Is Knife Lake Polluted?
Yes — Knife Lake in Kanabec County, Minnesota is on the EPA's Clean Water Act 303(d) impaired-waters list (2024 assessment cycle). It is cited for Mercury, Nutrients (phosphorus, nitrogen). A formal cleanup plan — a Total Maximum Daily Load (TMDL) — has been written. "Impaired" is a legal designation, separate from the lake's A–F water-quality grade: it means at least one designated use (such as swimming, aquatic life, or fish consumption) does not meet state standards for the listed pollutant.
EPA 303(d) Listing
| On 303(d) impaired list | Listed impaired |
| Cleanup plan (TMDL) | Completed |
| Assessment cycle | 2024 |
| EPA IR category | 4A |
| Location | Kanabec County, Minnesota |
Pollutants Cited
- Mercury — Mercury accumulates in fish tissue and is the single most common impairment in northern lakes. It usually arrives via atmospheric deposition from coal combustion rather than local discharge — which is why even remote, clear lakes can be mercury-impaired. It triggers fish-consumption advisories, not swimming closures.
- Nutrients (phosphorus, nitrogen) — Excess phosphorus and nitrogen fuel algae growth and summer blooms. This is the classic agricultural-runoff and shoreline-development impairment, and the one most likely to affect swimming via blue-green algae.
Specific parameters in the EPA record: MERCURY IN FISH TISSUE, NUTRIENTS.
What this means for using Knife Lake
Because Knife Lake is listed for Nutrients (phosphorus, nitrogen), swimming can be affected — check for posted beach advisories, especially after heavy rain or during visible algae. An impairment listing does not mean the lake is closed — most impaired lakes remain open for boating and swimming. It means a specific pollutant exceeds a standard for a specific use. Knife Lake carries an overall water-quality grade of F and phosphorus at 161.5 µg/L — see the full breakdown on the lake report. The official EPA assessment is available in the ATTAINS waterbody report.