William O'Brien State Park is a 1,520-acre state park on the St. Croix River north of Marine on St. Croix, valued for outdoor recreation and for its compact display of the valley’s ecological diversity, from glacial moraine and riparian ground to oak savanna, woods, and bogs. The area was first inhabited by Dakota and Ojibwe peoples, then reshaped by the fur trade and heavy logging. Much of the land later became William O'Brien’s estate, and his family’s donation in 1945 led to the park’s creation two years later. Expanded repeatedly, it has remained one of Minnesota’s five most-visited state parks.