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