The Elements of Geology. William Harmon Norton
flood plain (Fig. 71) shows that it is highest next the river, sloping gradually thence to the valley sides. These wide natural embankments are due to the fact that the river deposit is heavier near the bank, where the velocity of the silt-laden channel current is first checked by contact with the slower-moving overflow.
Fig. 72. Waste-filled Valley and Braided Channels of the Upper Mississippi
Thus banked off from the stream, the outer portions of a flood plain are often ill-drained and swampy, and here vegetal deposits, such as peat, may be interbedded with river silts.
A map of a wide flood plain, such as that of the Mississippi or the Missouri (Fig. 77), shows that the courses of the tributaries on entering it are deflected downstream. Why?
The aggrading streams by which flood plains are constructed gradually build their immediate banks and beds to higher and higher levels, and therefore find it easy at times of great floods to break their natural embankments and take new courses over the plain. In this way they aggrade each portion of it in turn by means of their shifting channels.
Braided channels. A river actively engaged in aggrading its valley with coarse waste builds a flood plain of comparatively steep gradient and often flows down it in a fairly direct course and through a network of braided channels. From time to time a channel becomes choked with waste, and the water no longer finding room in it breaks out and cuts and builds itself a new way which reunites down valley with the other channels. Thus there becomes established a network of ever-changing channels inclosing low islands of sand and gravel.
Fig. 73. Terraced Valley of River in Central Asia
Fig. 74. Terraces carved in Alluvial Deposits
Which is older, the rock floor of the valley or the river deposits which fill it? What are the relative ages of terraces a, b, c, and e? It will be noted that the remnants of the higher flood plains have not been swept away by the meandering river, as it swung from side to side of the valley at lower levels, because they have been defended by ledges of hard rock in the projecting spurs of the initial valley. The stream has encountered such defending ledges at the point marked d
Fig. 75. River Terraces of Rock covered with Alluvium
c, recent flood plain of the river. To what processes is it due? Account for the alluvium at a and b and on the opposite side of the valley at the same levels. Which is the older? Account for the flat rock floors on which these deposits of alluvium rest. Give the entire history which may be read in the section
Terraces. While aggrading streams thus tend to shift their channels, degrading streams, on the contrary, become more and more deeply intrenched in their valleys. It often occurs that a stream, after having built a flood plain, ceases to aggrade its bed because of a lessened load or for other reasons, such as an uplift of the region, and begins instead to degrade it. It leaves the original flood plain out of reach of even the highest floods. When again it reaches grade at a lower level it produces a new flood plain by lateral erosion in the older deposits, remnants of which stand as terraces on one or both sides of the valley. In this way a valley may be lined with a succession of terraces at different levels, each level representing an abandoned flood plain.
Fig. 76. Development of a Meander
The dotted line in a, b, and c shows the stage preceding that indicated by the unbroken line
Meanders. Valleys aggraded with fine waste form well-nigh level plains over which streams wind from side to side of a direct course in symmetric bends known as meanders, from the name of a winding river of Asia Minor. The giant Mississippi has developed meanders with a radius of one and one half miles, but a little creek may display on its meadow as perfect curves only a rod or so in radius. On the flood plain of either river or creek we may find examples of the successive stages in the development of the meander, from its beginning in the slight initial bend sufficient to deflect the current against the outer side. Eroding here and depositing on the inner side of the bend, it gradually reaches first the open bend (Fig. 76, a) whose width and length are not far from equal, and later that of the horseshoe meander (Fig. 76, b) whose diameter transverse to the course of the stream is much greater than that parallel with it. Little by little the neck of land projecting into the bend is narrowed, until at last it is cut through and a “cut-off” is established. The old channel is now silted up at both ends and becomes a crescentic lagoon (Fig. 76, c), or oxbow lake, which fills gradually to an arc-shaped shallow depression.
Fig. 77. Map of a portion of the Flood Plain of the Missouri River
Each small square represents one square mile. How wide is the flood plain of the Missouri? How wide is the flood plain of the Big Sioux? Why is the latter river deflected down valley on entering the flood plain of the master stream? How do the meanders of the two rivers compare in size? How does the width of each flood plain compare with the width of the belt occupied by the meanders of the river? Do you find traces of any former channels?
Flood plains characteristic of mature rivers. On reaching grade a stream planes a flat floor for its continually widening valley. Ever cutting on the outer bank of its curves, it deposits on the inner bank scroll-like flood-plain patches (Fig 60). For a while the valley bluffs do not give its growing meanders room to develop to their normal size, but as planation goes on, the bluffs are driven back to the full width of the meander belt and still later to a width which gives room for broad stretches of flood plain on either side (Fig. 77).
Usually a river first attains grade near its mouth, and here first sinks its bed to near baselevel. Extending its graded course upstream by cutting away barrier after barrier, it comes to have a widened and mature valley over its lower course, while its young headwaters are still busily eroding their beds. Its ungraded branches may thus bring down to its lower course more waste than it is competent to carry on to the sea, and here it aggrades its bed and builds a flood plain in order to gain a steeper gradient and velocity enough to transport its load.
As maturity is past and the relief of the land is lessened, a smaller and smaller load of waste is delivered to the river. It now has energy to spare and again degrades its valley, excavating its former flood plains and leaving them in terraces on either side, and at last in its old age sweeping them away.
Fig. 78. Alluvial Cones, Wyoming
Alluvial cones and fans. In hilly and mountainous countries one often sees on a valley side a conical or fan-shaped deposit of waste at the mouth of a lateral stream. The cause is obvious: the young branch has not been able as yet to wear its bed to accordant level with the already deepened valley of the master stream. It therefore builds its bed to grade at the point of juncture by depositing here its load of waste—a load too heavy to be carried