On June 3, 1805 Lewis and Clark were following the Missouri River in search of a route to the Pacific Ocean. They camped at the mouth of this river just east of here. They traveled northward to the mouth of the Milk River. At a large fork further up, Lewis found he was too far north for this to be the route they were seeking. Until exploration proved otherwise, most members of the party believed this river to be the main channel of Missouri.
He named it Maria's River after Maria Wood, a cousin with whom he thought he might be in love, ‘although its muddy waters... hardly compare with the pure celestial virtues and amiable qualities of that lovely fair one.' Later, the apostrophe was dropped and the name became Manias River.
On his return trip from the coast in 1806, Captain Lewis explored the Marias almost to its source.
In the fall of 1831 James Kipp of the American Fur Company built Fort Piegan at the mouth of the river, as a trading post for the Blackfeet Indians, and acquired 2,400 beaver ‘plews' or skins by trade during the first 10 days. In 1832 the post was abandoned and the Indians burned it.