Rivers, as is well known, are subject to floods when the volume of water is enormously increased and can no longer be contained in the ordinary channel, but spreads out over the level ground on each side. By this spreading, which may be for many miles in both directions, the velocity of the water is much diminished, and over the flooded area (flood plain) large quantities of material are thrown down, while the unchecked velocity in the channel may cause a scouring and deepening there or, under other conditions, the channel and flood plain may both be built up, especially if the river flow through a slowly subsiding region. The nature of the material deposited over the flood plain will depend on the character and swiftness of the flooded stream, and varies from the coarsest gravel to the finest silt. The latter is more usual, for the flood plain is widest along the lower course of the river. Flood-plain deposits attain great importance in interior basins which have no drainage outlet and consequently retain all the material which is washed into them from the surrounding mountains by the rain or rivers.
In such basins the rivers end in salt lakes or die out in the sands, but at intervals, it may be only rarely or during an annual wet season, the rivers are flooded and immense areas of the desert are converted into shallow seas. Near the mountains is formed a fringe of alluvial cones, which may coalesce into a continuous belt, and over the central parts of the basin is spread the finer material in even and regular stratification. The wind may carry away all this material and remove it beyond the limits of the basin, leaving only stony wastes, or the deposits may accumulate to a great thickness, according to circumstances.
Fig. 95. - Flood plain, Genesee River, N.Y. (Photograph by van Ingen).
Even in climates of heavy rainfall great interior basins, due to downwarping, are found, and though they drain to the sea, they may become filled to great depths with river deposits. The interior of South America, drained by the Orinoco, Amazon, Paraguay, etc., is an example of such basins where river deposition is actively progressing on a very large scale.
In climates with abundant rainfall (pluvial climates) the flood plains of rivers are covered with vegetation which protects the flood deposits, but in arid climates the flood plains are bare of vegetation for the whole or most of the year, and the river deposits are exposed to the sun. Great areas of mud and silt, thus exposed, shrink in drying and crack in deep fissures, which enclose polygonal areas? as may be seen in any mud puddle which is drying in the sun. The cracks thus formed are called sun or mud cracks and may be preserved indefinitely in rocks which are formed from flood-plain accumulations. In such deposits there is apt to be a difference in the material thrown down in the earlier and later stages of the flood, because of the difference in the velocity with which the waters move. After the river ceases to rise, the water over the flood plain becomes almost stagnant and lays down very fine material, which thus forms the cracked surface. When the next flood arrives, it carries coarser material, frequently sand, which fills up the cracks and thus preserves them. Mud cracks are formed under other conditions, as will be seen in the following pages, but probably nowhere on such an extensive scale as on the flood plains of rivers which flow through arid regions.
Footprints of land animals may be preserved in the same manner by being baked hard in the hot sun and then buried under the deposits of the next flood. Such cracks and footprints, and even the impression of raindrops, are frequently found in the rocks and give valuable assistance in determining the conditions under which those rocks were formed.
Fig. 96. - Sun cracks in Newark shale, about 14 natural size.
In ancient flood-plain deposits, which the rivers that made them have long since deserted, the old channels are indicated by coarser material, sands and gravel cemented into sandstone and conglomerate. When these old channels are plotted on a map, their sinuous course, great length in proportion to width, and their ramifying tributaries clearly mark them out as the records of an ancient system of drainage. Such channels and the accompanying broad and regularly stratified flood-plain deposits cover very extensive areas in South Dakota, Nebraska, and others of the Western States.