Mill River Dam Reservoir Report

Last Updated: December 6, 2025

Mill River Dam, located in Northampton, Massachusetts, is a concrete buttress dam completed in 1939 with a primary purpose of "other." The dam stands at a height of 40 feet and has a hydraulic height of 34.6 feet, providing a maximum storage capacity of 165 acre-feet and a normal storage capacity of 26 acre-feet.


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Summary

With a drainage area of 54 square miles, the dam serves as a crucial structure for the Mill River Diversion.

The dam is regulated by the Department of Conservation and Recreation in Massachusetts and is classified as having a "significant" hazard potential with a "fair" condition assessment as of 2015. The dam's spillway is of the uncontrolled type, with a width of 0 feet. Despite its moderate risk assessment rating, the dam has not been modified in recent years, indicating a need for ongoing monitoring and maintenance to ensure its structural integrity and the safety of downstream areas. The dam's inspection frequency is every 5 years, with the last inspection conducted in July 2015.

Overall, the Mill River Dam plays a vital role in water resource management in Hampshire County, Massachusetts, serving as a key infrastructure for flood control and water storage. Climate and water resource enthusiasts can appreciate the historical significance and engineering design of this concrete buttress dam, while also recognizing the importance of continued regulatory oversight and maintenance to mitigate risks and ensure the long-term functionality of this essential structure in the region.

Year Completed

1939

Dam Length

450

Dam Height

40

River Or Stream

MILL RIVER DIVERSION

Primary Dam Type

Concrete

Surface Area

12.8

Hydraulic Height

34.6

Drainage Area

54

Nid Storage

165

Hazard Potential

Significant

Foundations

Unlisted/Unknown

Nid Height

40
       
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.