Lake Wilhelm (Pa-475) Reservoir Report

Last Updated: December 6, 2025

Lake Wilhelm (Pa-475) in Mercer, Pennsylvania, is a state-regulated reservoir designed by the USDA NRCS for flood risk reduction along Sandy Creek.


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Summary

Completed in 1969, this earth dam stands at 50 feet in height and spans 500 feet in length, with a storage capacity of 66,400 acre-feet and a surface area of 1680 acres. The primary purpose of Lake Wilhelm is flood risk reduction, with secondary benefits including recreation opportunities for the surrounding community.

Managed by the Pennsylvania Department of Environmental Protection, Lake Wilhelm has a high hazard potential but is currently in satisfactory condition, as assessed during the last inspection in September 2020. The reservoir boasts a drainage area of 56.5 square miles and is a vital resource for water supply and flood control in the region. With its strategic location and effective design, Lake Wilhelm serves as a key component of the local water resource infrastructure, ensuring the safety and well-being of nearby residents during extreme weather events.

Year Completed

1969

Dam Length

500

Dam Height

50

River Or Stream

SANDY CREEK

Primary Dam Type

Earth

Surface Area

1680

Drainage Area

56.5

Nid Storage

66400

Hazard Potential

High

Foundations

Unlisted/Unknown

Nid Height

50
       
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.