Mcmillan Marsh Reservoir Report
Nearby: Kopp Lloyd Keller
Last Updated: December 13, 2025
Mcmillan Marsh, also known as Mcmillan Marsh Reservoir, is a state-owned water resource in Marathon, Wisconsin, with a primary purpose of water supply.
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Summary
This earth-type dam, completed in 1971, stands at a height of 11 feet and has a hydraulic height of 6 feet, serving as a vital structure for managing the water levels of the Little Eau Pleine river. The reservoir has a storage capacity of 4,900 acre-feet and covers a surface area of 690 acres, playing a crucial role in regulating water flow and maintaining water quality in the region.
Managed by the Wisconsin Department of Natural Resources (WIDNR), Mcmillan Marsh is regularly inspected and maintained to ensure its structural integrity and operational efficiency. Despite being classified as having a low hazard potential and a satisfactory condition assessment, the reservoir is considered to have a very high risk level due to its importance in water management. The controlled spillway, with a width of 456 feet, further enhances the safety and functionality of the dam, mitigating potential flood risks and protecting the surrounding area from water-related disasters.
Situated in a picturesque natural setting, Mcmillan Marsh serves as a vital lifeline for the local ecosystem and communities, showcasing the intersection of water resource management and climate resilience. With its strategic location and critical role in water supply, this reservoir stands as a testament to the importance of sustainable water resource management in the face of shifting environmental challenges and climate impacts.
Year Completed |
1971 |
Dam Length |
7900 |
Dam Height |
11 |
River Or Stream |
LITTLE EAU PLEINE |
Primary Dam Type |
Earth |
Surface Area |
690 |
Hydraulic Height |
6 |
Drainage Area |
49.5 |
Nid Storage |
4900 |
Structural Height |
11 |
Hazard Potential |
Low |
Nid Height |
11 |
Seasonal Comparison
Weather Forecast
Nearby Streamflow Levels
Big Eau Pleine River At Stratford
|
27cfs |
Rib River At Rib Falls
|
60cfs |
Black River At Neillsville
|
193cfs |
Wisconsin River At Rothschild
|
1960cfs |
Wisconsin River At Wisconsin Rapids
|
1900cfs |
Yellow River At Babcock
|
44cfs |
Dam Data Reference
Condition Assessment
SatisfactoryNo 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
HighDams 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.
Mcmillan Marsh