Fort Union Trading Post National Historic Site is a partial reconstruction of the most important fur trading post on the upper Missouri River from 1829 to 1867. The fort site is about two miles from the confluence of the Missouri River and its tributary, the Yellowstone River, on the Dakota side of the North Dakota/Montana border, 25 miles from Williston, North Dakota.

Williston is a city in and the county seat of Williams County, North Dakota, United States. The 2020 census gave its population as 29,160, making Williston the sixth-most populous city in North Dakota. The city's population nearly doubled between 2010 and 2020, due largely to the North Dakota oil boom.