Lower Great Falls Reservoir Report

Last Updated: December 19, 2025

Lower Great Falls, also known as Somersworth Dam, is a captivating hydroelectric structure located in Rollinsford, New Hampshire.


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Summary

Completed in 1984, this concrete, gravity, and masonry dam stands at a height of 36 feet and spans 270 feet in length. With a storage capacity of 472 acre-feet and a maximum discharge of 8460 cubic feet per second, it plays a crucial role in managing the Salmon Falls River's flow and supporting the surrounding ecosystem.

Managed by the Federal Energy Regulatory Commission, Lower Great Falls is a state-regulated dam that undergoes regular inspections to ensure its structural integrity and operational efficiency. The dam features a controlled spillway with a width of 178 feet and is equipped with two slide (sluice gate) outlet gates. Its hazard potential is assessed as significant, indicating the importance of proactive risk management measures to mitigate potential dangers and ensure the safety of downstream communities.

With a risk assessment rating of very high, Lower Great Falls poses a critical focal point for water resource and climate enthusiasts. Its strategic location, impressive design, and vital role in hydroelectric power generation make it a significant landmark in the region. As efforts continue to monitor and maintain this essential infrastructure, it stands as a testament to human ingenuity in harnessing the power of water resources while respecting the natural environment.

Year Completed

1984

Dam Length

270

Dam Height

36

River Or Stream

Salmon Falls River

Surface Area

32

Drainage Area

220

Nid Storage

472

Outlet Gates

Slide (sluice gate) - 2

Hazard Potential

Significant

Nid Height

36
       
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.