Is Alexander Reservoir Polluted?
Yes — Alexander Reservoir in Caribou County, Idaho is on the EPA's Clean Water Act 303(d) impaired-waters list (2022 assessment cycle). It is cited for Nutrients (phosphorus, nitrogen), Turbidity. A formal cleanup plan — a Total Maximum Daily Load (TMDL) — has been written. "Impaired" is a legal designation, separate from the lake's EPA condition class water-quality readings: 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 | 2022 |
| EPA IR category | 4A |
| Uses not supported | Aquatic life |
| EPA names this lake in the listing | Yes |
| Location | Caribou County, Idaho |
Pollutants Cited
- 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.
- Turbidity — listed by EPA as exceeding water-quality standards for at least one designated use.
Specific parameters in the EPA record: Phosphorus, Total, Total Suspended Solids (TSS).
What this means for using Alexander Reservoir
Because Alexander Reservoir 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. See the full breakdown on the lake report. The official EPA assessment is available in the ATTAINS waterbody report.
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