McHenry County, Illinois · Water Quality Report Card
Island Lake
Island Lake in McHenry County, Illinois has a water quality grade of F (Very Poor). Chlorophyll-a (algae): 136.5 µg/L.
- Mean depth
- ~8 ft est.
- Type
- Hypereutrophic
Island Lake is a highly nutrient-loaded hypereutrophic lake in McHenry County, Illinois with an estimated average depth of about 8 feet. Water quality is graded F based on 1 sampling years through 2021. Compared to the 4 other monitored lakes in McHenry County, Island Lake ranks #5 for water quality.
Source: EPA Water Quality Portal sampling records, Illinois EPA, last sampled 2021-08-10. Grade methodology: LakeGrade methodology.
Watch Island Lake water quality
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Swimming Safety
Avoid swimming, very poor water quality, potential algae toxins
Water Quality Grade: F (Very Poor)
No clarity data. Chlorophyll-a: 136.5 µg/L. Trophic State Index: 79.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No data | |
| Phosphorus | No data | |
| Chlorophyll-a (Algae) | 136.5 µg/L | Poor |
| Trophic State Index (TSI) | 79 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Lake Details
| Characteristic | Value |
|---|---|
| Average Depth | ~8 ft est. |
Depth marked “est.” is a modeled estimate from HydroLAKES, not a measured survey. Illinois has not published bathymetry for Island Lake.
Where to stay near Island Lake
Compare lakeside cabins, hotels, and vacation rentals near Island Lake in McHenry 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 Island Lake
Location
County Ranking
Ranked #5 of 5 lakes in McHenry County
Nearby Lakes in McHenry County
Explore Island Lake across LakeGrade
8.1 mi away · cable-pulled, no boat wake
Species activity outlook by water temperature and season
Top lakes and where to stay in the Chain O'Lakes area
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
More records, surveys & data sources
EPA Impairment Status
Island Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 5).
Causes of impairment
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit IL_RTZI · Official waterbody report
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth here is modelled, not surveyed — HydroLAKES v1.0 estimates lake depth from surrounding terrain rather than from soundings. Treat it as an approximation, and prefer a state bathymetric survey where one exists.
Where this data comes from
6 measurements from 1 monitoring station, 2021. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- illinois epa
IL_EPA_WQX6 samples · 2021
Analytical methods
- 10200 H 2 ~ Chlorophyll a-b-c Determination by spectrophotometer
Monitoring station identifiers (1)
Grade thresholds come from the U.S. EPA National Lakes Assessment 2022 Technical Support Document (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See how grades are calculated.
Data Sources
Water quality data from the EPA Water Quality Portal
Impairment status from EPA ATTAINS 303(d) database
Lake size & watershed from HydroLAKES (HydroSHEDS)
Grade thresholds from the EPA National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion
Most recent sample: 2021-08-10
Monitoring stations: 1