Big Rock Detention Dam Reservoir Report
Nearby: Jordan Dam Script Detention Dam
Last Updated: December 20, 2025
Big Rock Detention Dam, located in Valley County, Montana, serves as a crucial flood risk reduction structure along Willow Creek and the Milk River.
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Summary
Built in 1961 by the Bureau of Land Management, this gravity-type dam stands at a structural height of 13 feet and has a hydraulic height of 9 feet, with a length of 1793 feet and a storage capacity of 265 acre-feet. Despite its low hazard potential, the dam's condition assessment is currently marked as unsatisfactory, highlighting the need for ongoing maintenance and inspection.
The dam's primary purpose is flood risk reduction, with an emphasis on protecting the surrounding area from potential inundation during periods of heavy rainfall or snowmelt. While there are no associated locks or outlet gates, the dam features an uncontrolled spillway with a width of 3 feet. The risk assessment for Big Rock Detention Dam indicates a moderate level of risk, emphasizing the importance of continued risk management measures to ensure the safety and functionality of the structure.
As a federally-owned and operated dam under the jurisdiction of the Bureau of Land Management, Big Rock Detention Dam plays a crucial role in water resource management and climate resilience in the region. With its strategic location and design, the dam serves as a vital asset in protecting the local community and environment from the impacts of flooding, underscoring the importance of ongoing monitoring and maintenance to safeguard its effectiveness in mitigating flood risks.
Year Completed |
1961 |
Dam Length |
1793 |
River Or Stream |
WILLOW CK./MILK RIVER |
Primary Dam Type |
Gravity |
Surface Area |
62.21 |
Hydraulic Height |
9 |
Drainage Area |
0.1 |
Nid Storage |
265 |
Structural Height |
13 |
Hazard Potential |
Low |
Foundations |
Unlisted/Unknown |
Nid Height |
13 |
Seasonal Comparison
Weather Forecast
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.
Big Rock Detention Dam