Fig. 180.—Diagram illustrating an early stage in the development of meanders. The shaded part represents the area over which the stream has worked.
Flood-plain meanders. Cut-and-fill.—A stream with an alluvial plain is likely to meander widely ([Pl. XVI]). In general terms this may be said to be the result of low velocity, which allows it to be easily turned aside. Were the course of such a stream made straight, it would soon become crooked again. The manner of change is illustrated by Figs. [180] and [181]. If the banks be less resistant at some points than at others, as is always the case, the stream will cut in at those points. If the configuration of the channel is such as to direct a current against a given point, a ([Fig. 180]), the result is the same, even without inequality of material. Once a curve in the bank is started, it is increased by the current which is directed into it. Furthermore, as the current issues from the curve, it impinges against the opposite bank and develops a curve at that point. The water issuing from this curve develops another, and so on.
Once started, the curves or meanders tend to become more and more pronounced (compare Figs. [180] and [181]). In the case represented by [Fig. 1, Plate XVI] (Missouri River near Brunswick, Mo.) the narrow neck of land between curves is almost cut through. When this is accomplished, the stream will abandon its wide curve. A later stage in the process is shown in [Fig. 2, Plate XVI] (the Osage River near Schell, Mo.).
The straightening of the channel is often accomplished in another way. Even before the meanders reach the stage represented by [Fig. 1, Plate XVI], the position of the channel becomes unstable. In time of flood, the whole flat is covered with flowing water. The greater depth of water in the channel tends to give it a velocity greater than that of the water on the flat outside. But the distance from a to c via b ([Fig. 181]) is much greater than that in a direct line. It follows that the slope from a to c direct is greater than that by way of b. If the current between a and c in time of flood be strong enough to erode, it may deepen its bed, and thereby increase the volume of water following this course. The increased volume gives increased velocity, and the result may be the opening of a channel between a and c direct. The channel may be worn so deep that when the flood subsides, the stream will follow it. So long as the abandoned channel-curve remains unfilled with sediment, it is often called a cut-off. If it contains standing water and has the proper form, it is called an ox-bow lake ([Fig. 182]), or sometimes a bayou. The water-filled portions are not always bows ([Fig. 183], Osage River, near Butler, Mo.). Cut-offs, with or without standing water, are of common occurrence along most rivers with wide plains. Meandering is not confined to streams which are near sea-level. Even small creeks at high altitudes may meander, if so situated as to have slight velocity. Trout Creek in the Yellowstone Park ([Fig. 184]) is an example.
Fig. 181.—Diagram illustrating later stages in the development of meanders.
Fig. 182.—Meanders and cut-offs (ox-bow lakes) in the Mississippi Valley a little below Vicksburg. The figure also shows the migration of meanders down-stream, and their tendency to increase themselves. (From charts Nos. 18 and 19, Mississippi River Commission.)
Fig. 183.—Bayou Lakes, Osage River, near Butler, Mo.