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