Cheshire Mills Forebay Dam Reservoir Report

Last Updated: December 18, 2025

Cheshire Mills Forebay Dam, located in Harrisville, New Hampshire, is a historic structure built in 1886 primarily for hydroelectric purposes on the Nubanusit Brook.


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Summary

This concrete dam stands at a height of 29 feet and has a length of 94 feet, with a storage capacity of 1.8 billion gallons of water. The dam is under state regulation by the NHDES Dam Bureau and is subject to regular inspections to ensure its safety and integrity.

With a high hazard potential and a moderate risk assessment score of 3, Cheshire Mills Forebay Dam plays a crucial role in water resource management in Cheshire County, New Hampshire. The dam's uncontrolled spillway, with a width of 38 feet, is designed to handle a maximum discharge of 635 cubic feet per second. Despite being labeled as "Not Rated" in terms of condition assessment, the dam is closely monitored and maintained to prevent any potential risks or emergencies that may arise.

As a key feature in the region's hydroelectric infrastructure, Cheshire Mills Forebay Dam serves as a reminder of the importance of sustainable water resource management and climate resilience. With its long history and significant storage capacity, the dam continues to play a vital role in meeting the energy needs of the local community while also posing challenges that must be addressed through proactive risk management measures and ongoing regulatory oversight.

Year Completed

1886

Dam Length

94

Dam Height

29

River Or Stream

NUBANUSIT BROOK

Primary Dam Type

Concrete

Surface Area

0.2

Drainage Area

10.5

Nid Storage

1.8

Hazard Potential

High

Foundations

Unlisted/Unknown

Nid Height

29
       
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.