Of the many tributaries of the Missouri the most important is the Yellowstone, which rivals in size the main stream and joins it nearly eight hundred miles below the Three Forks. Like the Missouri it finds its source in and around that now famous region, where Nature has lavished without stint her most marvelous handiwork, and which the government of the United States has set apart for the common enjoyment of the people. Both the Missouri and the Yellowstone rivers, in the upper portions of their course, flow over immense cascades and rapids which have become well known among the cataracts of the globe. The Great Falls of the Missouri are located near the modern city of the same name. They comprise several cataracts and rapids, the highest perpendicular distance being 87 feet, and the total fall over 500 feet. The falls of the Yellowstone are within the National Park, the highest perpendicular distance being 310 feet, and the total descent from Yellowstone Lake to below the first cañon near Livingston, Mont., being about 3200 feet. [Sidenote: ALLUVIAL CHARACTER.] Upon emerging from the foothills of the mountains, both streams begin to assume that peculiar character which distinguishes them throughout the rest of their course to the sea. They flow through alluvial bottoms, built up of the detritus from the highlands and mountains, until the present bed of the river is in most places fifty to a hundred feet above the original bed in the solid rock. The usual characteristics of an alluvial river are here found in their highest development--a muddy current, freshly formed islands, sandbars innumerable, an unstable channel, and a shifting bed which is never in the same place for two years in succession. Among the most striking phenomena of a river like the Missouri is the constant change that is going on in the location of its channel. This seems to be in some places a periodical matter. The forces of the river get to working on particular lines and push their devastations for many years in one general direction. Being finally arrested by some insurmountable obstacle, or turned, it may be, by trifling causes, they work in another direction, and invade lands which have enjoyed long immunity, and upon which the cottonwood, walnut, and cedar have attained mature growth. [Illustration: A NEW “CUT OFF” Old course of river toward background Old course of river from background] [Sidenote: A WINDING WATERWAY.]