Surprise Lake Dam Reservoir Report

Last Updated: December 7, 2025

Surprise Lake Dam, located in Mountainside Borough, New Jersey, is a local government-owned structure primarily used for recreation purposes.


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Summary

Completed in 1928, this earth dam stands at a height of 20 feet and spans 200 feet in length, with a storage capacity of 71 acre-feet. Situated on Blue Brook, the dam is regulated by the New Jersey Department of Environmental Protection and undergoes regular inspections to ensure its satisfactory condition. With a low hazard potential, the dam poses minimal risk to the surrounding area.

Despite its age, Surprise Lake Dam continues to provide a safe and enjoyable recreational space for the community. With a surface area of 10 acres and a drainage area of 1.4 square miles, the dam offers a serene setting for outdoor activities. The dam's condition assessment in November 2021 deemed it to be in satisfactory shape, with an inspection frequency of every four years. While emergency action plans and risk assessments are not currently in place, the dam's low hazard potential and consistent maintenance contribute to its overall safety and resilience in the face of climate challenges. For water resource and climate enthusiasts, Surprise Lake Dam serves as a reminder of the importance of maintaining and monitoring aging infrastructure to ensure both public safety and environmental protection.

Year Completed

1928

Dam Length

200

Dam Height

20

River Or Stream

Blue Brook

Primary Dam Type

Earth

Surface Area

10

Drainage Area

1.4

Nid Storage

71

Hazard Potential

Low

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.