The _Yellowstone_ is the largest of these tributaries. Its sources are in the Rocky Mountains, near those of the Missouri and the Platte, and it may be navigated in canoes almost to its head. It runs first through a mountainous country, but in many parts fertile and well timbered; it then waters a rich, delightful land, broken into valleys and meadows, and well supplied with wood and water, till it reaches near the Missouri open meadows and low grounds, sufficiently timbered on its borders. In the upper country its course is said to be very rapid, but during the two last and largest portions, its current is much more gentle than that of the Missouri. On the sand-bars and along the margin of this river grows the small leafed willow; in the low grounds adjoining are scattered rose bushes three or four feet high, the red-berry, service-berry and redwood. The higher plains are either immediately on the river, in which case they are generally timbered, and have an undergrowth like that of the low grounds, with the addition of the broad leafed willow, gooseberry, purple currant and honeysuckle; or they are between the low grounds and the hills, and for the most part without wood, or any thing except large quantities of wild hyssop, a plant which rises to the height of about two feet, and, like the willow of the sand-bars, is a favorite food of the buffalo, elk, deer, grouse, porcupine, hare, and rabbit.[8] The _Platte_ is in fact much more rapid than the Missouri, and drives the current on the northern shore, on which it is constantly encroaching. At some distance below the confluence, the Missouri is two miles wide, with a rapid current of ten miles an hour in some parts, the rapidity increasing as we approach the mouth of the Platte; the velocity of which, combined with the vast quantity of rolling sands which are drifting from it into the Missouri, renders it completely unnavigable, unless for flats or rafts, though the Indians pass it in small flat canoes made of hides, and the Americans have contrived to navigate it by means of keel-boats, which, being constructed to draw but little water, and built upon a small keel, are remarkably well adapted for sailing up rapid and shallow streams. The Platte runs a course of fifteen degrees of longitude, from west to east, or more than eight hundred miles. The _Kansas River_ has a considerable resemblance to the Missouri, but its current is more moderate, and its water less turbid, except at times of high floods. Its valley, like that of the Missouri, has a deep and fertile soil, bearing forests of cotton-wood, sycamore, and other trees, interspersed with meadows; but in ascending, trees become more and more scattered, and at length disappear almost entirely, the country at its sources being one immense prairie.