Lake County, Illinois · Water Quality Report Card
Diamond Lake
Diamond Lake in Lake County, Illinois has a water quality grade of F (Very Poor). Chlorophyll-a (algae): 63.6 µg/L.
- Mean depth
- ~12 ft est.
- Type
- Hypereutrophic
Diamond Lake is a highly nutrient-loaded hypereutrophic lake in Lake County, Illinois with an estimated average depth of about 12 feet. Water quality is graded F based on 1 sampling years through 2020. Compared to the 18 other monitored lakes in Lake County, Diamond Lake ranks #16 for water quality.
Source: EPA Water Quality Portal sampling records, Illinois EPA, last sampled 2020-09-23. Grade methodology: LakeGrade methodology.
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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: 63.6 µg/L. Trophic State Index: 71.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No data | |
| Phosphorus | No data | |
| Chlorophyll-a (Algae) | 63.6 µg/L | Poor |
| Trophic State Index (TSI) | 71 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Lake Details
| Characteristic | Value |
|---|---|
| Average Depth | ~12 ft est. |
Depth marked “est.” is a modeled estimate from HydroLAKES, not a measured survey. Illinois has not published bathymetry for Diamond Lake.
Where to stay near Diamond Lake
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Fishing Diamond Lake
Location
County Ranking
Ranked #16 of 19 lakes in Lake County
Nearby Lakes in Lake County
Explore Diamond Lake across LakeGrade
Species activity outlook by water temperature and season
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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
Diamond Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 4A).
Causes of impairment
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit IL_RGB · 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
10 measurements from 1 monitoring station, 2020. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- illinois epa
IL_EPA_WQX10 samples · 2020
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: 2020-09-23
Monitoring stations: 1