Page:Popular Science Monthly Volume 25.djvu/620

604 gradually obliterated, and the valley seems to slope away with a gentle upward curve to the foot of the lowest hills. As soon as we are fairly out of the present river-bed the little willow gives way entirely to a large one (Salix cordata, Marshall), popularly known as the diamond willow. This species often grows very dense and in large clumps, forming an almost impenetrable thicket. It monopolizes the soil, and renders approach to the river difficult. It is at a point still more remote that the growth of cottonwoods begins, and these may form a belt half a mile to a mile in width. From the outer edge of these cotton-wood-forests the plain commences, and stretches back, not only across the remainder of the valley, but far away in an uninterrupted sea of grass, until another river system is reached (see Diagram No. III).

Such, in its general outline, may be conceived to be the normal character of the Missouri and Yellowstone Rivers after they pass the mountainous part of their course and enter the portion where wide valleys prevail. But there are, of course, many deviations from this normal type. The fires may have destroyed the cottonwoods and willows that line the river and occupy most of its bed, and an unbroken plain may extend down to the sand-bars upon its banks. These sandbars may form islands around which quite brisk currents flow even in the dry season. Sometimes, as at Spread-Eagle Bar, on the Missouri, a number of such bars occur, with shallow currents between them, wearing them away along clean-cut faces, and shifting their position from place to place, giving great width to the river. Large islands are often formed, which have accidentally escaped the denuding process, and, being beyond the reach of fires, become covered by a heavy growth of timber. Sometimes the bed of the river lies between two similar high banks, more or less central in the valley, showing that, instead of continuing to approach the bluffs on one side, its erosive action has from some cause been arrested or reversed. In such cases there is occasionally found a nearly equal current against each bank, but usually, even here, the main channel is snug against one of the walls, which it is rapidly carrying away, while the opposite wall has an ancient or obsolete appearance, with shoals or bars at its base. Of course, the entire configuration of the country is modified by the occurrence at short intervals of tributary streams with their valleys. These streams, in spring, contain considerable water; but, throughout the summer and autumn, most of them are perfectly dry, at least at their point of junction with the river, whatever water they receive from rains or springs being evaporated in their passage across the arid plains. One is greatly astonished to find no water, or only a rivulet, at the mouths of what are called rivers, and which drain hundreds of square miles of country.

But the Missouri and Yellowstone themselves never go dry. They are large and rapid streams at the dryest seasons of the year, and their turbid waters surge past like a resistless tide. They wear down their