Blackwater Dam Reservoir Report

Last Updated: December 19, 2025

Blackwater Dam, also known as Dry Reservoir, is a gravity-type dam located on the Blackwater River in Webster, New Hampshire.


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Summary

Built in 1941 by the US Army Corps of Engineers, the dam stands at a height of 75 feet and has a maximum storage capacity of 93,400 acre-feet. Its primary purpose is flood risk reduction, with a spillway width of 241 feet to manage water levels during periods of high discharge.

The dam is associated with two other structures and is owned, funded, designed, and regulated by the US Army Corps of Engineers. It has a high hazard potential and is subject to regular inspections and emergency action plans. USACE actively manages dam-related flood risks by monitoring its condition, prioritizing risk-reducing activities, and collaborating with local emergency managers to ensure preparedness and response in case of an emergency.

With a drainage area of 128 square miles and a moderate risk assessment score, Blackwater Dam plays a crucial role in water resource management in Merrimack, New Hampshire. As climate change continues to impact water resource availability and extreme weather events, the dam's role in flood risk reduction and water management becomes increasingly vital for ensuring the safety and resilience of the surrounding communities.

Year Completed

1941

Dam Length

1150

Dam Height

75

River Or Stream

BLACKWATER RIVER

Primary Dam Type

Gravity

Surface Area

1

Hydraulic Height

64

Drainage Area

128

Nid Storage

93400

Structural Height

69

Hazard Potential

High

Foundations

Rock

Nid Height

75
       
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.