Johnston City Lake Dam Reservoir Report

Last Updated: December 5, 2025

Johnston City Lake Dam, also known as Johnston City Reservoir, is a significant water supply structure located in Williamson County, Illinois.


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Summary

Completed in 1921, this earth dam stands at a height of 14 feet and spans 855 feet in length, with a storage capacity of 601 acre-feet. The dam serves the primary purpose of providing water supply to the local community, with a normal storage capacity of 466 acre-feet.

Managed by the local government and regulated by the Illinois Department of Natural Resources, Johnston City Lake Dam poses a significant hazard potential and is classified as having a moderate risk level. The spillway, with a width of 80 feet, is uncontrolled, and the dam has uncontrolled outlet gates. The dam's condition assessment is currently not available, and the last inspection was conducted in December 2020 with a frequency of every three years.

Enthusiasts of water resources and climate will find Johnston City Lake Dam an interesting structure to study, given its historical significance and the crucial role it plays in providing water supply to the area. With its unique design features, regulatory oversight, and risk assessment, this dam serves as a valuable example of how infrastructure can impact water management and community resilience in the face of climate challenges.

Year Completed

1921

Dam Length

855

Dam Height

14

River Or Stream

TRIB LAKE CREEK

Primary Dam Type

Earth

Nid Storage

601

Outlet Gates

Uncontrolled

Hazard Potential

Significant

Foundations

Unlisted/Unknown

Nid Height

14
       
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.