The Mississippi river may be regarded as occupying the lowest part of the valley, from its great estuary, the Gulf of Mexico, to its confluence with the Missouri and Illinois. Thence to Lake Michigan, {264} the immediate valley of the Illinois is to be viewed as the lowest part of the great valley under consideration. This conclusion necessarily results from an attentive consideration of the characters of the three rivers just mentioned. If the inclinations of the plains down which these rivers respectively flow, be in any degree proportionate to the velocities of their currents, the plain of the Illinois will be found to have far the least inclination, inasmuch as the velocity of its current is not more than one-fourth of that of either of the others. But in order to have a more distinct view of the matter, let us assume the parallel of latitude intersecting the Illinois at its head, or point of confluence of the Kankakee and Des Plaines rivers, and suppose a vertical section cut in the direction of the parallel. Such a section would intersect the Missouri at the distance of nearly seven hundred miles from its mouth, the Mississippi at about two hundred and sixty, and the Illinois at two hundred and fifty from the same point. Hence, allowing that the plains of each have the same inclination, the point of intersection on the Missouri would be at a greater elevation than that on the Mississippi, and that on the Illinois would be less elevated than either. But the difference of inclination in these plains is manifest, not only from the comparative velocities of the several streams alluded to, but from the circumstance, that the Illinois is destitute of any considerable rapids throughout its whole course, whereas the Mississippi, in addition to a current uniformly more rapid, is hurried down the De Moyen rapids, eleven miles in length; and the Missouri, without a perversion of terms, may be denominated a rapid throughout the distance above specified. By a similar course of reasoning it may also be made to appear, that the assumed point of intersection on the Illinois is less elevated than any other point in the same parallel of latitude between that river and Lake Erie, and even that it is somewhat lower than the surface of {265} the lake itself; for the aggregate descent, from the surface of Lake Michigan to the point under consideration, is evidently greater than from the surface of the same lake to that of Lake Erie; or, in other words, the descent of the Des Plaines, from Chicago to its confluence with the Kankakee, is greater by a few feet than that of the stream uniting Lakes Huron and Erie.