Earth Features and Their Meaning / An Introduction to Geology for the Student and the General Reader

by William Herbert Hobbs

Fig. 166.—Partially dissected upland (after Salisbury and Atwood).

With time the gully heads advance into the upland as they take on tributaries; and so at length they in part invest it and dissect it into numerous irregularly bounded and flat-topped tables which are separated by cañons ([Fig. 166]). At the same time the grade of the channel is becoming flatter, and its precipitous walls are being replaced by curving slopes, as will be more fully described in the sequel. It is because of this progressive reduction of grades with increasing age that the early stages of a river’s life are much the most turbulent of its history. The water then rushes down the steep grades in rapids, and is often at times opened out in some basin to form a lake where differences of uplift have been characteristic of neighboring sections. For several reasons such basins in the course of a stream are relatively short lived (Chapter XXX), and they disappear with the earlier stages of the river history.

The meshes of the river network.—From the continued throwing out of new tributaries by the streams, the meshes in the river network draw more closely together as the stages of its history advance. The closeness of texture which is at last developed upon the upland is in part determined by the quantity of rainfall, so that in New Jersey with heavy annual precipitation the meshes in the network are much smaller than they are, for example, upon the semiarid or arid plains of the western United States. Its design will, however, in either case more or less clearly express the plan of rock architecture which is hidden beneath the surface ([Chapter XVII]).

The upper and lower reaches of a river contrasted.—From the fact that the river progressively invades new portions of the upland and lays the acquired sections under more and more thorough investment, it has near its headwaters for a long time a frontier district which may be regarded as youthful even though the sections near its mouth have reached a somewhat advanced stage. The newly acquired sections of river valley may thus possess the steep grade and precipitous walls which are characteristic of early gullies and cañons and are in contrast with the more rounded and flat-bottomed sections below. Lateral streams, from the fact that they are newer than the main or trunk stream to which they are tributary, likewise descend upon somewhat steeper grades ([Fig. 167]).

Fig. 167.—Characteristic longitudinal sections of the upper portion of a river valley and its tributaries (after scaled sections by Nussbaum).

The balance between degradation and aggradation.—We have seen that the power to transport rock fragments is augmented at a most surprising rate with every increase in the current velocity. While the lighter particles of rock may be carried as high up as the surface of the water, the heavier ones are moved forward upon the bottom with a combined rolling and hopping motion aided by local eddies. Those particles which come in contact with the bottom or sides of the channel abrade its surface so as ever to deepen and widen the valley. This cutting accomplished by partially suspended débris in rapidly moving currents of water is known as corrasion and the stream is said to be incising its valley.

As the current is checked upon the lower and flatter grades, some of its load of sediment, and especially the coarser portion, will be deposited and so partially fill in the channel. A nice balance is thus established between degradation and the contrasted process known as aggradation. The older the river valley the flatter become the grades at any section of its course, and thus the point which separates the lower zone of aggradation from the upper one of degradation moves steadily upstream with the lapse of time.

The accordance of tributary valleys.—It is a consequence of the great sensitiveness of stream corrasion to current velocity that no side stream may enter the trunk valley at a level above that of the main stream—the tributary streams enter the trunk stream accordantly. Each has carved its own valley, and any abrupt increase in gradient of the side streams near where they enter the main stream would have increased the local corrasion at an accelerated rate and so have cut down the channel to the level of the trunk stream.

The grading of the flood plain.—All rivers are subject to seasonal variations in the volume of their waters. Where there are wet and dry seasons these differences are greatest, and for a large part of the year the valleys in such regions may be empty of water, and are in fact often utilized for thoroughfares. In the temperate climates of middle latitudes rivers are generally flooded in the spring when the winter snows are melted, though they may dwindle to comparatively small streams during the late summer. In the upper reaches of the river the current velocities are such that the usual river channel may carry all the water of flood time; but lower down and in the zone of aggradation, where the current has been checked, the level of the water rises in flood above the banks of its usual channel and spreads over the surrounding lowlands. As a deposit of sediment is spread upon the surface, the succession of the annual deposits from this source raises the general level as a broad floor described as the flood plain of the river.