Willow River is a river in St. Croix County, Wisconsin, United States and a tributary of the St. Croix River. Its headwaters are found near the village of Deer Park (near the northern border of the county) (45°10?35?N 92°20?02?W? / ?45.17639°N 92.33389°W? / 45.17639; -92.33389). The river at one time dumped into the lower St. Croix river in between Hudson and North Hudson (44°59?18?N 92°45?46?W? / ?44.98833°N 92.76278°W? / 44.98833; -92.76278). 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.[1] Subsequently, the Willow River Power Co. built a hydroelectric facility to utilize the same head of water, but the head was substantially decreased when the United States built the Red Wing Dam 30 miles downstream on the Mississippi River.[2]