Lower Ansonia Reservoir Dam Reservoir Report

Last Updated: December 5, 2025

The Lower Ansonia Reservoir Dam, also known as Lower Derby Hill Reservoir, is a vital structure owned by the local government in East Derby, Connecticut.


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Summary

Built in 1887, this earth dam stands at a height of 17.8 feet and serves the primary purpose of water supply for the area. With a normal storage capacity of 71 acre-feet and a maximum storage of 90 acre-feet, this dam plays a crucial role in regulating water flow and ensuring a stable water supply for the region.

Located on the Naugatuck River Tributary, the Lower Ansonia Reservoir Dam has a surface area of 9.6 acres and a drainage area of 0.56 square miles. Despite its age, the dam is regularly inspected and deemed to be in fair condition with a high hazard potential. The last inspection in August 2015 and a condition assessment in November 2015 indicate that the structure is well-maintained and meets state regulatory standards, with a designated emergency action plan in place for any unforeseen events.

With its historical significance and critical role in water resource management, the Lower Ansonia Reservoir Dam stands as a testament to effective dam engineering and maintenance practices. As a key component of the local water supply infrastructure, this dam plays a vital role in ensuring water security and resilience in the face of climate change challenges. Climate and water resource enthusiasts will appreciate the careful management and monitoring of this important structure to safeguard the surrounding community and environment.

Year Completed

1887

Dam Length

423

Dam Height

17.8

River Or Stream

NAUGATUCK RIVER TRIB

Primary Dam Type

Earth

Surface Area

9.6

Drainage Area

0.56

Nid Storage

90

Hazard Potential

High

Nid Height

18
       
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.