Flat Rock Dam Reservoir Report

Last Updated: December 22, 2025

Flat Rock Dam, also known as Ahles Dam, is a gravity dam located in Flat Rock, Michigan, along the Huron River.


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Summary

Built in 1924 for recreational purposes, this dam stands at 16.5 feet high with a length of 530 feet, providing a storage capacity of 2395 acre-feet. The dam has a spillway width of 492 feet and a maximum discharge of 14560 cubic feet per second, making it a vital structure for water resource management in the area.

Managed by the local government and regulated by the Michigan DEGLE, Flat Rock Dam has a high hazard potential but is currently in fair condition according to the latest assessment in July 2020. With a moderate risk level, the dam undergoes inspections every three years to ensure its safety and functionality. While the dam does not have any associated structures or locks, it remains a key feature in the region for recreational activities and water conservation efforts along the Huron River. Enthusiasts of water resources and climate will find Flat Rock Dam to be an intriguing structure that plays a significant role in the local ecosystem.

Year Completed

1924

Dam Length

530

Dam Height

16.5

River Or Stream

Huron River

Primary Dam Type

Gravity

Surface Area

188

Hydraulic Height

15.9

Drainage Area

876

Nid Storage

2395

Structural Height

16.5

Hazard Potential

High

Nid Height

17
       
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.