One Horse Gap Lake
Pope County, Illinois29 acresView mapDirections
Nutrients and algae run above the regional average.
↓ down from B at the last regrade
- Eutrophic
- #1 of 2 in the county
- Limited data
One Horse Gap Lake in Pope County, Illinois has a water quality grade of D (Nutrient-rich). Water clarity: 4 ft visibility. Phosphorus: 21 µg/L. Chlorophyll-a (algae): 14.7 µg/L. Surface area: 29 acres.
- Clarity
- 4 ft
- Typical in August
- 79°F
- Area
- 29 acres
- Type
- Eutrophic
- Phosphorus
- 21 µg/L
- Chlorophyll-a
- 14.7 µg/L
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Stays near One Horse Gap Lake
Compare lakeside cabins, hotels, and vacation rentals near One Horse Gap Lake in Pope 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 One Horse Gap Lake are listed below.
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One Horse Gap Lake is a 29-acre productive, nutrient-rich eutrophic lake in Pope County, Illinois. Water quality is graded D 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 EPA, last sampled 2020-11-09. 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 D is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
Water Quality Grade: D (Nutrient-rich)
Murky, only visible to about 4 ft. Phosphorus 21 µg/L. Chlorophyll-a 14.7 µg/L. TSI 54. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 4 ft | Not graded |
| Phosphorus | 21 µg/L | Fair |
| Chlorophyll-a (Algae) | 14.7 µg/L | Poor |
| Trophic State Index (TSI) | 54 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Water Temperature
One Horse Gap Lake typically sits around 79°F at the surface in August. Summer average is about 78°F. 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 →
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 29 acres |
No bathymetric survey has been published for One Horse Gap Lake, so its depth is not documented here.
Fishing One Horse Gap Lake
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.
County Ranking
Ranked #1 of 2 lakes in Pope County
Nearby Lakes in Pope County
Frequently asked questions about One Horse Gap Lake
Where is One Horse Gap Lake?
- One Horse Gap Lake is a lake in Pope County, Illinois. LakeQuality grades it D for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for One Horse Gap Lake?
- One Horse Gap Lake in Pope County, Illinois has a water quality grade of D (Nutrient-rich). Water clarity: 4 ft visibility. Phosphorus: 21 µg/L. Chlorophyll-a (algae): 14.7 µg/L. Surface area: 29 acres.
Is One Horse Gap Lake safe to swim in?
- The D is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
What is the water temperature of One Horse Gap Lake?
- One Horse Gap Lake typically sits around 79°F at the surface in August, from 286 EPA Water Quality Portal field readings (2010–2020). That is a typical value for the month, not a live reading.
Explore One Horse Gap Lake across LakeQuality
11.2 mi away · 100 ft
Species activity outlook by water temperature and season
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High nutrients, frequent algae, reduced clarity
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Where this data comes from
52 measurements from 3 monitoring stations, 2020. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- illinois epa
IL_EPA_WQX52 samples · 2020
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
Regulated discharger data from EPA ECHO (ICIS-NPDES)