In western North Dakota, right before you cross into Montana, the Yellowstone River joins the Missouri River from the south-west. On the journey westward Lewis and Clark passed the Yellowstone by, following Thomas Jefferson's order to follow the Missouri to its source. On the return journey Clark led a small exploration down the Yellowstone River, and the entire Corps of Discovery reunited at the confluence with the Missouri.
At that location there are actually a few places to drop in and visit.
At the Missouri-Yellowstone Confluence Interpretive Center there's an overlook where you can see the Yellowstone River come in straight towards you, joining the Missouri River flowing from right to left.
Fort Buford State Historic Site is located only half a mile eastward from that spot. It was a very large army post, established to protect settlers, and became a major supply depot for military field operations. It is perhaps most well known for being the place where Sitting Bull surrendered to the US Army. Managed by the state of North Dakota, the site has not received the attention that Fort Union Trading Post has, and has not been restored to nearly the same extent.
Located another two or three miles eastward, Fort Union Trading Post is a National Historic Site managed by the National Park Service. The fort was not a military site; it was a privately built and operated center for trading with the indigenous people of the region for furs.
It also was enormous.
Over the front gate.
A window next to the gate was used when the gates were shut.
A fur press was situated right outside of the front gate. Not far below was a dock from which goods were loaded onto boats traveling up and down the Missouri River. The Missouri has shifted away from the fort since that time. Here is a photo taken with my back to the gate, the channel cut by the Missouri is the the black strip that appears from side to side.