Turtlehead Lake is a productive, nutrient-rich eutrophic lake in Cook County, Illinois. Water quality is graded C based on 2 sampling years through 2021. Overall water quality is improving over recent sampling years. Compared to the 67 other monitored lakes in Cook County, Turtlehead Lake ranks #13 for water quality.
Source: EPA Water Quality Portal sampling records, Illinois DNR, last sampled 2021-09-21. Grade methodology: LakeGrade methodology.
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Swimming Safety
Generally safe for swimming, moderate water quality
Water Quality Grade: C, Fair
Very murky, less than 2.8 ft of visibility. Phosphorus level: 20.5 µg/L. Trophic State Index: 55.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 2.8 ft | F |
| Phosphorus | 20.5 µg/L | B |
| Chlorophyll-a (Algae) | No data | |
| Trophic State Index (TSI) | 55 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Where to stay near Turtlehead Lake
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Top-rated places to stay near Turtlehead Lake
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Fishing Turtlehead Lake
Water Quality Trend: ↑ Improving
Based on 2 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +9 m/yr | 2 |
| Phosphorus | ↑ Improving | -11 µg/L/yr | 2 |
Water Temperature
Turtlehead Lake averages about 80°F at the surface in summer, usually warmest in Jul (about 83°F) and coldest in May (about 64°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 51 field readings (2010–2021) via the EPA Water Quality Portal.
Location
County Ranking
Ranked #13 of 68 lakes in Cook County
Nearby Lakes in Cook County
Explore Turtlehead Lake across LakeGrade
More Illinois Lake Rankings
Other Grade C Lakes in Illinois
Eutrophic Lakes in Illinois
More records, surveys & data sources
EPA Impairment Status
Turtlehead 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_RHZE · Official waterbody report · Matched by proximity (1.32 km)
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: 2021-09-21
Monitoring stations: 2