Plymouth Reservoir Dam Reservoir Report

Last Updated: December 5, 2025

Plymouth Reservoir Dam, located in Thomaston, Connecticut, was completed in 1881 and serves as a vital water supply source for the region.


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Summary

This masonry dam stands at a height of 20 feet and spans 730 feet, with a storage capacity of 390 acre-feet. The dam is situated on the Naugatuck River Tributary and is owned by a Public Utility, regulated by the Connecticut Department of Energy and Environmental Protection (DEEP).

With a surface area of 36.7 acres and a drainage area of 0.57 square miles, the dam has a high hazard potential but is currently assessed as satisfactory in condition. The last inspection in May 2018 indicated a need for regular monitoring due to its risk potential. Despite its age, the dam has been well-maintained and continues to provide essential water resources for the community. Water resource and climate enthusiasts will appreciate the historical significance and engineering marvel of the Plymouth Reservoir Dam in managing water supply in Litchfield County, Connecticut.

Year Completed

1881

Dam Length

730

Dam Height

20

River Or Stream

NAUGATUCK RIVER TRIB

Primary Dam Type

Masonry

Surface Area

36.7

Drainage Area

0.57

Nid Storage

390

Hazard Potential

High

Foundations

Rock, Soil

Nid Height

20
       
Seasonal Comparison

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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.