Potomac River Dam #4 Reservoir Report

Last Updated: December 5, 2025

Potomac River Dam #4, located in Williamsport, Maryland, is a historic structure completed in 1869 by the Chesapeake & Ohio Canal Co.


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Summary

This gravity dam stands at a height of 20 feet and stretches 715 feet in length, creating a reservoir with a normal storage capacity of 7300 acre-feet. The dam serves a primary purpose of hydroelectric power generation and is owned and regulated by the National Park Service.

With a significant hazard potential and a state-regulated status, Potomac River Dam #4 is subject to regular inspections by the Bureau of Reclamation. The dam's maximum discharge capacity is 1,000,000 cubic feet per second, and it is responsible for managing a drainage area of 5,900 square miles. Despite its age, the dam's condition assessment is currently listed as "Not Available," highlighting the importance of ongoing monitoring and maintenance to ensure its structural integrity and safety.

For water resource and climate enthusiasts, Potomac River Dam #4 offers a glimpse into the intersection of history, engineering, and environmental stewardship. As a key component of the Potomac River's hydroelectric infrastructure, this gravity dam plays a crucial role in balancing water storage and release for both power generation and environmental conservation. Its location within the picturesque surroundings of Washington County, Maryland, makes it a focal point for those interested in sustainable water management practices and the preservation of natural resources in the face of changing climate conditions.

Year Completed

1869

Dam Length

715

Dam Height

20

River Or Stream

POTOMAC RIVER

Primary Dam Type

Gravity

Surface Area

675

Drainage Area

5900

Nid Storage

10000

Hazard Potential

Significant

Nid Height

20
       
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.