The Treaty of Traverse des Sioux in 1851 gave all of the Wahpeton and Sisseton Sioux ( upper Sioux ) land west of the Mississippi River to the U.S. government . The Treaty of Mendota that same year ceded the Mdewakanton and Wahpekute Sioux ( lower Sioux ) land in southern Minnesota , requiring relocation to an area near modern Morton . Both treaties , however , was amended to during the ratification process to eliminate the explicit guarantees of lands retained by the tribes . Additionally much of the promised payments was never delivered in part ostensibly because of debts owed by the Sioux to the fur traders .