18 | 24.00 | 239.80 | 34.36 | 0.99 | 10.00 |
| 2 | American Falls Reservoir | Idaho | 0.15 | 0.07 | 42.70 | 270.00 | 8.00 | 7.68 | |||
| 3 | Lake Texoma | Oklahoma-Texas | 4.42 | 0.06 | 164.05 | 753.59 | 14.00 | 0.90 | 7.16 | ||
| 4 | Eufaula Lake | Oklahoma | 6.41 | 0.13 | 0.08 | 29.84 | 170.94 | 34.23 | 0.64 | 6.92 | |
| 5 | Lake Clark | Alaska | 6.71 | 0.03 | 0.20 | 0.09 | 25.67 | 338.00 | 3.03 | 0.68 | 6.46 |
| 6 | Lake George | Florida | 7.55 | 0.01 | 0.20 | 0.06 | 56.70 | 658.00 | 5.57 | 1.32 | 6.38 |
| 7 | Utah Lake | Utah | 8.78 | 0.10 | 232.65 | 1,052.47 | 58.11 | 1.49 | 6.35 | ||
| 8 | Oneida Lake | New York | 4.59 | 0.13 | 0.02 | 23.00 | 1.78 | 6.04 | |||
| 9 | Pyramid Lake | Nevada | 6.00 | 5,488.44 | 2.36 | 5.84 | |||||
| 10 | Richland-Chambers Reservoir | Texas | 10.35 | 0.27 | 29.50 | 234.50 | 1.18 | 5.60 |
Lake Okeechobee in Florida is America’s dirtiest lake. 0.67 µg/L of lead was detected in its most recent samples. There is no safe level of lead and its presence in lakes is unnatural. Turbidity here was the second-highest of all evaluated lakes, measuring 34.36 NTU, after Utah Lake’s 58.11 NTU. This gives the impression of rather cloudy water. One cause for the murky appearance of Lake Okeechobee may be the high levels of phosphorus, estimated at 0.18 mg/L, which stimulates algae growth.
American Falls Reservoir in Idaho is the second dirtiest lake in the US. This 88-square-mile man-made reservoir appears clear to the human eye, at first, considering its turbidity levels measure a reasonable 8 NTU. However, chemical tests concluded it contained ammonia, which should not be present at all, estimated at 0.15 mg/L. When ammonia levels reach 0.53 mg/L, the water becomes harmful to fish.
Lake Texoma ranked third worst for cleanliness. Bordering Texas and Oklahoma, this reservoir is quite poorly oxygenated, solids-heavy and has a subtle rotten egg smell. Dissolved oxygen here is only 4.42 mg/L – if it were to drop below 2 mg/L, Lake Texoma would become hypoxic, meaning fish would suffocate. Sulfates of 164.05 mg/L, which are the fourth worst of all studied lakes, give the water an unpleasant odor. Lake Texoma has the fifth highest levels of TDS, estimated at 753.59 mg/L.
The absolute worst levels of TDS are recorded at the Pyramid Lake in Nevada, where they reach 5,488 mg/L. This is also where the pH deviates the most from the ideal level of 7 for lakes, with the last reading taken on February 12, 2020, showing an average pH level of 9.36.
Lake of the Woods, stretching across Canada’s Manitoba and Ontario provinces and into Minnesota, is the most lead-contaminated lake of the 100 included in the study. A sample taken on August 16, 2022, detected lead levels of 1 µg/L.
Devils Lake in North Dakota is the smelliest of all, where sulfate concentrations reach 745.17 mg/L. This may have happened due to its glacial deposits building up over thousands of years, with no natural outlet to dilute these sulfates over time.
Serge Selezen, CEO of Fishbox, commented on the findings:
“The health of fish populations is directly tied to water quality, and this data reveals some concerning trends. Dissolved oxygen is the most critical factor – when levels drop below 5 mg/L, fish become stressed, and below 2 mg/L, waters become hypoxic and can’t support most fish life.
“On average, America’s largest lakes have an acceptable pH balance of 7.96, good turbidity levels of 9.54 NTU, and healthy dissolved oxygen of 7.29 mg/L. They do, however, have quite significant TDS of 504.05 mg/L, a ninth of which (57.15 mg/L) comprise sulfates. Phosphorus levels average 0.05 mg/L. Ammonia was spotted in 24 lakes averaging 0.096 mg/L and lead – in 11, averaging 0.367 mg/L.
“We are pleased to find that, generally speaking, large lakes remain very clean and safe to swim and fish in, though monitoring any signage near swimming spots and adhering to the indicated advice may save you from catching a rare but nasty stomach bug.
“For anglers and anyone concerned about freshwater ecosystems, these numbers should serve as a wake-up call. Clean water isn’t just about human recreation, it’s about maintaining the delicate balance that keeps our fish populations thriving.”
