Stewart Gulch Main Fork Reservoir Report

Last Updated: December 14, 2025

Stewart Gulch Main Fork is a vital flood risk reduction structure located in Boise, Idaho.


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Summary

Completed in 1998 by Morrison Knudsen Corp, this earth dam stands at a height of 76.3 feet and has a hydraulic height of 73.4 feet, offering protection to the surrounding area from potential flooding events. With a storage capacity of 61 acre-feet and a maximum discharge of 168 cubic feet per second, this dam plays a crucial role in managing water flow and reducing the risk of flooding in the region.

Owned and regulated by the Idaho Department of Water Resources, Stewart Gulch Main Fork is inspected regularly to ensure its structural integrity and functionality. The dam is classified as having a significant hazard potential, but its condition is assessed as satisfactory. With a very high risk assessment ranking of 1, it is clear that the importance of this structure in mitigating flood risks in the area cannot be overstated. The dam's controlled spillway, with a width of 56 feet, further enhances its ability to manage water levels during periods of high flow.

As a key component in the flood risk reduction infrastructure of Boise, Stewart Gulch Main Fork serves as a critical line of defense against potential water-related disasters. With its robust design and satisfactory condition, this dam continues to play a vital role in protecting the local community and surrounding areas from the impacts of flooding. Its presence underscores the importance of proactive water resource management and climate resilience efforts in safeguarding communities against the growing challenges posed by a changing climate.

Year Completed

1998

Dam Length

570

Dam Height

76.3

River Or Stream

STEWART CREEK BOISE RIVER

Surface Area

4

Hydraulic Height

73.4

Drainage Area

4.2

Nid Storage

61

Outlet Gates

None

Hazard Potential

Significant

Foundations

Rock, Soil

Nid Height

76
       
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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.