Lake Mills Reservoir Report

Last Updated: December 5, 2025

Lake Mills, also known as Rock Lake, is a picturesque reservoir located in Jefferson County, Wisconsin.


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Summary

Owned and regulated by the local government, this gravity dam was completed in 1865 for the primary purpose of recreation. With a height of 17.2 feet and a storage capacity of 28,330 acre-feet, the dam sits on Rock Creek and covers a surface area of 1,371 acres.

Despite its age, Lake Mills remains in satisfactory condition with a high hazard potential due to its location and structural specifications. Regular inspections are conducted by the Wisconsin Department of Natural Resources, with the last assessment taking place in October 2019. The dam has a controlled spillway and a maximum discharge of 115 cubic feet per second, serving as a vital resource for the community while also posing a very high risk in the event of a failure.

In recent years, Lake Mills has undergone modifications in 1962 and 2016 to ensure its continued safety and functionality. With its scenic surroundings and recreational opportunities, this reservoir serves as a key feature in the region's water resource management efforts, attracting enthusiasts interested in both water conservation and climate resilience.

Years Modified

1962 - Other, 2016 - Other

Year Completed

1865

Dam Length

400

Dam Height

17.2

River Or Stream

ROCK CREEK

Primary Dam Type

Gravity

Surface Area

1371

Hydraulic Height

3

Drainage Area

11

Nid Storage

28330

Structural Height

17.2

Hazard Potential

High

Nid Height

17
       
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.