Gem County Lake Quality
Idaho, 2 lakes, average grade A (Excellent)
Gem County holds 2 monitored lakes. With a sample that small the county-level summary describes those specific waters, not necessarily every water body in the county. 1 of those lake carry an A–F grade. The county-level summary describes that subset, not every water body listed. On average, Gem County lakes grade an A. That puts it near the top of the Idaho county ranking.
The county's lakes lean shallow, which makes the grades sensitive to wind, sediment-resuspension, and any nutrient inputs from the watershed. The cleanest lake on the books is Black Cnyn Reservoir Ns of (A); the most-stressed is Black Cnyn Reservoir Ns of (A).
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Quick Answers for Gem County
Planning a trip? Check special fishing regulations for Black Cnyn Reservoir Ns of — or check the official Idaho fishing regulations.
All Lakes in Gem County
2 lakes · 1 graded, 1 not rated1 graded lake · 1 not rated, hidden
| # | Lake | Grade | Clarity | Phosphorus | Algae | Trophic | Access |
|---|---|---|---|---|---|---|---|
| 1 | ABlack Cnyn Reservoir Ns ofGem | A | 7.5 ft | 29.5 µg/L | 3.1 µg/L | No recent samples | ✓ Public |
| NRBlack Canyon ReservoirGem | NR | No recent samples | No recent samples | No recent samples | No recent samples | Not recorded |
NR = not rated: there is not enough recent sampling to support a letter. That is EPA Integrated Reporting Category 3 (insufficient information), not a failing grade. How grading works.
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Other Counties in Idaho
See how Gem County stacks up in the national county water-quality rankings, or browse the full directory of graded lakes across all United States (49 states), Canada (Ontario, Québec, Nova Scotia, British Columbia and Newfoundland and Labrador), and 38 other country hubs..
Source: EPA National Aquatic Resource Surveys, 2026.