It is by no means intended to follow these initial facts by recitals of the progress of the subsequent local discoveries in the Mississippi valley, which were made respectively under French, British, and American rule. Sufficient is it, for the present purpose, to say, that the thread of the discovery of the Mississippi, north and west of the points named, was not taken up effectively, till the acquisition of Louisiana. Mr. Jefferson determined to explore the newly acquired territories, and directed the several expeditions of discovery under Lewis and Clark, and Lieut. Z. M. Pike. The former traced out the Missouri to its sources, and followed the Columbia to the Pacific; while the latter continued the discovery of the Mississippi river above St. Anthony's falls where Hennepin, and perhaps Carver, had respectively left it. The map which Pike published in 1810 contained, however, an error of a capital geographical point, in regard to the actual source of the Mississippi. He placed it in Turtle lake, at the source of Turtle river of upper _Lac Cedre Rouge_, or Cass lake, which lies in the portage to Red lake of the great Red River of the North, being in the ordinary route of the fur trade to that region.
In 1820, Mr. Calhoun, who determined to erect a cordon of military posts to cover the remotest of the western settlements, at the same time that he despatched Major Long to ascend to the Yellowstone of the Missouri, directed the extreme upper Mississippi to be examined and traced out to its source. This expedition, led by Gov. Cass, through the upper lakes, reached the mouth of Turtle river of the large lake beyond the upper cataract of the Mississippi, which has since borne the name of the intrepid leader of the party. It was satisfactorily determined that Turtle lake was not the source, nor even one of the main sources, of the Mississippi; but that this river was discharged, in the integrity of its volume, into the western end of Cass lake. To determine this point more positively, and trace the river to its source, another expedition was organized by the Department of War in 1832, and committed to me. Taking up the line of discovery where it had been left in 1820, the river was ascended up a series of rapids about forty miles north, to a large lake called the Amigegoma; a few miles above which, it is constituted by two forks, having a southern and western origin, the largest and longest of which was found[2] to originate in Itasca lake, in north latitude 37° 13'--a position not far north of Ottertail lake, in the highlands of HAUTEUR DES TERRES.