Thunderbird Lake 2 Dam Reservoir Report

Last Updated: December 5, 2025

Thunderbird Lake 2 Dam, located in McHenry, Illinois, is a privately owned structure built in 1965 for recreational purposes.


°F

°F

mph

Wind

%

Humidity

Summary

The dam, standing at 10 feet tall and stretching 360 feet in length, serves as a barrier on Sleepy Hollow Creek to create Thunderbird Lake. With a storage capacity of 395 acre-feet, the dam offers opportunities for outdoor activities such as boating and fishing in the surrounding area.

Managed by the Illinois Department of Natural Resources, Thunderbird Lake 2 Dam is subject to regular state inspections and enforcement measures to ensure its safety and compliance with regulations. Despite being classified as having a low hazard potential, the dam is considered to have a moderate risk level, underscoring the importance of ongoing risk assessment and management. Though details on the dam's condition assessment and emergency action plan are not currently available, the structure continues to provide recreational enjoyment while being monitored for any potential issues.

For water resource and climate enthusiasts, Thunderbird Lake 2 Dam presents a unique case study of a privately owned earth dam designed for recreational purposes. Situated in a picturesque setting and regulated by the state of Illinois, the dam exemplifies the intersection of human-made infrastructure with natural resources. As efforts to mitigate risks and ensure safety continue, Thunderbird Lake 2 Dam remains a focal point for outdoor enthusiasts seeking adventure and relaxation in the heart of McHenry County.

Year Completed

1965

Dam Length

360

Dam Height

10

River Or Stream

SLEEPY HOLLOW CREEK

Primary Dam Type

Earth

Nid Storage

395

Outlet Gates

None

Hazard Potential

Low

Foundations

Unlisted/Unknown

Nid Height

10
       
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.