Thunderbird Lake 1 Dam Reservoir Report

Last Updated: December 5, 2025

Thunderbird Lake 1 Dam, located in McHenry, Illinois, is a privately owned earth dam constructed in 1965 for recreational purposes on Sleepy Hollow Creek.


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Summary

Standing at a height of 7 feet and a length of 420 feet, the dam has a storage capacity of 395 acre-feet, with a primary purpose of providing recreational opportunities to visitors. The dam's spillway is uncontrolled with a width of 21 feet, and it is classified as having a low hazard potential and moderate risk assessment.

While the dam is state-regulated and inspected, its current condition assessment is not available, and there is limited information on emergency preparedness measures such as an Emergency Action Plan (EAP) or inundation maps. Despite these gaps, Thunderbird Lake 1 Dam continues to serve as a popular recreational spot in the region, offering a picturesque setting for water resource and climate enthusiasts to enjoy outdoor activities. The dam's historical significance, combined with its scenic location and moderate risk assessment, make it a notable site for those interested in the intersection of water management and environmental conservation.

Year Completed

1965

Dam Length

420

Dam Height

7

River Or Stream

SLEEPY HOLLOW CREEK

Primary Dam Type

Earth

Nid Storage

395

Outlet Gates

None

Hazard Potential

Low

Foundations

Unlisted/Unknown

Nid Height

7
       
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.