Simpson Lake Dam Reservoir Report

Last Updated: December 5, 2025

Simpson Lake Dam, located in Johnson, Illinois, is a privately owned earth dam completed in 1963 by designer Walter Evitts.


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Summary

The dam, standing at a height of 23 feet and stretching 690 feet in length, serves primarily for recreational purposes, offering a serene spot for water activities and scenic views. The dam is regulated by the Illinois Department of Natural Resources (IDNR) and is subject to regular inspections to ensure its structural integrity and compliance with state regulations.

With a low hazard potential and moderate risk assessment rating, Simpson Lake Dam poses minimal danger to the surrounding area. While its condition assessment is currently listed as not available, the dam's emergency action plan status and risk management measures are yet to be determined. Despite its uncontrolled spillway and outlet gates, the dam continues to provide a secure and enjoyable environment for visitors seeking relaxation and leisure activities in the tranquil waters of Simpson Lake. Enthusiasts of water resources and climate will find Simpson Lake Dam a fascinating example of a privately owned recreational dam in the heart of Illinois.

Year Completed

1963

Dam Length

690

Dam Height

23

River Or Stream

TRIB BAY CREEK

Primary Dam Type

Earth

Nid Storage

222

Outlet Gates

Uncontrolled

Hazard Potential

Low

Foundations

Unlisted/Unknown

Nid Height

23
       
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.