To such a moving, overpowering liquid mass, there are still, below the Missouri, rocky banks, and occasionally isolated cliffs, to stand up and resist its sweep; but its alluvions become wider and deeper opposite to these rocky barriers. Its bends stretch over greater distances, and its channel grows deeper at every accession of a tributary. The chief of these, after passing the Missouri, are from the Rocky Mountains and Ozark slopes, the St. Francis, White, Arkansas, and Red Rivers; and from the other bank the Kaskaskia, the Ohio, Wolf, and Yazoo. It is estimated to flow twelve hundred miles below the Missouri. Its width is about one mile opposite St. Louis. It is narrower but more than twice the depth at New Orleans, and yet narrower, because more divided, at its embouchure at the Balize, where a bar prevents ships drawing over eighteen feet of water from entering. No attempt has heretofore been made to determine the elevation of that part of the American continent which gives rise to the Mississippi River. From the observations made on the expedition, the elevation is confessedly less than would _à priori_ be supposed. If it is not, like the Nile, cradled among mountains, whose very altitude and position are unknown, there is enough of the unknown about its origin to wish for more information. Originating on a vast continental plateau, or watershed, the superabundance of its waters are drained off by the three greatest rivers of North America, namely, the St. Lawrence, the Nelson's rivers of Hudson's Bay, and the Mississippi. Yet the apex of this height of land is moderate, although its distance from the sea at either point is immense. From the best data at command, I have endeavored to come at the probable altitude of this plateau, availing myself at the same time of the judgment of the several members of the expedition. Taking the elevation of Lake Erie above tide-water, as instrumentally determined, in the New York surveys, as a basis, we find Lake Superior lying at an altitude of six hundred and forty-one feet above the Atlantic. From thence, through the valley of the St. Louis, and across the Savanna summit, to the Mississippi, at the confluence of the Sandy Lake River, estimates noted on the route, indicate an aggregate rise of four hundred and ninety feet. The ascent of the river, from this point to Cass Lake, is estimated to be one hundred and sixty-two feet; giving this lake an aggregate elevation of thirteen hundred and ninety-three feet above the Atlantic. Barometrical admeasurements made in 1836, by Mr. Nicollet, in the service of the United States Topographical Bureau, place the elevation of this lake at fourteen hundred and two feet above the Gulf of Mexico,[79] being just twelve feet above these early estimates. The same authority estimates its length from the Balize, at twenty-seven hundred and fifty miles. Its velocity below Cass Lake may be estimated to result from a mean descent of a fraction over five inches per mile.