One Horse Gap Lake
Pope County, Illinois29 acresView mapDirections
Illinois EPA publishes the water-quality measurements for One Horse Gap Lake. Here is what has been measured.
- Eutrophic
- Limited data
One Horse Gap Lake in Pope County, Illinois has water-quality measurements published by Illinois EPA, shown here as reported. Water clarity: 4 ft visibility. Phosphorus: 21 µg/L. Chlorophyll-a (algae): 14.7 µg/L. Surface area: 29 acres.
One Horse Gap Lake is a 29-acre productive, nutrient-rich eutrophic lake in Pope County, Illinois. 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. Methodology: how these readings are compared.
Swimming conditions
Readings onlyLimited data
Based on clarity and algae readings from past sampling — there is no live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
Nutrient and algae readings are not a swimming clearance. Check local advisories before you swim.
Whether One Horse Gap Lake is safe to swim in today is a call for the Illinois Environmental Protection Agency and the county health department, who post current advisories. LakeQuality does not issue swimming clearances.
Water quality measurements for One Horse Gap Lake
These are the measurements Illinois EPA publishes for One Horse Gap Lake, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Murky, only visible to about 4 ft. Phosphorus 21 µg/L. Chlorophyll-a 14.7 µg/L. TSI 54.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 4ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 21µg/L | Fair GoodFairPoor |
| Chlorophyll-a (algae) | 14.7µg/L | Poor GoodFairPoor |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 54 | Eutrophic OligotrophicMesotrophicEutrophicHypereutrophic |
High nutrients, frequent algae, reduced clarity
Ask about One Horse Gap Lake
Answers come straight from the public measurements and survey records on this page.
Water Temperature
One Horse Gap Lake typically sits around 65°F at the surface in October. 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.
Column height runs from this lake’s coldest month (56°F) to its warmest (79°F).
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
Surface area
29 acres
No bathymetric survey has been published for One Horse Gap Lake, so its depth is not documented here.
Stays via Stay22
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 readings cover nutrients and algae, not lodging. Cabins and rentals around One Horse Gap Lake are listed below.
Top-rated places to stay near One Horse Gap Lake
Prices for Oct 23–25, 4 adults. Change dates on the map below.
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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.
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. Its water-quality measurements are published by Illinois EPA and shown on this page as reported.
What is the water quality of One Horse Gap Lake?
- One Horse Gap Lake in Pope County, Illinois has water-quality measurements published by Illinois EPA, shown here as reported. 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?
- Nutrient and algae readings are 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 65°F at the surface in October, from 286 EPA Water Quality Portal field readings (2010–2020). That is a typical value for the month, not a live reading.
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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)
EPA condition classes from the National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See where the data comes from and data sources.
Other records on this page
Regulated discharger data from EPA ECHO (ICIS-NPDES)