Mandan are a Native American tribe that historically lived along the banks of the Missouri River and its tributaries, the Heart and Knife Rivers in present-day Dakotas. Unlike many neighboring tribes in the Great Plains region, the Mandan practiced agriculture and established permanent villages. These villages were composed of round earthen lodges surrounding a central plaza. In addition to farming, the Mandan gathered wild plants and berries and hunted buffalo. In contrast to the other tribes in the region, which led a nomadic existence following herds of buffalo, the Mandan developed a religious ceremony known as the Okipa, with the dual purpose of attracting buffalo and renewing the world for another year.
Archaeological research suggests the people migrated from the Ohio River valley to the banks of the Missouri River. They were first encountered by Europeans along the Missouri in 1738. Their friendliness and willingness to trade brought many traders and fur trappers to their villages over the next century. By the turn of the 19th century, because of attacks by neighboring tribes and epidemics of smallpox and whooping cough, the numbers of the Mandan had diminished dramatically. Beginning in 1837, a major smallpox outbreak reduced the number of Mandan to approximately 125. With such meager numbers, the Mandan banded together with two neighboring tribes, the Arikara and Hidatsa.
In an effort to establish good relations, the U.S. government founded the Fort Berthold Agency to care for the combined tribes. The Agency soon set up the Fort Berthold Reservation. With the 1934 Indian Reorganization Act, the Mandan officially merged with the Hidatsa and the Arikara into the ” Three Affiliated Tribes,” known as the Mandan, Hidatsa and Arikara Nation. About half of the Mandan still reside in the area of the reservation, the rest residing around the United States and in Canada.
(Canadian Source) https://www.cs.mcgill.ca/~rwest/wikispeedia/wpcd/wp/m/Mandan.htm