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LakeQuality
Late afternoon sun over Slim Lake, scattering a bright path across wind-textured water
D

Fremont County Lake Quality

Idaho, 18 lakes, average grade D (Nutrient-rich)

There are 18 monitored lakes in Fremont County. The mix is varied enough that the county-level average tells you the broad strokes, not the choose-this-cabin specifics. 2 of those lakes carry an A–F grade. The county-level summary describes that subset, not every water body listed. On average, Fremont County lakes grade D, with phosphorus loading and reduced clarity showing up across most of the dataset.

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 Island Park Lake (D); the most-stressed is Island Park Lake (D).

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All Lakes in Fremont County

18 lakes · 2 graded, 16 not rated2 graded lakes · 16 not rated, hidden

#LakeGradeClarityPhosphorusAlgaeTrophicAccess
1DIsland Park LakeFremontD9.8 ft24 µg/L11.8 µg/LNo recent samples✓ Public
2DIsland Pk ReservoirFremontD2.3 ft56 µg/L9.8 µg/LNo recent samplesNot recorded
NRHenry'S Lake @ Boat RampFremontNRNo recent samples11 µg/L1.5 µg/LNo recent samples✓ Public
NRHenrys LakeFremontNRNo recent samplesNo recent samplesNo recent samplesNo recent samples✓ Public
NRAshton ReservoirFremontNRNo recent samplesNo recent samplesNo recent samplesNo recent samplesNot recorded
NRHorseshoe LakeFremontNRNo recent samplesNo recent samplesNo recent samplesNo recent samplesNot recorded
NRRobinson LakeFremontNRNo recent samplesNo recent samplesNo recent samplesNo recent samplesNot recorded
NRBeaver PondFremontNRNo recent samplesNo recent samplesNo recent samplesNo recent samplesNot recorded
NRFish PondFremontNRNo recent samplesNo recent samplesNo recent samplesNo recent samplesNot recorded
NRSwan LakeFremontNRNo recent samplesNo recent samplesNo recent samplesNo recent samplesNot recorded
NRSilver LakeFremontNRNo recent samplesNo recent samplesNo recent samplesNo recent samplesNot recorded
NRGolden LakeFremontNRNo recent samplesNo recent samplesNo recent samplesNo recent samplesNot recorded
NRSand Creek ReservoirFremontNRNo recent samplesNo recent samplesNo recent samplesNo recent samplesNot recorded
NRUpper Arcadia ReservoirFremontNRNo recent samplesNo recent samplesNo recent samplesNo recent samplesNot recorded
NRLower Arcadia ReservoirFremontNRNo recent samplesNo recent samplesNo recent samplesNo recent samplesNot recorded
NRLemon LakeFremontNRNo recent samplesNo recent samplesNo recent samplesNo recent samplesNot recorded
NREgin LakesFremontNRNo recent samplesNo recent samplesNo recent samplesNo recent samplesNot recorded
NRQuayles LakeFremontNRNo recent samplesNo recent samplesNo recent samplesNo recent samplesNot 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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See how Fremont 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.