Thayer Lake Dam, located in Thayer, Iowa, is a state-owned structure designed by Howard R. Green and completed in 1930. This earth dam, standing at 30 feet tall with a length of 590 feet, serves primarily for recreational purposes and is regulated by the Iowa Department of Natural Resources. Situated on TR-Four Mile Creek, the dam has a storage capacity of 222 acre-feet and covers a surface area of 14.1 acres within a drainage area of 0.79 square miles.

Despite being classified as having a low hazard potential, the dam's risk assessment indicates a moderate level of risk. The condition of the dam is currently not rated, with the last inspection conducted in October 2003. The spillway type is uncontrolled with a width of 3 feet. Thayer Lake Dam is an essential part of the local water resource infrastructure, contributing to the conservation efforts and recreational opportunities in Union County. With its historical significance and environmental impact, the dam remains a focal point for water resource and climate enthusiasts in the region.