The Willow River is a 40-mile tributary of the St. Croix River that winds through the low-lying hills of St. Croix County in western Wisconsin. Originating in and around the Cylon Wildlife Area near Deer Park, the river flows through New Richmond, Boardman, and Burkhardt before entering Willow River State Park and eventually joining the St. Croix River near Hudson, Wisconsin. Approximately 25 miles of the Willow support fishing opportunities, though the river has experienced general warming since the 1980s due to urbanization and loss of groundwater flow from residential wells and large commercial farming operations. Once considered prime fly fishing water within 25 miles of the Twin Cities metro area, the Willow now presents a diverse fishery ranging from quality trout water in the upper sections to warmwater species downstream.