Grub Detention Dam Reservoir Report
Nearby: Jordan Dam Script Detention Dam
Last Updated: December 20, 2025
Grub Detention Dam, located in Nashua, Montana, serves as a critical structure managed by the Bureau of Land Management for flood risk reduction along the Willow Creek/Milk River.
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Summary
Completed in 1964, this gravity dam stands at a hydraulic height of 15 feet and a structural height of 21 feet, with a storage capacity of 4,109 acre-feet. With a surface area of 689.45 acres and a drainage area of 62.8 square miles, the dam plays a crucial role in managing water flow in the region.
Despite its low hazard potential, Grub Detention Dam has been assessed as unsatisfactory in condition, indicating a need for maintenance and improvement. The dam features an uncontrolled spillway with a width of 5 feet and has not been modified since its completion. The inspection frequency is set at every five years, with the last assessment conducted in December 2019. While the risk assessment is moderate, the dam's risk management measures and emergency action plan status remain unclear, highlighting the importance of ongoing monitoring and maintenance to ensure its effectiveness in flood control.
Water resource and climate enthusiasts can appreciate Grub Detention Dam's contribution to mitigating flood risks in the Valley County region of Montana. As a federal structure owned, funded, designed, and regulated by the Bureau of Land Management, this dam stands as a testament to proactive water management strategies. Despite its current condition assessment of unsatisfactory, ongoing monitoring and potential improvements can enhance the dam's functionality and resilience in the face of changing climate patterns and increased water-related challenges in the future.
Year Completed |
1964 |
Dam Length |
2339 |
River Or Stream |
WILLOW CK./MILK RIVER |
Primary Dam Type |
Gravity |
Surface Area |
689.45 |
Hydraulic Height |
15 |
Drainage Area |
62.8 |
Nid Storage |
4109 |
Structural Height |
21 |
Hazard Potential |
Low |
Foundations |
Soil |
Nid Height |
21 |
Seasonal Comparison
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Nearby Streamflow Levels
Dam Data Reference
Condition Assessment
SatisfactoryNo 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
HighDams 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.
Grub Detention Dam