Turtlehead Lake
Cook County, IllinoisView mapDirections
Nutrients and algae run above the regional average.
- Eutrophic
- Impaired · Cause unknown
- #7 of 16 in the county
- Limited data
Turtlehead Lake in Cook County, Illinois has a water quality grade of D (Nutrient-rich). Water clarity: 2.8 ft visibility. Phosphorus: 54 µg/L. Chlorophyll-a (algae): 24.4 µg/L.
- Clarity
- 2.8 ft
- Typical in September
- 74°F
- Type
- Eutrophic
- Phosphorus
- 54 µg/L
- Chlorophyll-a
- 24.4 µg/L
- Trend
- ↑ Improving
Stays via Stay22
Stays near Turtlehead Lake
Compare lakeside cabins, hotels, and vacation rentals near Turtlehead 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. The water-quality grade measures nutrients and algae, not lodging — cabins and rentals around Turtlehead Lake are listed below.
Top-rated places to stay near Turtlehead Lake
Prices for Sep 11–13, 4 adults. Change dates on the map below.
Availability and prices from Stay22 partners (Booking.com, Vrbo, Expedia). We may earn a commission from bookings, at no extra cost to you.
Planning a whole trip? See where to stay at Illinois’s top lakes →
Watch Turtlehead Lake’s letter
Email me if this lake’s grade moves. Grades stay free.
Or email
Turtlehead Lake is a productive, nutrient-rich eutrophic lake in Cook County, Illinois. Water quality is graded D based on 2 sampling years through 2023. Overall water quality is improving over recent sampling years. Compared to the 15 other monitored lakes in Cook County, Turtlehead Lake ranks #7 for water quality.
Source: EPA Water Quality Portal sampling records, Illinois EPA, last sampled 2023-04-18. Grade methodology: LakeQuality methodology.
Swimming Safety
Grade onlyLimited data
Based on clarity and algae grades from past sampling — we do not hold live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
The D is nutrients and algae, not a swimming clearance. Turtlehead Lake is listed impaired under Clean Water Act §303(d) (IR 5) for cause unknown, mercury. Check local advisories before you swim.
Water Quality Grade: D (Nutrient-rich)
Very murky, less than 2.8 ft of visibility. Phosphorus 54 µg/L. Chlorophyll-a 24.4 µg/L. TSI 62. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 2.8 ft | Not graded |
| Phosphorus | 54 µg/L | Fair |
| Chlorophyll-a (Algae) | 24.4 µg/L | Poor |
| Trophic State Index (TSI) | 62 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Water Temperature
Turtlehead Lake typically sits around 74°F at the surface in September. Summer average is about 74°F. Usually warmest in Aug (about 76°F) and coldest in May (about 61°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 103 field readings (2010–2023) via the EPA Water Quality Portal. See the full Turtlehead Lake water temperature breakdown →
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 |
| Chlorophyll-a | ↑ Improving | -13.2 µg/L/yr | 2 |
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 (1.34 km)
County Ranking
Ranked #7 of 16 lakes in Cook County
Nearby Lakes in Cook County
Frequently asked questions about Turtlehead Lake
Where is Turtlehead Lake?
- Turtlehead Lake is a lake in Cook County, Illinois. LakeQuality grades it D for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Turtlehead Lake?
- Turtlehead Lake in Cook County, Illinois has a water quality grade of D (Nutrient-rich). Water clarity: 2.8 ft visibility. Phosphorus: 54 µg/L. Chlorophyll-a (algae): 24.4 µg/L.
Is Turtlehead Lake safe to swim in?
- The D is nutrients and algae, not a swimming clearance. Turtlehead Lake is listed impaired under Clean Water Act §303(d) (IR 5) for cause unknown, mercury. Check local advisories before you swim.
What is the water temperature of Turtlehead Lake?
- Turtlehead Lake typically sits around 74°F at the surface in September, from 103 EPA Water Quality Portal field readings (2010–2023). That is a typical value for the month, not a live reading.
Explore Turtlehead Lake across LakeQuality
More Illinois Lake Rankings
Other Grade D Lakes in Illinois
Eutrophic Lakes in Illinois
Where this data comes from
37 measurements from 3 monitoring stations, 2021–2023. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- illinois epa
IL_EPA_WQX36 samples · 2021 - USGS Illinois Water Science Center
USGS-IL1 sample · 2023
Analytical methods
- Secchi disk method
- MEASURED IN FIELD
- Phosphorus by Colorimetry
- 10200 H 2 ~ Chlorophyll a-b-c Determination by spectrophotometer
Monitoring station identifiers (3)
Grade thresholds from the EPA National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See how grades are calculated and data sources.
Other records on this page
Impairment status from EPA ATTAINS 303(d) database