Horsethief Lake Reservoir Report

Last Updated: December 5, 2025

Horsethief Lake, located in Keystone, South Dakota, is a stunning water resource managed by the USDA Forest Service.


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Summary

Originally completed in 1940, this Earth dam stands at a height of 62 feet, with a hydraulic height of 49 feet, and a structural height of 65 feet. The reservoir has a normal storage capacity of 464 acre-feet and serves primarily for recreational purposes, including fishing and wildlife viewing.

With a drainage area of 4 square miles and a maximum discharge of 990 cubic feet per second, Horsethief Lake offers a serene setting for outdoor enthusiasts to enjoy the beauty of nature. Despite its high hazard potential, the dam's condition assessment is currently not rated, and it last underwent inspection in September 2019. The reservoir's spillway is uncontrolled, and it has a mechanical modification dating back to 2005.

For those interested in exploring water resources and climate in South Dakota, Horsethief Lake is a captivating destination that offers both recreational opportunities and a glimpse into dam engineering and management. In the heart of Pennington County, this reservoir on Pine Creek provides a tranquil escape surrounded by lush landscapes. As stewarded by the Forest Service, this Earth dam stands as a testament to the harmonious coexistence of human activity and natural beauty in the region.

Years Modified

2005 - Mechanical

Year Completed

1940

Dam Length

325

Dam Height

62

River Or Stream

PINE CREEK

Primary Dam Type

Earth

Hydraulic Height

49

Drainage Area

4

Nid Storage

590

Structural Height

65

Hazard Potential

High

Nid Height

65
       
Seasonal Comparison

Weather Forecast

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.