After the Army left the area, having subdued Native Americans, the fort was used by the US as the Indian Agency for the regional Arikara, Hidatsa, and Mandan Affiliated Tribes and their reservation.
In the mid-1950s both of the former fort sites were submerged under Lake Sakakawea, created by extensive flooding of the bottomlands after the Garrison Dam was constructed on the Missouri River.
The forts were named after Italian-born Bartholomew Berthold (1780–1831),[1] a prominent merchant and fur trader of St. Louis.
The first Fort Berthold was founded in 1845 on the upper Missouri River by the American Fur Company (controlled until 1830 by John Jacob Astor).
In the 1950s, these peoples lost most of their fertile farmland, homes, and several towns they had long established in the bottomlands along the river, in addition to cemeteries.
They were forced to give up these lands to be flooded by the government's creation of Lake Sakakawea following construction of the Garrison Dam in 1953.