When a stream swings around a curve, the swiftest part of the current is on the outside of the curve and the slowest on the inside. A river that is carrying about all the load that it can, on passing around a curve, is able in its outer part to carry more than before and cuts into the bank, while on its inner part it flows less rapidly and is able to carry less, thus being forced to drop some of its load. As a river flows along its graded stretches, eroding in some places and filling in others, itRIVER PLAIN.
broadens its valley floor, leaving at the border of its channel a low plain which in time of flood may be covered with water. Such a river-made plain is called a flood plain.
If a river once begins to swing on its valley floor, it continues to doso, since whenever it strikes the bank, it is reflected toward the other side, and is made to move in the direction of the opposite bank as well as downstream. The windings that it thus assumes on a flat valley floor are roughly S-shaped and are called meanders, from the name of a river in Asia Minor which was, in very ancient time, noted for having such swinging curves. The size of these curves will be proportional to the size of the river.
Great rivers like the Mississippi have a swing of several miles, while a small stream may have a swing of only a few feet or rods. These meanders are continually changing their shape, owing to theRIVER MEANDERING IN ITS FLOOD PLAIN.
cutting and filling. Since they strike the bank with the greatest force on the downstream side of the curve, they also move downstream themselves. This can be seen from maps ofthe Mississippi taken at different times.
The meanders sometimes become so tortuous that the downstream side of one curve approaches the upstream side of another and even cuts into it, thus causing the river to desert its curved path and straighten itself at this point. The old deserted winding looks something like an oxbow, and when filled with water,is called an oxbow lake. Sometimes the
THE MISSISSIPPI AND SOME OF ITS
ABANDONED MEANDERS.
meanders are artificially straightened, as has been done in the lower Rhine valley, and much arable land reclaimed.
In time of flood, when a river spreads over its flood plain, the velocity of the water is checked outside the channel and some of the sediment it carries is deposited. The most sudden check in velocityLEVEE ALONG THE LOWER MISSISSIPPI.
occurs where it leaves the channel, so more material will be deposited here than elsewhere on the flood plain. The banks of the channel will thus be built up more rapidly, and the flood plain near the river will slope away from the channel instead of toward it.
This is well shown in the lower Mississippi, where the river is found to be flowing on a natural embankment, the side streams running away from the river instead of into it. In some places the embankment isLEVEE OF THE SACRAMENTO.
fifteen or twenty feet above the rest of the flood plain. These natural levees, as they are called, often force the tributary streams to flow for long distances upon the flood plain before they can enter the main river. The Yazoo River is forced to flow along the flood plain some 200 miles before it can enter the Mississippi. Artificial levees are often built to keep rivers from overflowing their flood plains. Such are the high levees along the Lower Mississippi and Sacramento rivers.
Sometimes the flood plain of the main river is built up more rapidly than the tributaries can build theirs, so that they are dammed up as they enter the flood plain of the main stream and form a series of fringing lakes along its border. A fine example of this is found in the lower course of the Red River of Louisiana.
A river is said to be mature when it has reduced its valley to gradeand is able to meander freely upon its flood plain, its side streams having appropriated all the undrained upland which they are able to obtain. The river is now carrying off in the easiest and most effective way the drainage which falls upon its drainage basin.
When a river has graded itself and built its flood plain, its own active work consists largely in carrying off the materials brought to it by its side streams. Although these are now able to appropriate no new territory they continue to wear down the country and round off thedivides till the whole region, unless re?levated, is reduced to an almost level plain with its entire drainage system nearly at grade. Most of the material now carried by the river is in solution, and there is but little erosion. The river has accomplished its life work, it has borne tothe sea all the burden it has
AN OLD RIVER.
This river has done its work and has completed a cycle of erosion.
to bear, its labors are ended,
it has reached old age. It has reduced its drainage area to a base level of erosion. When a river has thus done all the wearing down of which it is capable it is said to have completed a cycle of erosion.
159.Rivers in Dry Climates. -In a region where the climate is very dry, rivers are often intermittent in their flow. They contain water only after rains. Such rivers may dry up before they reach any other body of water, their water entirely evaporating or sinking into the dry soil. Their development is therefore somewhat irregular.
RESULTS OF A SUDDEN FLOOD.
If the slopes are steep and there is little vegetation to protect them and hinder the quick run-off of the water, rivers flood very rapidly, eroding their channels and washing away their banks. Where they descend upon level ground they silt up their old courses and acquire new channels. Thus a river which for the larger part of the year is a mere brook may after a rain become a devastating torrent, bursting its banks and carrying destruction to settlements and farm lands along its course. It may even change its entire lower course.