Horsetail Lake
Cook County, IllinoisView mapDirections
Nutrients and algae run well above the regional average.
↓ down from D at the last regrade
- Hypereutrophic
- #10 of 16 in the county
- Limited data
Horsetail Lake in Cook County, Illinois has a water quality grade of F (Algae-prone). Water clarity: 2.2 ft visibility. Phosphorus: 45 µg/L. Chlorophyll-a (algae): 39.6 µg/L.
- Clarity
- 2.2 ft
- Typical in September
- 68°F
- Type
- Hypereutrophic
Stays via Stay22
Stays near Horsetail Lake
Compare lakeside cabins, hotels, and vacation rentals near Horsetail 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 Horsetail Lake are listed below.
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Horsetail Lake is a highly nutrient-loaded hypereutrophic lake in Cook County, Illinois. Water quality is graded F based on 2 sampling years through 2022. Compared to the 15 other monitored lakes in Cook County, Horsetail Lake ranks #10 for water quality.
Source: EPA Water Quality Portal sampling records, Illinois EPA, last sampled 2022-03-24. 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 F is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
Water Quality Grade: F (Algae-prone)
Very murky, less than 2.2 ft of visibility. Phosphorus 45 µg/L. Chlorophyll-a 39.6 µg/L. TSI 64. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 2.2 ft | Not graded |
| Phosphorus | 45 µg/L | Fair |
| Chlorophyll-a (Algae) | 39.6 µg/L | Poor |
| Trophic State Index (TSI) | 64 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Water Temperature
Horsetail Lake typically sits around 68°F at the surface in September. Summer average is about 81°F. Usually warmest in Jul (about 84°F) and coldest in Oct (about 58°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 42 field readings (2010–2022) via the EPA Water Quality Portal. See the full Horsetail Lake water temperature breakdown →
Fishing Horsetail Lake
County Ranking
Ranked #10 of 16 lakes in Cook County
Nearby Lakes in Cook County
Frequently asked questions about Horsetail Lake
Where is Horsetail Lake?
- Horsetail Lake is a lake in Cook County, Illinois. LakeQuality grades it F for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Horsetail Lake?
- Horsetail Lake in Cook County, Illinois has a water quality grade of F (Algae-prone). Water clarity: 2.2 ft visibility. Phosphorus: 45 µg/L. Chlorophyll-a (algae): 39.6 µg/L.
Is Horsetail Lake safe to swim in?
- The F is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
What is the water temperature of Horsetail Lake?
- Horsetail Lake typically sits around 68°F at the surface in September, from 42 EPA Water Quality Portal field readings (2010–2022). That is a typical value for the month, not a live reading.
Explore Horsetail Lake across LakeQuality
Species activity outlook by water temperature and season
Where to stay at Illinois's top lakes, each with its grade
Very high nutrients, dense algae, poor clarity
Illinois's cleanest and most-polluted lakes, ranked
More Illinois Lake Rankings
Other Grade F Lakes in Illinois
Hypereutrophic Lakes in Illinois
Where this data comes from
37 measurements from 3 monitoring stations, 2020–2022. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- illinois epa
IL_EPA_WQX36 samples · 2020 - USGS Illinois Water Science Center
USGS-IL1 sample · 2022
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.