Published April 7, 2026 · Updated annually
Lakes with the Worst Water Quality in the Midwest (2026)
Of the 3,770 monitored lakes across Minnesota and Wisconsin, 716 receive a grade of D or F — indicating poor to very poor water quality. These 15 lakes have the lowest overall scores based on EPA Water Quality Portal measurements of clarity, phosphorus, and algae levels.
Bottom 15 Lakes by Water Quality
| Rank | Lake | State | County | Grade | Clarity (ft) | Phosphorus (ug/L) |
|---|---|---|---|---|---|---|
| 1 | Kabetogama Lake | MN | St. Louis | F | 2.6 | — |
| 2 | North Long Lake | MN | Ramsey | F | 2.5 | 102 |
| 3 | Mud Lake | MN | Hennepin | F | 0.9 | 150 |
| 4 | Mud Lake | MN | Hennepin | F | 3 | 196 |
| 5 | Mud Lake | MN | Otter Tail | F | 1.6 | — |
| 6 | Mud Lake | MN | Stearns | F | 3 | 166 |
| 7 | Snelling Lake | MN | Hennepin | F | 1.6 | — |
| 8 | Pike Lake | MN | Scott | F | 2 | 182 |
| 9 | Pike Lake | MN | Ramsey | F | 2.3 | 125 |
| 10 | Horseshoe Lake | MN | Rice | F | 2 | — |
| 11 | Long Lake | MN | Cottonwood | F | 1 | — |
| 12 | Unnamed Lake | MN | Ramsey | F | 1 | 173 |
| 13 | Unnamed Lake | MN | Meeker | F | 0.7 | 224 |
| 14 | Unnamed Lake | MN | Benton | F | 1.5 | — |
| 15 | Unnamed Lake | MN | Becker | F | 1.2 | — |
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.
Frequently Asked Questions
Swimming is not recommended in lakes graded D or F. These lakes frequently have high algae levels that may include toxic cyanobacteria (blue-green algae). If you see green scum, discolored water, or a paint-like sheen on the surface, stay out of the water and keep pets away.
Persistently green lakes have high phosphorus concentrations — often from decades of agricultural runoff, failing septic systems, or internal loading from nutrient-rich sediments. The phosphorus fuels continuous algae growth. Even when external sources are reduced, phosphorus stored in bottom sediments can continue feeding algae for years.
Yes, but it takes time and coordinated effort. Reducing external nutrient inputs (agricultural runoff, stormwater, septic systems) is essential. In-lake treatments like alum applications can bind phosphorus in sediments. Some Minnesota lakes have improved by a full letter grade over 10-15 years of active management.
/methodology