Fig. 180.—Comparison of the cross sections of river valleys for the different stages of the erosion cycle.
The river cross sections of successive stages.—To the successive stages of a river’s life it has been common to carry over the names from the well-marked periods of a human life. If neglecting for the moment the general aspect of the upland, we fix our attention upon the characteristic cross sections of the river valley, we find that here also there are clearly marked characters to distinguish each stage of the river’s life ([Fig. 180]). In infancy the steep, narrow, and sharp-angled cañon is a characteristic; with youth the wider V-form has already developed; in adolescence the angles of the cañon are transformed into well-rounded shoulders, and the valley broadens so as in the lower reaches to lay down a flood plain; in maturity the divides and the double curves of the line of beauty appear; while in the decline of old age the valleys are extremely broad and flat and are floored by an extended flood plain.
The entrenchment of meanders with renewed uplift.—Upon the reduced grades which are characteristic of the declining stage of a river’s life, the current has little power to modify the surface configuration. On the old land of this stage a renewed uplift starts the streams again into action. This infusion of driving power into moving water, regarded as a machine capable of accomplishing certain work, is like winding up a clock that has run down. Once more the streams acquire a velocity sufficient to enable them to cut their valleys into the land surface, and so a new erosional cycle may be inaugurated upon the old land surface—the peneplain. After such an uplift has been accomplished and the rivers have sunk their early valleys within the new upland, we may look out from this now elevated surface and the eye take in but a single horizontal line, since we view the plain along its edge.
Fig. 181.—The Beavertail Bend of the Yakima Cañon in central Washington (after George Otis Smith).
By the uplift the meanders of the earlier rivers may become entrenched in the new upland, the wide lobes of the individual meanders being now separated by mountains where before had been plains of silt only. The New River of the Cumberland plateau and the Yakima River of central Washington ([Fig. 181]) furnish excellent American examples of intrenched meanders, as the Moselle River does in Europe. Upon the course of the latter river near the town of Zell a tunnel of the railroad a quarter of a mile in length pierces a mountain in the neck of a meander lobe in which the river itself travels a distance of more than six miles in order to make the same advance. The Kaiser Wilhelm tunnel in the same district penetrates a larger mountain included in a double meander of the river. Although intrenched, river meanders are still competent to scour and so undermine the outer bank, and with favoring conditions they may by this process erode extended “bottoms” out of the plateau. (See Lockport quadrangle, U. S. G. S.)
The valley of the rejuvenated river.—Whenever a new uplift occurs before an erosional cycle has been completed, the rivers become intrenched, not in a peneplain, but in the bottoms of broad valleys. The sweeping curves which characterize mature landscapes may thus be brought into striking contrast with the straight lines of youthful cañons which with V-sections descend from their lowest levels ([Fig. 182]). The full cross section of such a valley shows a central V whose sharp shoulders are extended outward and upward in the softened curves of later erosion stages.
Fig. 182.—A rejuvenated river valley (after a photograph by Fairbanks).