Powder Mill Reservoir Report

Last Updated: December 19, 2025

Powder Mill, located in Bennington, New Hampshire, is a privately owned dam on the Contoocook River that serves the dual purpose of hydroelectric power generation and water supply.


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Summary

Built in 1924, this concrete dam stands at a height of 20.6 feet and has a length of 366 feet. With a storage capacity of 8,600 acre-feet and a drainage area of 184 square miles, Powder Mill plays a crucial role in managing water resources in the region.

Managed by the NHDES Dam Bureau and regulated by the Federal Energy Regulatory Commission, Powder Mill has a low hazard potential and a moderate risk assessment rating. The dam features an uncontrolled spillway with a width of 228 feet and is equipped with five slide (sluice gate) outlet gates. Despite its age, Powder Mill has undergone regular inspections, with the last one conducted in June 2017. The dam's Emergency Action Plan was last revised in February 2018, ensuring that necessary measures are in place to address any potential risks associated with the structure.

As a key component of the region's water infrastructure, Powder Mill stands as a testament to the intersection of water resource management and climate resilience. Its role in providing clean energy and reliable water supply underscores the importance of maintaining and monitoring such structures to ensure the safety and sustainability of our water resources in the face of a changing climate.

Year Completed

1924

Dam Length

366

Dam Height

20.6

River Or Stream

Contoocook River

Primary Dam Type

Concrete

Surface Area

435

Drainage Area

184

Nid Storage

8600

Outlet Gates

Slide (sluice gate) - 5

Hazard Potential

Low

Nid Height

21
       
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.