One Horse Gap Reservoir Report

Last Updated: December 5, 2025

One Horse Gap, located in Pope County, Illinois, is a federal-owned earth dam completed in 1973 for the primary purpose of recreation.


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Summary

Standing at a height of 30 feet with a hydraulic height of 23 feet, the dam boasts a length of 443 feet and a storage capacity of 304 acre-feet. Managed by the USDA Forest Service, the dam is regulated, inspected, and enforced by the agency to ensure its safety and functionality.

With a significant hazard potential and a fair condition assessment as of August 2019, One Horse Gap poses moderate risk according to a risk assessment rating of 3. The spillway type is uncontrolled, with a width of 20 feet, and the dam is situated on the TR-GRAND PIERRE river. Despite its age, the dam remains a key feature for recreation in the area, offering opportunities for water-based activities and scenic views for water resource and climate enthusiasts to enjoy.

Overall, One Horse Gap serves as an important structure within the federal-owned land, providing both recreational opportunities and water management benefits. As a well-maintained and regulated earth dam, it stands as a testament to the ongoing efforts of the USDA Forest Service to ensure the safety and sustainability of water resources in Illinois.

Year Completed

1973

Dam Length

443

Dam Height

30

River Or Stream

TR-GRAND PIERRE

Primary Dam Type

Earth

Surface Area

29

Hydraulic Height

23

Nid Storage

304

Structural Height

36

Hazard Potential

Significant

Nid Height

36
       
Seasonal Comparison

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Dam Data Reference

Condition Assessment

Satisfactory
No existing or potential dam safety deficiencies are recognized. Acceptable performance is expected under all loading conditions (static, hydrologic, seismic) in accordance with the minimum applicable state or federal regulatory criteria or tolerable risk guidelines.
Fair
No existing dam safety deficiencies are recognized for normal operating conditions. Rare or extreme hydrologic and/or seismic events may result in a dam safety deficiency. Risk may be in the range to take further action. Note: Rare or extreme event is defined by the regulatory agency based on their minimum
Poor A dam safety deficiency is recognized for normal operating conditions which may realistically occur. Remedial action is necessary. POOR may also be used when uncertainties exist as to critical analysis parameters which identify a potential dam safety deficiency. Investigations and studies are necessary.
Unsatisfactory
A dam safety deficiency is recognized that requires immediate or emergency remedial action for problem resolution.
Not Rated
The dam has not been inspected, is not under state or federal jurisdiction, or has been inspected but, for whatever reason, has not been rated.
Not Available
Dams for which the condition assessment is restricted to approved government users.

Hazard Potential Classification

High
Dams assigned the high hazard potential classification are those where failure or mis-operation will probably cause loss of human life.
Significant
Dams assigned the significant hazard potential classification are those dams where failure or mis-operation results in no probable loss of human life but can cause economic loss, environment damage, disruption of lifeline facilities, or impact other concerns. Significant hazard potential classification dams are often located in predominantly rural or agricultural areas but could be in areas with population and significant infrastructure.
Low
Dams assigned the low hazard potential classification are those where failure or mis-operation results in no probable loss of human life and low economic and/or environmental losses. Losses are principally limited to the owner's property.
Undetermined
Dams for which a downstream hazard potential has not been designated or is not provided.
Not Available
Dams for which the downstream hazard potential is restricted to approved government users.