One Horse Gap Lake is a 29-acre productive, nutrient-rich eutrophic lake in Pope County, Illinois. Water quality is graded B based on 2 sampling years through 2020. Compared to the 1 other monitored lakes in Pope County, One Horse Gap Lake ranks #1 for water quality.
Source: EPA Water Quality Portal sampling records, Illinois DNR, last sampled 2020-11-09. Grade methodology: LakeGrade methodology.
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Swimming Safety
Good for swimming, clear water with low algae levels
Water Quality Grade: B, Good
Murky, only visible to about 4 ft. Phosphorus level: 17 µg/L. Trophic State Index: 51.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 4 ft | D |
| Phosphorus | 17 µg/L | A |
| Chlorophyll-a (Algae) | No data | |
| Trophic State Index (TSI) | 51 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 29 acres |
Where to stay near One Horse Gap Lake
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Fishing One Horse Gap Lake
Water Temperature
One Horse Gap Lake averages about 78°F at the surface in summer, usually warmest in Aug (about 79°F) and coldest in Nov (about 56°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 286 field readings (2010–2020) via the EPA Water Quality Portal. See the full One Horse Gap Lake water temperature breakdown →
Location
County Ranking
Ranked #1 of 2 lakes in Pope County
Nearby Lakes in Pope County
Explore One Horse Gap Lake across LakeGrade
More Illinois Lake Rankings
Other Grade B Lakes in Illinois
Eutrophic Lakes in Illinois
More records, surveys & data sources
Reservoir Info (USACE NID)
One Horse Gap Lake is a man-made reservoir impounded by the One Horse Gap (completed 1973), built primarily for recreation on the TR-GRAND PIERRE; earth-type dam, 36 ft tall and 443 ft long.
- Surface area
- 29 ac
- Normal storage
- 193 ac-ft
- Max storage
- 304 ac-ft
- Drainage area
- 0 sq mi
- Hazard class
- Significant
- Owner
- USDA FS
Source: USACE National Inventory of Dams, NID ID IL50615 · Operator website
Regulated Pollution Sources Nearby
1 EPA-permitted facility operate within about 5 miles of One Horse Gap Lake under Clean Water Act (NPDES) discharge permits.
- Kuhlmann & Keene Fisheries LlcElizabethtown · 3.8 mi away · NPDES IL0080136
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
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Grading methodology based on Metropolitan Council standards
Most recent sample: 2020-11-09
Monitoring stations: 3