Four Mile Dam Reservoir Report

Last Updated: April 13, 2026

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Four Mile Dam, located in Alpena, Michigan, was completed in 1902 and serves as a crucial hydroelectric structure on Thunder Bay.


Summary

With a height of 27 feet and a length of 520 feet, this gravity dam with multi-arch core types and rock foundations provides a storage capacity of 900 acre-feet and a maximum discharge of 27,000 cubic feet per second. The dam also offers recreational opportunities, covering a surface area of 90 acres within a drainage area of 1,260 square miles.

Managed by the Federal Energy Regulatory Commission, Four Mile Dam is classified as a high hazard potential structure due to its age and condition assessment not being available. Despite its risk level being assessed as very high, the dam has controlled spillways with a width of 361 feet and a series of 7 needle and 17 slide gates for water flow control. While the dam does not have a state jurisdiction, permitting, or enforcement oversight, it remains a significant feature in water resource management and climate adaptation efforts in the region, attracting water resource and climate enthusiasts with its historical significance and engineering marvel.

15-Day Long Term Forecast


Year Completed

1902

Dam Length

520

Dam Height

27

River Or Stream

Thunder Bay

Surface Area

90

Drainage Area

1260

Nid Storage

900

Outlet Gates

Needle - 7, Slide (sluice gate) - 17

Hazard Potential

High

Foundations

Rock

Nid Height

27
       
Seasonal Comparison

5-Day Hourly Forecast Detail

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.
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