6o4 THE POPULAR SCIENCE MONTHLY. 



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 edsje 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 wade 

 valleys prevail. But there are, of course, many deviations from this 

 normal type. The fires may have destroyed the cottonwoods and wil- 

 lows that line the river and occupy most of its bed, and an unbroken 

 plain may extend down to the sand-bars uj^on its banks. These sand- 

 bars 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 pro- 

 cess, 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 hun- 

 dreds 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 



