Page:Popular Science Monthly Volume 25.djvu/615

Rh on the globe does there exist a better example from which to study these principles of surface erosion than in the Upper Missouri River system. This will be best seen when we consider a little more closely the proper bed of the river.

For two or three months of each year, between March and June, the river is high, and this state of high water is tolerably uniform from year to year, so as to be in a manner normal. Supplied chiefly from melting snows at greater and greater altitudes as the season advances, it persists with only slight fluctuations until the supply is exhausted, when the water slowly falls to its low-water mark, where it remains the rest of the year with only a small amount of variation, because the rainfall is so light. There thus exist two distinct and somewhat uniform conditions of the water, each occupying its regular part of the year. Owing to this regularity of high water, the maximum bed of the river produced by it is somewhat uniform and clearly marked, while it also bears a tolerably uniform relation to the deeper channel represented by the low-water state. Examined in time of low water, this river-bed seems to be three or four times as wide as the river itself. The stream, then, usually flows in serpentine curves which cross and recross the bed. The bed itself is also crooked much as is the channel, only its curves are as much longer as it is wider. The whole valley is usually also winding with much more ample curves, and the river-bed crosses and recrosses it in a manner similar to that in which the channel crosses and recrosses the bed. The river itself generally hugs one of the banks of the bed, but it is always at a curve, or bend, such as will tend to wear the bed on the convex side and thus render it more crooked. The distance traversed by the channel in crossing from one side of the bed to the other is small, compared with the distance traversed while in close contact with the bank of the river-bed, which it is perpetually extending into the general valley. The reason why it does not constantly grow wider is, that on the abandoned side the surface is being constantly raised by deposits of material which the water, more sluggish on this side, can no longer hold. As the river shifts its position in the valley, a strip of land of varying width is formed each year to be gradually assimilated to the permanent valley (see Diagram No. II).

If, now, we take the more general view and regard the entire valley as one homogeneous product, we can better study the process by which it has been formed. Beginning with the channel of the river we shall find that, except where crossing the bed, its cross-section presents a figure approaching more or less closely to a right-angled triangle with the right angle at the bottom, or deepest place. One side will then be formed by a steep wall or bank, which may become perpendicular above the surface of the water, but is not usually so below. The other side of the triangle represents the general bottom of the river, which gradually grows more shallow toward the remote side of the river-bed.