The first Fort Assiniboine in Montana was a temporary post established in 1834.
It was located at a point some distance above Fort Union, North Dakota where the steamer Assiniboine ran aground in the summer of 1834. Owned by the American Fur Company, the Assiniboine was put into service by the company in 1833. The boat was forced to remain through the winter at the mouth of the Poplar River, where the temporary post was established.
The crew of the steamer Assiniboine built the post which was occupied that winter by Daniel Lamont, whose party secured many pelts and meat in trade with the area Indians.
The temporary post was abandoned in the spring of 1835 and burned by the Indians. During the steamboat’s return to St. Louis, Missouri a fire broke out, and the vessel and its cargo of furs and skins as well as the natural history collection of Prince Maximillian were all destroyed. Initial plans to make the site an outpost of Fort Union were abandoned in April 1835.