South Dike -Willow Reservoir Report

Last Updated: December 7, 2025

South Dike-Willow is a vital earth dam located in Tomahawk, Wisconsin, along the Tomahawk River.


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Summary

Built in 1927 for hydroelectric purposes, this dam stands at a height of 11 feet and spans 3500 feet in length. With a storage capacity of 67,126 acre-feet and a drainage area of 310 square miles, it serves multiple functions including flood risk reduction and hydroelectric power generation.

Managed by the Federal Energy Regulatory Commission, South Dike-Willow has a high hazard potential due to its critical role in protecting the surrounding area from flooding. Although its condition assessment is currently not available, regular inspections are conducted to ensure its structural integrity. The dam's emergency action plan was last revised in December 2018, highlighting the ongoing efforts to enhance its safety measures and preparedness for any potential risks.

South Dike-Willow's significance lies in its contribution to water resource management and climate resilience in the region. As a key infrastructure for both flood control and renewable energy production, it underscores the interconnectedness of water resources and climate adaptation efforts. The dam's strategic location, design, and operational oversight make it a crucial asset for safeguarding the community and environment against the impacts of extreme weather events and ensuring sustainable water use for future generations.

Year Completed

1927

Dam Length

3500

Dam Height

11

River Or Stream

Tomahawk River

Primary Dam Type

Earth

Drainage Area

310

Nid Storage

67126

Hazard Potential

High

Foundations

Soil

Nid Height

11
       
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.