Longwood Lake Dam Reservoir Report

Last Updated: December 9, 2025

Longwood Lake Dam in Jefferson Township, Morris County, New Jersey, stands as a historic stone structure completed in 1898 for the primary purpose of recreation along the Rockaway River.


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Summary

With a height of 21.5 feet and a length of 91 feet, this dam stores a maximum of 351 acre-feet of water with a normal storage capacity of 214 acre-feet, covering a surface area of 43 acres. Despite its picturesque setting, Longwood Lake Dam has been classified with a high hazard potential and poor condition assessment, signaling a need for regular inspections and potential maintenance.

Managed by the local government, Longwood Lake Dam falls under the regulatory oversight of the New Jersey Department of Environmental Protection (NJDEP), ensuring compliance with state permitting, inspection, and enforcement requirements. The dam's last inspection in July 2021 revealed its poor condition, highlighting the urgency for necessary repairs or maintenance to mitigate the risks associated with its high hazard potential. It is crucial for water resource and climate enthusiasts to monitor the status of Longwood Lake Dam and advocate for its preservation to ensure the safety of surrounding communities and the sustainability of the Rockaway River ecosystem.

Year Completed

1898

Dam Length

91

Dam Height

21.5

River Or Stream

Rockaway River

Primary Dam Type

Stone

Surface Area

43

Drainage Area

19

Nid Storage

351

Hazard Potential

High

Nid Height

22
       
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.