The St. Croix begins modestly at Upper St. Croix Lake in Douglas County, Wisconsin, near Solon Springs, roughly south of Lake Superior. It flows south to Gordon, then turns southwest, joined by the Namekagon River in northern Burnett County where it broadens considerably. But the river's current channel tells only part of the story. Between 19,000 and 14,000 years ago, the Laurentide Ice Sheet's Superior Lobe retreated from this region, leaving behind reddish-brown glacial tills and creating proglacial Lake Duluth in the western Lake Superior basin. When the Brule outlet opened, a massive surge of glacial meltwater roared down the St. Croix corridor, carving the deep inner channel known as the Dalles and sculpting the giant potholes of Interstate Park between 10,800 and 10,600 years before present. When the ice retreated further and opened lower eastern outlets, the flood ceased abruptly, severing the St. Croix from the Lake Superior basin and creating the northward-flowing Bois Brule River.

Below its junction with the Namekagon, the St. Croix becomes a border river, separating Wisconsin from Minnesota for roughly 125 miles. Major tributaries feed in from both sides: the Kettle, Snake, and Sunrise Rivers from the Minnesota west bank, the Apple, Willow, and Kinnickinnic Rivers from the Wisconsin east bank. Just below Stillwater, Minnesota, the river broadens into Lake St. Croix, a wide natural impoundment that stretches south until the St. Croix finally meets the Mississippi at Prescott, Wisconsin, southeast of St. Paul. That confluence is visible and striking -- the clearer waters of the St. Croix meet the muddier Mississippi in a distinct line of contrasting color. More than a dozen bridges span the river along its length, from the Scott Bridge in Douglas County down to the Prescott Drawbridge and BNSF Railway lift bridge at the mouth.

It was along the banks of the St. Croix, in the milltown of Stillwater, that the state of Minnesota was first proposed in 1848. The river valley had drawn French fur traders after the end of the French and Indian War in 1763, followed by British traders backed by the powerful North West Company. By the mid-nineteenth century, logging had transformed the upper watershed. Sawmills lined the riverbanks, and the St. Croix carried timber downstream to feed the construction boom of a growing nation. Stillwater became a prosperous hub of that industry, and it was civic ambition born of that prosperity that led to the Stillwater Convention and the eventual creation of Minnesota Territory. The 1931 Stillwater lift bridge, connecting Stillwater to Houlton, Wisconsin, still stands as an artifact of that era, though the newer St. Croix Crossing bridge, completed in August 2017, now carries highway traffic.

The St. Croix earned federal protection early. It was among the original eight rivers designated under the National Wild and Scenic Rivers Act of 1968. The upper reaches below the St. Croix Flowage, along with the Namekagon River, became the St. Croix National Scenic Riverway. The lower stretch below the Saint Croix Falls Dam received protection as the Lower St. Croix National Scenic Riverway, encompassing the dramatic Dalles gorge near Interstate Park. Only one interruption breaks the free-flowing character: the hydroelectric Saint Croix Falls Dam at St. Croix Falls, Wisconsin, operated by Northern States Power Company. Conservation has not been without conflict. The construction of the St. Croix Crossing bridge required an amendment to the Wild and Scenic Rivers Act itself, over objections from residents, conservation groups, and the National Park Service, who cited concerns about light and noise pollution in the protected corridor.

The St. Croix today is one of the Upper Midwest's premier recreational waterways. Canoeists, kayakers, and boaters share the river with anglers pursuing walleye, smallmouth bass, and muskie. Scenic highways on both banks connect small towns known for antique shops, bed-and-breakfasts, and historical tours. Public lands line the corridor: Governor Knowles State Forest on the Wisconsin side, Wild River State Park and Saint Croix State Park on the Minnesota side, and Interstate Park straddling both states at the Dalles. The Wild Rivers Conservancy of the St. Croix and Namekagon, founded in 1911 as a volunteer citizens' group, continues to advocate for the river's ecological integrity as the official friends group of the National Scenic Riverway. From its quiet headwaters near Lake Superior to its dramatic meeting with the Mississippi, the St. Croix remains a living corridor through the heart of the northwoods.