Nilan North Dam Reservoir Report

Last Updated: December 22, 2025

Located in Lewis and Clark, Montana, the Nilan North Dam is a state-regulated irrigation structure on the TR-WILLOW CREEK river.


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Summary

Completed in 1951, this earth dam stands at a height of 51 feet and has a storage capacity of 15,600 acre-feet, serving primarily for irrigation purposes. With a surface area of 525 acres and a drainage area of 3.55 square miles, the dam has a spillway width of 72 feet and a maximum discharge capacity of 395 cubic feet per second.

With a high hazard potential and a satisfactory condition assessment as of June 2018, Nilan North Dam undergoes inspections every 5 years to ensure its safety and functionality. The dam's emergency action plan was last revised in January 2022, highlighting the importance of preparedness and risk management for this critical infrastructure. Despite its moderate risk assessment rating, the dam plays a crucial role in water resource management in the region, showcasing the intersection of environmental stewardship and agricultural needs in the face of changing climates.

Year Completed

1951

Dam Length

530

Dam Height

51

River Or Stream

TR-WILLOW CREEK

Primary Dam Type

Earth

Surface Area

525

Hydraulic Height

51

Drainage Area

3.55

Nid Storage

15600

Hazard Potential

High

Foundations

Rock, Soil

Nid Height

51
       
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.