Before shipping goods down the Mississippi river. what river did most cart trains follow?
The West Plains Trail had originated with Native Americans, and before the ox cart traffic it connected the fur-trading posts of the Columbia Fur Company.  In fact, that company introduced the Red River ox cart to haul its furs and goods. It also developed the trails, and by the early 1830s, an expedition from the Selkirk settlement driving a flock of sheep from Kentucky to the Assiniboine found the trail to be well-marked.From the Red River Settlement, the trail went south upstream along the Red River's west bank to Pembina, just across the international border. Pembina had been a fur-trading post since the last decade of the eighteenth century.  From there, some traffic continued south along the river, but most cart trains went west along the Pembina River to St. Joseph near the border and then south, or else cut the corner to the southwest in order to intercept the southbound trail from St. Joseph. This north-south trail paralleled the Red River about thirty miles (50 km) to the west.  By staying on the uplands west of the Red River, this route avoided crossing the tributaries of that river near their confluences with the Red, and also kept out of the swampy, flood-prone, and mosquito-ridden bottomlands in the lakebed of Glacial Lake Agassiz which the river drained.
In what is now southeastern North Dakota, the trail veered to the south-southeast to close with the Red River at Georgetown, Fort Abercrombie, and Breckenridge, Minnesota, all of which came into existence in consequence of the passing cart traffic.  From Breckenridge, the trail continued upstream along the east bank of the Red and Bois des Sioux Rivers to the continental divide at Lake Traverse. Some traffic went along the lakeshore through the Traverse Gap on the continental divide, then down either side of Big Stone Lake, source of the Minnesota River, while other carters took a short cut directly south from the Bois des Sioux across the open prairie through modern Graceville, Minnesota thereby avoiding the wet country in the Traverse Gap.The trail continued on intertwined routes down both sides of the valley of the Minnesota River past fur posts at Lac qui Parle and downstream locations, and the Upper Sioux and Lower Sioux Indian Agencies and Fort Ridgely, all established in the 1850s. From Fort Ridgely, the trail struck across the open prairie to the Minnesota River at Traverse des Sioux near modern-day St. Peter, Minnesota, where the furs and goods were, at first, usually transshipped to flatboats.  In later years, most cart trains crossed to the east bank and proceeded northeast along the wooded river bottoms and uplands to Fort Snelling or Mendota, where the Minnesota River joined the Mississippi.  From there furs were shipped down the Mississippi River to Saint Louis and other markets.
Minnesota River