Grays Lake
Lake County, IllinoisView mapDirections
Clarity, phosphorus, and algae all sit in the cleanest band for this region.
↑ up from B at the last regrade
- Mesotrophic
- Impaired · Noxious aquatic plants
- #1 of 19 in the county
- 48 ft max depth
- Limited data
Grays Lake in Lake County, Illinois has a water quality grade of A (Excellent). Water clarity: 3.5 ft visibility. Phosphorus: 23 µg/L. Chlorophyll-a (algae): 3.1 µg/L.
- Clarity
- 3.5 ft
- Typical in October
- 67°F
- Max depth
- ~48 ft est.
- Type
- Mesotrophic
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Watch Grays Lake’s letter
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Grays Lake is a mesotrophic lake in Lake County, Illinois with an estimated average depth of about 48 feet. Water quality is graded A based on 2 sampling years through 2021. Compared to the 18 other monitored lakes in Lake County, Grays Lake ranks #1 for water quality.
Source: EPA Water Quality Portal sampling records, Illinois EPA, last sampled 2021-10-12. 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 A is nutrients and algae, not a swimming clearance. Grays Lake is listed impaired under Clean Water Act §303(d) (IR 5) for noxious aquatic plants, nutrients (phosphorus, nitrogen). Check local advisories before you swim.
Water Quality Grade: A (Excellent)
Murky, only visible to about 3.5 ft. Phosphorus 23 µg/L. Chlorophyll-a 3.1 µg/L. TSI 50. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 3.5 ft | Not graded |
| Phosphorus | 23 µg/L | Good |
| Chlorophyll-a (Algae) | 3.1 µg/L | Good |
| Trophic State Index (TSI) | 50 | Mesotrophic |
Moderate nutrients, good water quality
Water Temperature
Grays Lake typically sits around 67°F at the surface in October. Summer average is about 73°F. Usually warmest in Jul (about 80°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 54 field readings (2018–2021) via the EPA Water Quality Portal. See the full Grays Lake water temperature breakdown →
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | ~48 ft est. |
| Average Depth | ~11 ft est. |
Depth marked “est.” is a modeled maximum from GLOBathy (Khazaei et al. 2022, CC0), not a measured survey. Illinois has not published bathymetry for Grays Lake.
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.
Fishing Grays Lake
EPA Impairment Status
Grays 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_UTL · Official waterbody report · Matched by proximity (2.22 km)
Regulated Pollution Sources Nearby
1 EPA-permitted facility operate within about 5 miles of Grays Lake under Clean Water Act (NPDES) discharge permits. 1 facility has a reported Clean Water Act violation in the past three years.
- Countryside LandfillGrayslake · 1.1 mi away · NPDES IL0075558Recent violation
Facilities permitted to discharge under the Clean Water Act within ~5 miles; proximity does not by itself mean a facility affects this lake. Source: EPA ECHO (ICIS-NPDES), as of 2026-07.
County Ranking
Ranked #1 of 19 lakes in Lake County
Nearby Lakes in Lake County
Frequently asked questions about Grays Lake
Where is Grays Lake?
- Grays Lake is a lake in Lake County, Illinois. LakeQuality grades it A for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Grays Lake?
- Grays Lake in Lake County, Illinois has a water quality grade of A (Excellent). Water clarity: 3.5 ft visibility. Phosphorus: 23 µg/L. Chlorophyll-a (algae): 3.1 µg/L.
Is Grays Lake safe to swim in?
- The A is nutrients and algae, not a swimming clearance. Grays Lake is listed impaired under Clean Water Act §303(d) (IR 5) for noxious aquatic plants, nutrients (phosphorus, nitrogen). Check local advisories before you swim.
What is the water temperature of Grays Lake?
- Grays Lake typically sits around 67°F at the surface in October, from 54 EPA Water Quality Portal field readings (2018–2021). That is a typical value for the month, not a live reading.
How deep is Grays Lake?
- No published bathymetric survey covers Grays Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 48 feet, but that is a modelled estimate derived from surrounding terrain rather than a measurement on the water, so treat it as approximate.
Explore Grays Lake across LakeQuality
Species activity outlook by water temperature and season
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Moderate nutrients, good water quality
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Where this data comes from
50 measurements from 3 monitoring stations, 2021. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- illinois epa
IL_EPA_WQX50 samples · 2021
Analytical methods
- 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
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Lake size & watershed from HydroLAKES (HydroSHEDS)