Updated Aug 2026
Idaho Lake Water Quality
Water quality grades for 50 monitored lakes across 17 counties.
50 Idaho lakes carry a grade. Most of the input data flows through monitoring run by the Idaho Department of Environmental Quality, which feeds state, federal, and volunteer measurements into the EPA Water Quality Portal. About 16% of graded Idaho lakes earn an A. The bulk sit in the middle B–C range; 32% fall into D or F, almost all of those in counties where row-crop agriculture dominates the watershed.
Eutrophic conditions dominate — nutrient-loaded, productive, with algal pressure that shows up most years in late summer. These lakes tend to be shallow, agricultural-watershed, and over-warmed by the time July ends. Gem ranks highest on average county grade (A). Blaine sits lowest. With only 17 counties in the state, the cross-county spread is narrower than the within-county spread for most lakes.
Grade Distribution
All 50 Idaho lakes in the index: 29 (58.0%) carry an A–F grade and 21 (42.0%) are not rated, because they lack enough recent nutrient or algae sampling to support a letter.
Where to stay near Idaho
Top-rated places to stay near Idaho
Availability and prices from Stay22 partners (Booking.com, Vrbo, Expedia). We may earn a commission from bookings, at no extra cost to you.
Top-Graded Idaho Lakes
The highest water-quality grades in Idaho — clarity, depth, and A-F grade at a glance.
Lake Map
Explore Idaho Lakes
Idaho reports, fishing & travel
Reservoirs in Idaho
View all 17 reservoirs →17 of 50 Idaho lakes (34%) are man-made reservoirs in the USACE National Inventory of Dams. 17 carry a High or Significant hazard rating. The oldest matched dam dates to 1906.
EPA-Impaired Lakes in Idaho
View all 9 impaired lakes →9 of 50 Idaho lakes (18%) are officially listed as impaired under Clean Water Act §303(d) in the most recent EPA ATTAINS reporting cycle. 7 have an approved Total Maximum Daily Load (TMDL) restoration plan.
Idaho Fishing & Recreation
Licenses, regulations, records, and where to fish across Idaho.
Counties
All 17 Idaho counties, ranked by average lake water quality. Compare them against every county nationwide in the county rankings, browse the full directory of graded lakes, or pick a different state.
Top 25 Cleanest Lakes
25 lakes25 graded lakes
| # | Lake | County | Grade | Clarity | Trophic | Access |
|---|---|---|---|---|---|---|
| 1 | ABlack Cnyn Reservoir Ns of | Gem | A | 7.5 ft | Mesotrophic | ✓ Public |
| 2 | ADeadwood Reservoir | Valley | A | 17.2 ft | Oligotrophic | Not recorded |
| 3 | ACascade Reservoir | Valley | A | 8.2 ft | Oligotrophic | ✓ Public |
| 4 | ALake Lowell Across From Upper Embankment | Canyon | A | 7.2 ft | Mesotrophic | ✓ Public |
| 5 | ALake Lowell | Canyon | A | 7.2 ft | Oligotrophic | Not recorded |
| 6 | ACascade Reservoir A Sugar Loaf Island | Valley | A | 8.5 ft | Oligotrophic | ✓ Public |
| 7 | ABear Lake | Bear Lake | A | 19 ft | Oligotrophic | Not recorded |
| 8 | AAmerican Falls R Lake | Bannock | A | No recent samples | Mesotrophic | Not recorded |
| 9 | BBear Lake Off North Beach | Bear Lake | B | 21.7 ft | Oligotrophic | ✓ Public |
| 10 | CPasisades Reservoir | Bonneville | C | 10.5 ft | Mesotrophic | Not recorded |
| 11 | CPalisades Lake | Bonneville | C | 13.1 ft | Mesotrophic | Not recorded |
| 12 | CPalisades Reservoir | Bonneville | C | 12.5 ft | Oligotrophic | Not recorded |
| 13 | CMud Lake | Bear Lake | C | No recent samples | Eutrophic | Not recorded |
| 14 | DCas Reservoir Inlet Between Lf And | Valley | D | 4.3 ft | Eutrophic | ✓ Public |
| 15 | DLake Walcott | Cassia | D | 4.3 ft | Eutrophic | ✓ Public |
| 16 | DCascade R Lake | Valley | D | 9.2 ft | Mesotrophic | ✓ Public |
| 17 | DCascade R Lake | Valley | D | 5.7 ft | Eutrophic | ✓ Public |
| 18 | DIsland Park Lake | Fremont | D | 9.8 ft | Eutrophic | ✓ Public |
| 19 | DCascade R W of Sugarloaf Island Lake | Valley | D | 8.2 ft | Eutrophic | ✓ Public |
| 20 | DIsland Pk Reservoir | Fremont | D | 2.3 ft | Eutrophic | Not recorded |
| 21 | DLittlewood Lake | Blaine | D | 12.5 ft | Eutrophic | ✓ Public |
| 22 | DRirie Lake | Bonneville | D | 4.6 ft | Eutrophic | ✓ Public |
| 23 | DCascade Reservoir Inlet | Valley | D | 7.3 ft | Eutrophic | Not recorded |
| 24 | DAmerican Falls R-Sportsmans Cove Lake | Power | D | No recent samples | Eutrophic | ✓ Public |
| 25 | DAmerican Falls Reservoir | Power | D | No recent samples | Eutrophic | ✓ Public |
25 Lakes Needing Attention
25 lakes25 graded lakes
Lake Match
Which lake suits how you actually use one?
Six questions — swimming, fishing, quiet, cabins, drive time — and a shortlist of graded lakes that fit, with the water quality behind each one.
Source: EPA National Aquatic Resource Surveys and the EPA Water Quality Portal.
More on lake water quality
The top-graded lake in every state we cover, picked grade-first with water clarity as the tie-breaker — plus the states whose winner is still not clean.
State-by-state Secchi champions: the clearest measured lake in each U.S. state we grade, which is not the same as the cleanest nutrient grade.
A data snapshot of the lakes we grade: how the A–F distribution breaks down, the clearest and the greenest states, and where the water is improving.
How lakes are graded A–F on nutrients and algae using EPA ecoregion condition classes, what each letter means in the water, and why an A is not a clearance.