Glenn Shoals Lake
Montgomery County, IllinoisView mapDirections
Nutrients and algae run well above the regional average.
- Hypereutrophic
- Impaired · Algal growth
- #1 of 2 in the county
- 63 ft max depth
- Public access
- Limited data
Glenn Shoals Lake in Montgomery County, Illinois has a water quality grade of F (Algae-prone). Water clarity: 1 ft visibility. Phosphorus: 122 µg/L. Chlorophyll-a (algae): 72.1 µg/L.
- Clarity
- 1 ft
- Typical in August
- 78°F
- Max depth
- ~63 ft est.
- Type
- Hypereutrophic
- Phosphorus
- 122 µg/L
- Chlorophyll-a
- 72.1 µg/L
- Public access
- Yes
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Glenn Shoals Lake is a highly nutrient-loaded hypereutrophic lake in Montgomery County, Illinois with an estimated average depth of about 63 feet. Water quality is graded F based on 2 sampling years through 2022. Limited fish-species data available. Public access is available with 1 public boat landing. 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 EPA, last sampled 2022-11-01. Grade methodology: LakeQuality methodology.
What Glenn Shoals Lake is good for
From access and monitoring records, not editorial picks.
- Boat launch1 documented ramp
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. Glenn Shoals Lake is listed impaired under Clean Water Act §303(d) (IR 5) for algal growth, mercury, nutrients (phosphorus, nitrogen), turbidity. Check local advisories before you swim.
Water Quality Grade: F (Algae-prone)
Very murky, less than 1 ft of visibility. Phosphorus 122 µg/L. Chlorophyll-a 72.1 µg/L. TSI 74. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1 ft | Not graded |
| Phosphorus | 122 µg/L | Poor |
| Chlorophyll-a (Algae) | 72.1 µg/L | Poor |
| Trophic State Index (TSI) | 74 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Water Temperature
Glenn Shoals Lake typically sits around 78°F at the surface in August. Summer average is about 80°F. 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 →
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | ~63 ft est. |
| Average Depth | ~39 ft est. |
| Public Access | Yes |
Depth marked “est.” is a modeled maximum from GLOBathy (Khazaei et al. 2022, CC0), not a measured survey. Illinois has not published bathymetry for Glenn Shoals 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.
Recreation & Access
- Glenn Shoals LakePublic
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Fishing Glenn Shoals Lake
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
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.
County Ranking
Ranked #1 of 2 lakes in Montgomery County
Nearby Lakes in Montgomery County
Frequently asked questions about Glenn Shoals Lake
Where is Glenn Shoals Lake?
- Glenn Shoals Lake is a lake in Montgomery County, Illinois. LakeQuality grades it F for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Glenn Shoals Lake?
- Glenn Shoals Lake in Montgomery County, Illinois has a water quality grade of F (Algae-prone). Water clarity: 1 ft visibility. Phosphorus: 122 µg/L. Chlorophyll-a (algae): 72.1 µg/L.
Is Glenn Shoals Lake safe to swim in?
- The F is nutrients and algae, not a swimming clearance. Glenn Shoals Lake is listed impaired under Clean Water Act §303(d) (IR 5) for algal growth, mercury, nutrients (phosphorus, nitrogen), turbidity. Check local advisories before you swim.
What is the water temperature of Glenn Shoals Lake?
- Glenn Shoals Lake typically sits around 78°F at the surface in August, from 77 EPA Water Quality Portal field readings (2012–2022). That is a typical value for the month, not a live reading.
How deep is Glenn Shoals Lake?
- No published bathymetric survey covers Glenn Shoals Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 63 feet, but that is a modelled estimate derived from surrounding terrain rather than a measurement on the water, so treat it as approximate.
Are there boat ramps on Glenn Shoals Lake?
- Glenn Shoals Lake has 1 documented boat landing in the USGS National Boat Ramps inventory. Named access points include Glenn Shoals Lake. Public access is listed as yes.
Explore Glenn Shoals 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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Where this data comes from
48 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_WQX48 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)
Lake size & watershed from HydroLAKES (HydroSHEDS)
Named boat-access points from the USGS National Boat Ramps inventory