Hillsborough Mill Reservoir Report

Last Updated: December 17, 2025

Hillsborough Mill is a historic hydroelectric dam located on the Souhegan River in Wilton, New Hampshire.


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Summary

Built in 1925, this concrete dam stands at a height of 22 feet and spans 200 feet in length, with a storage capacity of 70 acre-feet. The dam is regulated by the New Hampshire Department of Environmental Services Dam Bureau and is inspected regularly by the Federal Energy Regulatory Commission.

The primary purpose of the Hillsborough Mill dam is hydroelectric power generation, with three slide gates for outlet control. The dam has a significant hazard potential and is classified as having a moderate risk level. Despite its age, the condition assessment for the dam is not available, but emergency action plans are in place and regularly reviewed to ensure public safety in the event of a dam failure. The dam's spillway type is uncontrolled, with a width of 115 feet, and it has a maximum discharge capacity of 13,100 cubic feet per second.

Overall, Hillsborough Mill represents an important part of New Hampshire's water resource infrastructure, contributing to the state's renewable energy production and water management efforts. With its rich history and ongoing regulatory oversight, the dam serves as a reminder of the importance of maintaining and monitoring our water resources in the face of climate change and increasing water demands.

Year Completed

1925

Dam Length

200

Dam Height

22

River Or Stream

Souhegan River

Primary Dam Type

Concrete

Surface Area

7

Drainage Area

97

Nid Storage

70

Outlet Gates

Slide (sluice gate) - 3

Hazard Potential

Significant

Nid Height

22
       
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.