Lake Opeka
Cook County, IllinoisView mapDirections
Clarity, phosphorus, and algae all sit in the cleanest band for this region.
↑ up from D at the last regrade
- Mesotrophic
- Impaired · Cause unknown
- #1 of 16 in the county
- 49 ft max depth
- Limited data
Lake Opeka in Cook County, Illinois has a water quality grade of A (Excellent). Water clarity: 4 ft visibility. Phosphorus: 22.5 µg/L. Chlorophyll-a (algae): 5.3 µg/L.
- Clarity
- 4 ft
- Typical in September
- 72°F
- Max depth
- ~49 ft est.
- Type
- Mesotrophic
- Phosphorus
- 22.5 µg/L
- Chlorophyll-a
- 5.3 µg/L
- Trend
- ↓ Declining
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Watch Lake Opeka’s letter
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Lake Opeka is a mesotrophic lake in Cook County, Illinois with an estimated average depth of about 49 feet. Water quality is graded A based on 2 sampling years through 2022. Overall water quality is declining over recent sampling years. Compared to the 15 other monitored lakes in Cook County, Lake Opeka ranks #1 for water quality.
Source: EPA Water Quality Portal sampling records, Illinois EPA, last sampled 2022-09-22. 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. Lake Opeka is listed impaired under Clean Water Act §303(d) (IR 5) for cause unknown. Check local advisories before you swim.
Water Quality Grade: A (Excellent)
Murky, only visible to about 4 ft. Phosphorus 22.5 µg/L. Chlorophyll-a 5.3 µg/L. TSI 51. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 4 ft | Not graded |
| Phosphorus | 22.5 µg/L | Good |
| Chlorophyll-a (Algae) | 5.3 µg/L | Good |
| Trophic State Index (TSI) | 51 | Mesotrophic |
Moderate nutrients, good water quality
Water Temperature
Lake Opeka typically sits around 72°F at the surface in September. Summer average is about 77°F. Usually warmest in Jul (about 79°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 43 field readings (2020–2022) via the EPA Water Quality Portal. See the full Lake Opeka water temperature breakdown →
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | ~49 ft est. |
| Average Depth | ~7 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 Lake Opeka.
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 Lake Opeka
Water Quality Trend: ↓ Declining
Based on 2 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -21.5 m/yr | 2 |
EPA Impairment Status
Lake Opeka 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_RGF · Official waterbody report
Regulated Pollution Sources Nearby
12 EPA-permitted facilities operate within about 5 miles of Lake Opeka under Clean Water Act (NPDES) discharge permits, including 2 classified by the EPA as major dischargers. 7 facilities have a reported Clean Water Act violation in the past three years.
- Mwrdgc- James Kirie WrpDes Plaines · 2 mi away · NPDES IL0047741MajorRecent violation
- Bensenville - South Stp, Village OfBensenville · 4.8 mi away · NPDES IL0021849MajorRecent violation
- Blackhawk Estates Mobile Home Community StpChicago · 1.8 mi away · NPDES IL0049859Recent violation
- Des Plaines Mobile Home Park StpDes Plaines · 2.2 mi away · NPDES IL0054160Recent violation
- Aircraft Service International Inc. D/B/A Menzies AviationChicago · 2.3 mi away · NPDES IL0066567Recent violation
- Aircraft Service International IncChicago · 2.4 mi away · NPDES IL0080235Recent 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 16 lakes in Cook County
Nearby Lakes in Cook County
Frequently asked questions about Lake Opeka
Where is Lake Opeka?
- Lake Opeka is a lake in Cook County, Illinois. LakeQuality grades it A for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Lake Opeka?
- Lake Opeka in Cook County, Illinois has a water quality grade of A (Excellent). Water clarity: 4 ft visibility. Phosphorus: 22.5 µg/L. Chlorophyll-a (algae): 5.3 µg/L.
Is Lake Opeka safe to swim in?
- The A is nutrients and algae, not a swimming clearance. Lake Opeka is listed impaired under Clean Water Act §303(d) (IR 5) for cause unknown. Check local advisories before you swim.
What is the water temperature of Lake Opeka?
- Lake Opeka typically sits around 72°F at the surface in September, from 43 EPA Water Quality Portal field readings (2020–2022). That is a typical value for the month, not a live reading.
How deep is Lake Opeka?
- No published bathymetric survey covers Lake Opeka, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 49 feet, but that is a modelled estimate derived from surrounding terrain rather than a measurement on the water, so treat it as approximate.
Explore Lake Opeka across LakeQuality
More Illinois Lake Rankings
Other Grade A Lakes in Illinois
Mesotrophic Lakes in Illinois
Where this data comes from
53 measurements from 4 monitoring stations, 2020–2022. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- illinois epa
IL_EPA_WQX52 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 (4)
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)