Ethnographers (people who study cultural societies) group people by the languages they used or were likely to be used by a single group at one time. Many of the sites were of Mandan, Hidatsa, and Sahnish origins. In the 1995 the North Dakota Historical Society completed the Missouri Trench National Historical Landmark Theme Study, that summarized the archeological investigation of the Missouri River area from southern South Dakota through North Dakota to Montana. Many of these sites, although collapsed and abandoned long before, were excavated along the Missouri River during the 1950’s and 1960’s. Dates of migrations all Three Tribes have been, determined by archeological investigation of village sites constructed along the Missouri and elsewhere. According to anthropologists, the Sahnish people lived in an area that extended from the Gulf of Mexico, across Kansas, Nebraska, South Dakota.

The Mandan and Hidatsa believe they were, created in this area and have always lived here. The Hidatsa moved from central Minnesota to the eastern part of North Dakota near Devils Lake, and moved to join the Mandan at the Missouri River about 1600 A.D. One theory is the Mandan moved from the area of southern Minnesota and northern Iowa to the plains in South Dakota about 900 A.D., and slowly migrated north along the Missouri River to North Dakota about 1000 A.D. The Mandan and Hidatsa people were originally woodland people who moved to the plains at various times. Historians document the first tribe, to occupy this area was the Mandan with the Hidatsa, and the Sahnish moving up the river later. The Mandan, Hidatsa, and Sahnish live in the Missouri River area.

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