According to a study by the vacation rental platform Lake.com, Florida’s Lake Okeechobee is the dirtiest lake in America, while Lake Superior is the nation’s cleanest. The recent study analyzed chemical data from the National Water Quality Monitoring Council for 46 of the 100 largest U.S. lakes. Their goal was to assess how safe the lakes are for swimming. The researchers used the latest available figures for each indicator, reported up until July 15, 2025. Lakes with less than three factors available in the official data were excluded, resulting in a list of 46 of the largest lakes. The lakes were ranked based on a total pollution score — the higher the pollution score, the dirtier the lake was considered.
Anglers can learn plenty from the results. The study evaluated eight commonly measured characteristics of a lake’s cleanliness level: dissolved oxygen, ammonia, lead, phosphorus, sulfate, total dissolved solids, turbidity and pH. For example, high levels of phosphorus and low levels of dissolved oxygen often contribute to a lake’s distress, leading to unfavorable fish populations and even potential health concerns for watermen.
The Dirtiest U.S. Lakes
Lake Okeechobee’s designation as the dirtiest lake is due to a combination of factors. Its water is visibly murky, with high turbidity levels of 34.36 NTU (Nephelometric Turbidity Unit). Lake Okeechobee has been plagued for decades with nutrient pollution leading to harmful algae blooms and sediment resuspension from its muck bottom.
The lake also contains lead at 0.67 mg/L (micrograms per liter), which is unnatural and has no safe level. High levels of phosphorus (0.18 mg/L) and nitrogen also contribute to the algae growth and blooms that occur regularly.
American Falls Reservoir in Idaho ranks as the second dirtiest lake, primarily due to the presence of ammonia. Manmade factors and agricultural practices lead to regular unsafe swimming conditions and warnings from the state. Lake Texoma, on the border of Texas and Oklahoma, is the third worst, characterized by low dissolved oxygen, high Total Dissolved Solids (TDS), and an unpleasant odor from high sulfate levels.
Most anglers know this all too well, but when dissolved oxygen levels fall too low within a water body, fish kills are a rather common and unfortunate side effect. Fountains, aerators and diffusers are in increasing use, especially in shallower lakes, to help add oxygen to a lake or pond.
While not in the top three, other lakes showed concerning levels of specific pollutants. Nevada’s Pyramid Lake has the highest TDS levels at 5,488 mg/L, and North Dakota’s Devils Lake is the smelliest, with the highest sulfate concentrations.
The Cleanest U.S. Lakes
Living up to its name, Lake Superior is the cleanest lake in the study. It has very clear water with low turbidity (0.46 NTU) and low TDS (44.04 mg/L). It is also exceptionally rich in dissolved oxygen (10.45 mg/L) and has nearly undetectable levels of phosphorus. Lake Chelan in Washington State, with the lowest TDS of all lakes at 30.20 mg/L, was ranked the second cleanest.
How to Determine if Your Home Fishing Lake is Healthy
Anglers who fish certain bodies of water definitely notice if it’s lagging or thriving in health. Obvious signs of distress include fish kills, algae blooms and oil slicks at the water’s surface. Water clarity can be an indicator too if a clear water lake clouds up.
Excessive nutrients can be a disaster for lakes. Nitrogen and phosphorus are the major culprits, often found in fertilizers and agricultural runoff. But localized runoff from roadways, dilapidated septic systems, stormwater runoff, and infrastructure failures such as sewage leaks also introduce pollution into waters.
While fishing, notice if your lake has a healthy diversity of fish species. For example, in the South, invasive tilapia seem to be the only species able to handle ponds and lakes with low dissolved oxygen — so if a lake or pond is full of tilapia mouthing the surface and nothing else, that could be a sign of an unhealthy water body.
Your best bet if you believe your lake is in trouble is to consider getting professional water testing done by a local agency such as a health department or state environmental organization.
