The Willow River is a 61.1-mile-long (98.3 km) river in St. Croix County, Wisconsin, United States, and a tributary of the St. Croix River. Its source is in southern Polk County east of the village of Clear Lake. The river at one time flowed unobstructed into the lower St. Croix River in between Hudson and North Hudson (44°59′18″N92°45′46″W). But a dam was built to block off its mouth, a narrow channel was cut above the dam to the St. Croix River, the channel was dammed, and a mill was built to exploit the head of water that the two dams created. Subsequently, the Willow River Power Co.