Onion River, also known as Waldo, is a gravity dam located in Sheboygan, Wisconsin, with a primary purpose of recreation. Completed in 1856, this dam stands at a height of 15 feet and has a hydraulic height of 8 feet, providing a storage capacity of 200 acre-feet. With a structural height of 15 feet and a length of 300 feet, Onion River serves as a controlled spillway with a width of 62 feet, ensuring proper water management in times of high discharge.

Despite its fair condition assessment and high hazard potential, Onion River continues to be regulated, inspected, and enforced by the Wisconsin Department of Natural Resources. The dam's risk assessment has been classified as very high, emphasizing the importance of ongoing monitoring and management measures to ensure the safety of the surrounding community. With a drainage area of 22 square miles and a maximum discharge of 1165 cubic feet per second, Onion River plays a crucial role in water resource management and climate resilience efforts in the region.