Pool No. 21, located in Thief River Falls, Minnesota, is a federally owned earth dam completed in 1971 with a primary purpose of "Other." This dam stands at a height of 8 feet and spans a length of 1300 feet, providing a maximum storage capacity of 5370 acre-feet. The dam is situated on the Mud River and is under the jurisdiction of the Omaha District of the US Army Corps of Engineers.

Despite its low hazard potential, Pool No. 21 plays a crucial role in managing water resources in the area. With a normal storage capacity of 2100 acre-feet and a maximum discharge capability of 1400 cubic feet per second, this dam helps regulate water flow and mitigate flooding risks in the region. Although not currently rated for condition assessment, regular inspections are conducted every 8 years to ensure the safety and functionality of the structure.

Water resource and climate enthusiasts interested in the infrastructure that supports sustainable water management practices will find Pool No. 21 a fascinating feat of engineering. This structure not only serves as a vital component of the local water management system but also reflects the importance of maintaining and monitoring such critical infrastructure to ensure the resilience of water resources in the face of changing climate conditions.