Sibley Reservoir Report

Last Updated: December 5, 2025

Sibley is a captivating water resource nestled in the heart of Wyoming, specifically in Sheridan County.


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Summary

Managed by the Forest Service, this federal-owned dam on Prune Creek serves primarily for recreational purposes, including fish and wildlife pond activities. Built-in 1938, the Earth-type dam stands at 30 feet tall and stretches 420 feet in length, holding a storage capacity of 635 acre-feet and covering a surface area of 34 acres. With a high hazard potential and a moderate risk assessment rating, Sibley plays a crucial role in maintaining the ecological balance and water supply for the region.

Despite its age, Sibley remains under the vigilant oversight of the Forest Service, with regular inspections and state-regulated maintenance in place. The dam boasts a maximum discharge capacity of 1360 cubic feet per second and an uncontrolled spillway width of 22 feet. While its condition assessment is labeled as "Not Rated," the dam's structural integrity and operational efficiency are upheld through continuous monitoring and risk management measures. In the event of an emergency, the dam is equipped with emergency action plans, ensuring the safety of downstream communities and enhancing overall disaster preparedness.

Located in the scenic city of Dayton, Sibley stands as a testament to sustainable water resource management and environmental stewardship. With its picturesque surroundings and vital role in supporting local wildlife habitats, the dam serves as a key hub for outdoor recreation and nature appreciation. As climate change continues to impact water resources, Sibley remains a resilient structure, safeguarding against potential hazards and contributing to the ecological resilience of the region.

Year Completed

1938

Dam Length

420

Dam Height

30

River Or Stream

PRUNE CREEK

Primary Dam Type

Earth

Surface Area

34

Hydraulic Height

22

Drainage Area

5

Nid Storage

635

Structural Height

35

Outlet Gates

Uncontrolled

Hazard Potential

High

Nid Height

35
       
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.