Mcmillan Marsh, also known as Mcmillan Marsh Reservoir, is a state-owned water resource in Marathon, Wisconsin, with a primary purpose of water supply. 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.