Nepaug Dam Reservoir Report

Last Updated: December 9, 2025

Nepaug Dam, located in Collinsville, Connecticut, stands as a vital structure for water supply purposes, serving as a key reservoir along the Nepaug River.



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Summary

Built in 1918, this concrete dam boasts a height of 113 feet and a length of 650 feet, with a storage capacity of 40,540 acre-feet and a drainage area of 31.9 square miles. The dam plays a crucial role in managing water resources in the region, with a normal storage capacity of 29,300 acre-feet and a maximum discharge of 7,200 cubic feet per second.

With its high hazard potential and satisfactory condition assessment, Nepaug Dam remains under state regulation and inspection by the Department of Energy and Environmental Protection (DEEP) in Connecticut. The dam features an uncontrolled spillway, valve outlet gates, and is primarily constructed on rock foundations. Despite its age, the dam continues to meet regulatory guidelines, with a moderate risk assessment rating of 3. The associated structures, CT00370S02 and CT00370S03, further support the functionality and resilience of the Nepaug Dam complex, ensuring the efficient management of water resources and mitigating potential risks to the surrounding communities.

Nepaug Dam's historical significance, engineering design, and operational efficiency make it a noteworthy landmark in water resource management within Litchfield County, Connecticut. As a key component of the region's infrastructure, the dam exemplifies the importance of sustainable water supply practices and effective risk management strategies in the face of climate change and evolving environmental challenges. With a strong regulatory framework in place and a commitment to ongoing inspections and maintenance, Nepaug Dam continues to play a crucial role in ensuring the resilience and reliability of water resources for the local community and beyond.

Year Completed

1918

Dam Length

650

Dam Height

113

River Or Stream

NEPAUG RIVER

Primary Dam Type

Concrete

Surface Area

850

Drainage Area

31.9

Nid Storage

40540

Outlet Gates

Valve

Hazard Potential

High

Foundations

Rock

Nid Height

113
       
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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.