RIVER WORK. In draining from the land the water carries a load of mineral matter in solu tion and in suspension. The former is mainly supplied from the underground water, though some is obtained from the river bed. The sus pended material is in part derived from the in wash of soil by the rains, in part from materials obtained by weathering of the valley sides, and in part by the direct work of the rivers, using the rock materials as tools of excavation. There is a great variation in the work which rivers are doing. Some have such a rock load, on so gentle a slope, that they cannot cut, but must build up their beds. Others are rapidly excavating their beds and have cut deep gorges and canyons, which they are still deepening. Their rate of work varies with the volume of water, the slope, the nature of the rock, and the amount and nature of the load of rock waste being transported. Since the rate of work varies with the kind of rock which is being excavated, rivers that are engaged in deepening their valleys are liable to have falls and rapids because of variations in rock texture.
One of the great geological results of this river work is the formation of valleys. Where a stream is rapidly cutting. its valley is narrow and steep-sided. Even in this case the valley is broader than the stream, partly because, by its meandering course, the river undercuts its bank.
and partly because weathering is broadening the valley. Weathering continues even after the stream has ceased its down-cutting, and therefore the valley continues to grow broader and broader, the stream removing the materials which this weathering supplies. Thus as a transporting agent rivers, in cooperation with weathering, which prepares and supplies the materials, are important factors long after they have ceased to cut directly into the rock.
In the transfer of the waste of the land to the sea some of the material halts on the way. Even the most rapid of streams, bearing the coarsest of fragments, furnish illustrations of this in their beds, in bars, and in narrow strips of de posit on their margins. The larger streams. especially near their mouths, are often bordered by flood-plains in which sediment is laid aside in flood time and in which, as the stream slowly changes its course by meandering, portions are being taken up on the side of cutting while other portions of the river load are being deposited on the opposite side. Such flood-plain deposits are built of fine-grained fragments, making a very fertile soil. See EROSION; VALLEY; FLOOD-PLAIN; DELTA ; etc.