The Willow River originates in southern Polk County, Wisconsin, east of the village of Clear Lake, and flows approximately 61 miles through St. Croix County before joining the St. Croix River between Hudson and North Hudson. The river winds through varied terrain, transitioning from slow-moving upper reaches through the Cylon Marsh State Wildlife Area to a more dynamic character as it passes communities including Deer Park, New Richmond, Boardman, and Burkhardt.

The river's most dramatic features concentrate in its lower section within Willow River State Park, where it has carved a stunning 200-foot gorge through limestone bedrock. Willow Falls, the park's namesake cascade, creates a spectacular series of drops that were once blocked by dams but have been restored to their natural state.

The river historically powered lumber and flour mills in the late 1800s, with three dams constructed to harness its energy. Two of these—Willow Falls Dam (removed 1992) and Mounds Dam (removed 1998)—have been eliminated, restoring four miles of trout stream habitat.

Paddling opportunities exist primarily on the navigable section north of Burkhardt, with river access available at New Richmond's Sather Nature Center. The segment from Highway 64 to Willow River State Park offers Class I-II water, with Class II rapids appearing below County Road E. This approximately 6-mile stretch features a 200-foot elevation drop and includes the dramatic approach to Willow Falls. The river is best paddled during spring and early summer when flows are adequate, as it can become too shallow during late summer months.

The restored river now supports populations of rainbow, brook, and brown trout, with the Wisconsin Department of Natural Resources stocking these species at various locations. Fish surveys following dam removals have documented healthy populations, including 13-inch brown trout and 15-inch rainbow trout.

The river corridor, positioned along the Mississippi Flyway, provides habitat for over 200 species of birds. Within Willow River State Park's 2,800 acres, visitors encounter prairie remnants, cold springs emerging from the limestone, and the restored gorge ecosystem. Little Falls Lake, formed by the remaining Little Falls Dam near the river's mouth, provides additional recreational opportunities.

The park, one of Wisconsin's most visited with nearly one million annual guests, offers extensive hiking trails, camping facilities, and a nature center. The river's proximity to the Twin Cities metropolitan area—just across the border from Minneapolis-St. Paul—makes it an accessible destination for urban populations seeking natural experiences. Historical significance includes the river's role in 19th-century industrial development and its transformation from a working waterway to a protected recreational resource demonstrating successful ecological restoration.