Island Park Reservoir Report

Last Updated: December 5, 2025

Island Park, located in Idaho along the Henrys Fork River, is a federal-owned irrigation dam managed by the Bureau of Reclamation.


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Summary

Built in 1937, this earth dam stands at a height of 93.4 feet and has a storage capacity of 169,646 acre-feet, making it crucial for agricultural irrigation in the region. The dam, with a spillway width of 257 feet, also serves as a high-hazard potential structure due to its significant hydraulic height of 73 feet.

Over the years, Island Park has undergone multiple modifications to enhance its structural integrity and seismic resilience. The dam has been inspected regularly by the Bureau of Reclamation, with the last inspection conducted in July 2020. Despite its high hazard potential, the dam is rated to have a moderate risk level (3), indicating the need for ongoing risk management measures to ensure the safety of surrounding communities in Saint Anthony, Idaho. With its strategic location and critical role in water resource management, Island Park remains a vital infrastructure for sustaining agricultural activities and safeguarding against potential flood risks in the region.

Years Modified

1980 - Structural, 1984 - Hydraulic, 1984 - Other, 1984 - Seismic, 1984 - Structural

Year Completed

1937

Dam Length

9448

Dam Height

93.4

River Or Stream

HENRYS FORK

Primary Dam Type

Earth

Surface Area

8680

Hydraulic Height

73

Drainage Area

481

Nid Storage

169646

Structural Height

93

Hazard Potential

High

Foundations

Rock

Nid Height

93
       
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.