Lakes with the Worst Water Quality in the U.S. (2026)

Bottom 15 Lakes by Water Quality

What Causes Poor Water Quality?

Lakes with D and F grades share common problems, most of them driven by human activity in the surrounding watershed:

• Agricultural runoff, Fertilizer and manure from nearby farms wash phosphorus and nitrogen into lakes, fueling explosive algae growth. This is the single largest source of lake nutrient pollution in the Midwest, according to the EPA.
• Shallow depth, Shallow lakes warm quickly in summer, creating ideal conditions for algae. They also have less volume to dilute incoming nutrients. Wind easily stirs up bottom sediments, releasing stored phosphorus.
• Urban stormwater, Impervious surfaces (roads, parking lots, roofs) funnel stormwater directly into lakes carrying oil, fertilizer, pet waste, and sediment.
• Internal phosphorus loading, In oxygen-depleted bottom water, sediments release stored phosphorus back into the water column. This creates a feedback loop where poor conditions perpetuate themselves even after external sources are reduced.
• Failing septic systems, Older lakeshore properties with aging septic systems can leach nutrients directly into groundwater that feeds the lake.

Health Risks of Poor Water Quality

Lakes graded D or F frequently develop harmful algal blooms (HABs) dominated by cyanobacteria (blue-green algae). These organisms can produce toxins, called cyanotoxins, that cause:

• Skin rashes and eye irritation from contact
• Gastrointestinal illness from swallowing water
• Liver damage from prolonged exposure
• Risk to pets, dogs are especially vulnerable to cyanotoxin poisoning

Both the Minnesota Department of Health and Wisconsin DHS recommend avoiding contact with water that appears green, scummy, or paint-like.

Can These Lakes Improve?

Yes. Lake restoration is possible but requires sustained effort. Common approaches include:

• Buffer strips, Planting native vegetation along shorelines to filter runoff
• Alum treatments, Binding phosphorus in lake sediments to prevent internal loading
• Watershed management plans, Coordinated efforts to reduce nutrient inputs across the entire drainage area
• Cover crops and conservation tillage, Agricultural practices that reduce soil erosion and nutrient runoff

Several Minnesota lakes have improved by a full letter grade after sustained watershed management. Track trend data on individual lake pages to see which lakes are improving or declining over time.