Glenn Shoals Lake is a highly nutrient-loaded hypereutrophic lake in Montgomery County, Illinois. Water quality is graded F based on 2 sampling years through 2022. Limited fish-species data available. Public access is available with 3 public boat landings. Compared to the 1 other monitored lakes in Montgomery County, Glenn Shoals Lake ranks #1 for water quality.
Source: EPA Water Quality Portal sampling records, Illinois DNR, last sampled 2022-11-01. 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
Very murky, less than 1 ft of visibility. Phosphorus level: 91 µg/L. Trophic State Index: 73.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1 ft | F |
| Phosphorus | 91 µg/L | F |
| Chlorophyll-a (Algae) | No data | |
| Trophic State Index (TSI) | 73 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Where to stay near Glenn Shoals Lake
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Fishing Glenn Shoals Lake
Recreation & Access
Water Temperature
Glenn Shoals Lake averages about 80°F at the surface in summer, usually warmest in Jul (about 85°F) and coldest in Nov (about 57°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 77 field readings (2012–2022) via the EPA Water Quality Portal. See the full Glenn Shoals Lake water temperature breakdown →
Location
County Ranking
Ranked #1 of 2 lakes in Montgomery County
Cleaner Lakes Within 30 Miles
Glenn Shoals Lake holds Grade F. One nearby lake has a higher grade.
See full comparison →Nearby Lakes in Montgomery County
Explore Glenn Shoals Lake across LakeGrade
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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More records, surveys & data sources
Reservoir Info (USACE NID)
Glenn Shoals Lake is a man-made reservoir impounded by the Lake Glenn Shoals Dam (completed 1978), built primarily for flood risk reduction on the MIDDLE FORK OF SHOAL CREEK; earth-type dam, 56 ft tall and 1,430 ft long.
- Normal storage
- 13,000 ac-ft
- Max storage
- 38,900 ac-ft
- Hazard class
- Significant
- Owner
- CITY OF HILLSBORO
All listed purposes: Flood Risk Reduction;Water Supply.
Source: USACE National Inventory of Dams, NID ID IL00971 · Operator website
EPA Impairment Status
Glenn Shoals 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_ROL · Official waterbody report
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth is a modeled estimate.
Regulated Pollution Sources Nearby
4 EPA-permitted facilities operate within about 5 miles of Glenn Shoals Lake under Clean Water Act (NPDES) discharge permits, including 1 classified by the EPA as major discharger. 4 facilities have a reported Clean Water Act violation in the past three years.
- Hillsboro Stp, City OfHillsboro · 3.2 mi away · NPDES IL0029203MajorRecent violation
- Irving Stp, Village OfIrving · 3 mi away · NPDES IL0053881Recent violation
- Hillsboro Energy LlcHillsboro · 4.3 mi away · NPDES IL0080039Recent violation
- Hillsboro Energy LlcHillsboro · 4.4 mi away · NPDES IL0078727Recent 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.
Data Sources
Water quality data from the EPA Water Quality Portal
Impairment status from EPA ATTAINS 303(d) database
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Lake size & watershed from HydroLAKES (HydroSHEDS)
Grading methodology based on Metropolitan Council standards
Public boat-access points from the USGS National Boat Ramps inventory
Most recent sample: 2022-11-01
Monitoring stations: 3