Johnson Reservoir Report

Last Updated: December 5, 2025

Johnson is a privately owned irrigation dam located in Franklin, Idaho, near the city of Preston.


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Summary

Built in 1860, this earth dam stands at a height of 27.6 feet and has a storage capacity of 396 acre-feet. The dam is primarily used for irrigation purposes and is regulated by the Idaho Department of Water Resources.

With a significant hazard potential and a fair condition assessment as of July 2018, Johnson Dam is closely monitored and inspected by state regulatory agencies to ensure its safety and functionality. The dam has a spillway width of 20 feet and features slide gates as outlet controls. Despite being modified over the years, the dam still maintains its original purpose and design, serving as a vital water resource for the surrounding area.

Located along Worm Creek and Cub River, Johnson Dam plays a crucial role in water management and agricultural activities in the region. With a moderate risk assessment and regular inspections, this historic structure continues to contribute to the local economy and environment while highlighting the importance of sustainable water resource management in the face of changing climate conditions.

Year Completed

1860

Dam Length

450

Dam Height

27.6

River Or Stream

WORM CREEK CUB RIVER

Surface Area

40

Hydraulic Height

24.6

Drainage Area

0.9

Nid Storage

396

Outlet Gates

Slide (sluice gate)

Hazard Potential

Significant

Foundations

Rock, Soil

Nid Height

28
       
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.