Impaired Lakes in Idaho
6 of 29 graded Idaho lakes (21%) are officially listed as impaired under Clean Water Act §303(d) in the most recent EPA ATTAINS reporting cycle. 5 have an approved TMDL restoration plan.
Top Causes of Impairment in Idaho
Reading the Idaho impairment landscape
6 of Idaho's 29 tracked lakes carry impaired-waters listings — 21%. That's a meaningful minority worth checking before specific trips, particularly for swimming, fishing, or drinking-water source identification. Most Idaho lakes are unimpaired; the impaired set tends to concentrate in agricultural, post-industrial, or high-development watersheds rather than being randomly distributed.
The leading impairment category in Idaho is Nutrients (phosphorus, nitrogen) — nutrient enrichment, typically from agricultural fertilizer runoff, septic discharge, or stormwater carrying lawn-care residue. Nutrient-impaired lakes can still be safe for contact recreation in many conditions, but they're prone to algal blooms (including potentially toxic cyanobacteria) during warm-water periods. Check the state's beach-monitoring or harmful-algal-bloom dashboard before swimming on nutrient-impaired waters in summer.
Impaired-waters listings come from the state environmental agency's §303(d) submission to EPA, refreshed on a roughly 2-year cycle. A listed lake stays on the list until either a TMDL is established and the cleanup completes, or subsequent monitoring shows the pollutant concentrations no longer exceed water-quality standards. The list below reflects the most recent available Idaho submission to the EPA Assessment, Total Maximum Daily Load Tracking and Implementation System (ATTAINS) dataset.
All 6 EPA-Listed Impaired Lakes in Idaho
| Lake | County | Grade | Causes | TMDL |
|---|---|---|---|---|
| Cascade R W of Sugarloaf Island Lake | Valley | D | Nutrients (phosphorus, nitrogen), pH / acidity | ✓ |
| Littlewood Lake | Blaine | D | Hydrologic alteration | ✓ |
| American Falls R-Little Hole Draw Lake | Power | D | Algal growth, Nutrients (phosphorus, nitrogen), Oxygen depletion (low DO) +1 more | ✓ |
| Ideq Lake | Kootenai | NR | Mercury, Metals (other than mercury) | ✓ |
| Little Wood Reservoir | Blaine | NR | Hydrologic alteration | ✓ |
| Cat Creek Reference Lake | Elmore | NR | Mercury | — |
Frequently Asked Questions
How many lakes are impaired in Idaho?
6 of 29 graded Idaho lakes (21%) are listed as impaired in the most recent EPA ATTAINS reporting cycle. Of those, 3 are on the federal Clean Water Act §303(d) list and 5 have an approved Total Maximum Daily Load (TMDL) restoration plan.
What does Clean Water Act §303(d) mean?
Section 303(d) of the federal Clean Water Act requires every state to identify waters that don't meet water quality standards even after pollution control measures. Each listed water must then have a Total Maximum Daily Load (TMDL) developed — a plan that caps how much of each pollutant can enter the waterbody. The "303(d) list" is the impaired-waters list. The EPA reviews each state's list every two years.
What are the top causes of impairment in Idaho?
The most common causes flagged across Idaho's impaired lakes are: Nutrients (phosphorus, nitrogen) (2 lakes); Hydrologic alteration (2 lakes); Mercury (2 lakes); pH / acidity (1 lakes); Metals (other than mercury) (1 lakes); Algal growth (1 lakes). Mercury is widespread in fish tissue across the upper Midwest from atmospheric deposition. Nutrient impairment (phosphorus, nitrogen) typically reflects agricultural runoff and urban stormwater.
Can I swim in an impaired lake?
It depends on the cause of impairment. A lake impaired for "mercury in fish tissue" is generally safe to swim in — the warning is about long-term consumption of fish. A lake impaired for "pathogens" (E. coli) or "algal growth" may pose a swimming risk and often has a separate beach advisory. Check the cause categories listed for each lake below, and consult your state health department's beach monitoring program for current advisories.
Where does this impairment data come from?
Every impairment record on this page comes from the EPA's ATTAINS (Assessment and TMDL Tracking and Implementation System) public dataset, the federal repository of state-reported water quality assessments. EPA ATTAINS is published as GeoParquet files on the EPA Office of Water S3 bucket and refreshed several times per year. Causes and use-support codes follow EPA's standard taxonomy.
More on impaired waters
What it means when a lake is listed as impaired, why mercury is the most common reason, and why a listed lake can still be clear water with a good grade.
Why states issue fish-consumption advisories, which contaminants — mercury, PCBs, PFAS — drive them, and how to eat the fish you catch more safely.
How phosphorus and nutrient runoff drive algae growth and turn lakes green, where the nutrients come from, and what it means for a lake's grade.
Which lakes are actually improving: multi-year EPA monitoring trends, the named recoveries still in progress, and what it takes to move a letter grade.
Data source
Impairment data from the EPA ATTAINS (Assessment and TMDL Tracking and Implementation System) public dataset. Assessment unit IDs are matched to LakeQuality lakes by name and proximity; some lakes may not appear here if their ATTAINS name doesn't match our naming convention. For an official query, consult .