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

Montgomery County Lake Quality

Illinois, 8 lakes, average grade F (Algae-prone)

Montgomery County holds 8 monitored lakes. With a sample that small the county-level summary describes those specific waters, not necessarily every water body in the county. 2 of those lakes carry an A–F grade. The county-level summary describes that subset, not every water body listed. The county average is F — the most-stressed bracket in our rubric. Most monitored lakes here show persistent nutrient loading and algal pressure.

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 Glenn Shoals Lake (F); the most-stressed is Glenn Shoals Lake (F).

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All Lakes in Montgomery County

8 lakes · 2 graded, 6 not rated2 graded lakes · 6 not rated, hidden

#LakeGradeClarityPhosphorusAlgaeTrophicAccess
1FGlenn Shoals LakeMontgomeryF1 ft122 µg/L72.1 µg/LNo recent samples✓ Public
2FLou Yaeger LakeMontgomeryF1.8 ft255 µg/L72.6 µg/LNo recent samples✓ Public
NRCoffeen LakeMontgomeryNRNo recent samplesNo recent samplesNo recent samplesNo recent samples✓ Public
NRLake HillsboroMontgomeryNRNo recent samplesNo recent samplesNo recent samplesNo recent samples✓ Public
NRLake Glenn ShoalsMontgomeryNRNo recent samplesNo recent samplesNo recent samplesNo recent samples✓ Public
NRFivemile LakeMontgomeryNRNo recent samplesNo recent samplesNo recent samplesNo recent samplesNot recorded
NRLake Lou YaegerMontgomeryNRNo recent samplesNo recent samplesNo recent samplesNo recent samplesNot recorded
NRWalton LakeMontgomeryNRNo 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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Source: EPA National Aquatic Resource Surveys, 2026.