Cox Hollow Number Two Reservoir Report

Last Updated: December 5, 2025

Cox Hollow Number Two, also known as Twin Valley Lake at Governor Dodge State Park in Wisconsin, is a state-owned recreational dam on Mill Creek.


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Summary

Built in 1965 by the USDA NRCS, this earth dam stands at a height of 55 feet and has a hydraulic height of 37 feet, providing a storage capacity of 5,370 acre-feet. With a surface area of 152 acres and a drainage area of 13.9 square miles, the dam serves the primary purpose of recreation.

Despite its age, Cox Hollow Number Two is in fair condition with a high hazard potential, prompting regular inspections by the Wisconsin Department of Natural Resources. The dam's spillway is controlled with a width of 5 feet, and it has a maximum discharge capacity of 606 cubic feet per second. The risk assessment for the dam is categorized as very high, emphasizing the need for continued monitoring and maintenance to ensure public safety and water resource protection in the surrounding area.

For water resource and climate enthusiasts, Cox Hollow Number Two offers a fascinating glimpse into the intersection of human infrastructure and natural ecosystems. This recreational dam not only provides opportunities for outdoor activities but also serves as a vital component of the local watershed, contributing to flood control and water storage. As we continue to navigate the challenges of climate change, understanding and maintaining dams like Cox Hollow Number Two will be crucial in safeguarding our water resources for future generations.

Years Modified

1966 - Other

Year Completed

1965

Dam Length

630

Dam Height

55

River Or Stream

Mill Creek

Primary Dam Type

Earth

Surface Area

152

Hydraulic Height

37

Drainage Area

13.9

Nid Storage

5370

Structural Height

55

Hazard Potential

High

Nid Height

55
       
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.