Coulterville Lake
Randolph County, IllinoisView mapDirections
Nutrients and algae run well above the regional average.
- Hypereutrophic
- Impaired · Nutrients (phosphorus, nitrogen)
- #3 of 3 in the county
- Limited data
Coulterville Lake in Randolph County, Illinois has a water quality grade of F (Algae-prone). Water clarity: 2.7 ft visibility. Phosphorus: 452.5 µg/L. Chlorophyll-a (algae): 51 µg/L.
- Clarity
- 2.7 ft
- Typical in August
- 86°F
- Type
- Hypereutrophic
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Stays near Coulterville Lake
Compare lakeside cabins, hotels, and vacation rentals near Coulterville Lake in Randolph 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 Coulterville Lake are listed below.
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Watch Coulterville Lake’s letter
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Coulterville Lake is a highly nutrient-loaded hypereutrophic lake in Randolph County, Illinois. Water quality is graded F based on 2 sampling years through 2022. Compared to the 2 other monitored lakes in Randolph County, Coulterville Lake ranks #3 for water quality.
Source: EPA Water Quality Portal sampling records, Illinois EPA, last sampled 2022-10-18. 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. Coulterville Lake is listed impaired under Clean Water Act §303(d) (IR 4A) for nutrients (phosphorus, nitrogen). Check local advisories before you swim.
Water Quality Grade: F (Algae-prone)
Very murky, less than 2.7 ft of visibility. Phosphorus 452.5 µg/L. Chlorophyll-a 51 µg/L. TSI 75. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 2.7 ft | Not graded |
| Phosphorus | 452.5 µg/L | Poor |
| Chlorophyll-a (Algae) | 51 µg/L | Poor |
| Trophic State Index (TSI) | 75 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Water Temperature
Coulterville Lake typically sits around 86°F at the surface in August. Summer average is about 84°F. Usually warmest in Jul (about 86°F) and coldest in Oct (about 60°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 260 field readings (2012–2022) via the EPA Water Quality Portal. See the full Coulterville Lake water temperature breakdown →
Fishing Coulterville Lake
Reservoir Info (USACE NID)
Coulterville Lake is a man-made reservoir impounded by the Lake Coulterville Dam (completed 1917), built primarily for recreation on the TRIB SOUTH FORK MUD CREEK; earth-type dam, 18 ft tall and 460 ft long.
- Normal storage
- 109 ac-ft
- Max storage
- 181 ac-ft
- Hazard class
- Low
- Owner
- LINCOLN TRAIL BANK & TRUST COMPANY
Source: USACE National Inventory of Dams, NID ID IL00561 · Operator website
EPA Impairment Status
Coulterville 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_ROV · Official waterbody report
Regulated Pollution Sources Nearby
3 EPA-permitted facilities operate within about 5 miles of Coulterville Lake under Clean Water Act (NPDES) discharge permits. 3 facilities have a reported Clean Water Act violation in the past three years.
- Peabody Gateway North Mining LlcCoulterville · 1.2 mi away · NPDES IL0079481Recent violation
- Peabody Coulterville Mining LlcCoulterville · 2.9 mi away · NPDES IL0062189Recent violation
- Prairie State Generating Company LlcMarissa · 4.7 mi away · NPDES IL0077526Recent 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 #3 of 3 lakes in Randolph County
Nearby Lakes With Higher Grades
Coulterville Lake holds Grade F. One nearby lake has a higher nutrient-and-algae grade — useful for a swim day, not a reason to skip lodging here.
Nearby Lakes in Randolph County
Frequently asked questions about Coulterville Lake
Where is Coulterville Lake?
- Coulterville Lake is a lake in Randolph County, Illinois. LakeQuality grades it F for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Coulterville Lake?
- Coulterville Lake in Randolph County, Illinois has a water quality grade of F (Algae-prone). Water clarity: 2.7 ft visibility. Phosphorus: 452.5 µg/L. Chlorophyll-a (algae): 51 µg/L.
Is Coulterville Lake safe to swim in?
- The F is nutrients and algae, not a swimming clearance. Coulterville Lake is listed impaired under Clean Water Act §303(d) (IR 4A) for nutrients (phosphorus, nitrogen). Check local advisories before you swim.
What is the water temperature of Coulterville Lake?
- Coulterville Lake typically sits around 86°F at the surface in August, from 260 EPA Water Quality Portal field readings (2012–2022). That is a typical value for the month, not a live reading.
Explore Coulterville 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
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Other Grade F Lakes in Illinois
Hypereutrophic Lakes in Illinois
Where this data comes from
49 measurements from 3 monitoring stations, 2022. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- illinois epa
IL_EPA_WQX49 samples · 2022
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)