Flatfoot Lake is a mesotrophic lake in Cook County, Illinois. Water quality is graded B based on 2 sampling years through 2020. Overall water quality is declining over recent sampling years. Compared to the 67 other monitored lakes in Cook County, Flatfoot Lake ranks #7 for water quality.
Source: EPA Water Quality Portal sampling records, Illinois DNR, last sampled 2020-10-05. Grade methodology: LakeGrade methodology.
Watch Flatfoot Lake water quality
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Swimming Safety
Good for swimming, clear water with low algae levels
Water Quality Grade: B, Good
Murky, only visible to about 6.3 ft. Phosphorus level: 10.5 µg/L. Trophic State Index: 44.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 6.3 ft | D |
| Phosphorus | 10.5 µg/L | A |
| Chlorophyll-a (Algae) | No data | |
| Trophic State Index (TSI) | 44 | Mesotrophic |
Moderate nutrients, good water quality
Where to stay near Flatfoot Lake
Compare lakeside cabins, hotels, and vacation rentals near Flatfoot Lake in Cook County, Illinois — live availability and prices for your trip. Switch the map to Satellite to see the lake and how close each stay is to the water.
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Fishing Flatfoot Lake
Invasive & Introduced Species
Red chips are ecologically harmful invasive species. Gray chips are stocked or introduced fish that aren't a current ecological concern.
Source: USGS Nonindigenous Aquatic Species (NAS) Database. Hover a chip to see scientific name, ecological context, status, and first-observed year.
Water Quality Trend: ↓ Declining
Based on 2 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -35 m/yr | 2 |
Water Temperature
Flatfoot Lake averages about 76°F at the surface in summer, usually warmest in Jul (about 82°F) and coldest in Oct (about 60°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 48 field readings (2016–2020) via the EPA Water Quality Portal.
Location
County Ranking
Ranked #7 of 68 lakes in Cook County
Cleaner Lakes Within 30 Miles
Flatfoot Lake holds Grade B. One nearby lake has a higher grade.
See full comparison →Nearby Lakes in Cook County
Explore Flatfoot Lake across LakeGrade
More Illinois Lake Rankings
Other Grade B Lakes in Illinois
Mesotrophic Lakes in Illinois
More records, surveys & data sources
EPA Impairment Status
Flatfoot Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 5).
Causes of impairment
Source: EPA ATTAINS assessment unit IL_RHZJ · Official waterbody report
Data Sources
Water quality data from the EPA Water Quality Portal
Impairment status from EPA ATTAINS 303(d) database
Grading methodology based on Metropolitan Council standards
Most recent sample: 2020-10-05
Monitoring stations: 2