=Scour-and-fill.=—It has already been shown that aggrading streams cut laterally at the same time that they build up their plains. It is now to be added that they periodically deepen their channels to a notable extent, and that the deepening of the channel takes place at the very time when the flood-plain is being aggraded. In other words, the stream in flood aggrades its plain, and degrades its channel. This follows from the fact that the current is sluggish in the former position, where the water is shallow, and rapid in the latter, where it is deep. When the flood subsides, the channel, deepened while the current was torrential, is filled again by the feebler current which follows. This alternate deepening and filling is known as scour-and-fill. It is well illustrated by the Missouri River. At Nebraska City, scour is believed to occasionally reach depths of 70 to 90 feet.[76] At Blair, about 25 miles above Omaha, the same river is believed to cut to bed-rock (about 40 feet below the bottom of the channel in low water) twice a year, that is, during floods.[77] Fig. 185 shows the changes recorded in the channel of the river at this point during the year 1883. It shows that the scour-and-fill during this year amounted to almost 40 feet. All streams similarly situated do a like work. The material thus eroded is shifted down-stream, some of it for short distances only, and some of it to the sea. Even an aggrading stream therefore is not without erosive activity; it is a stream whose fill exceeds its scour, not one which has ceased to erode. [Illustration: +Fig.+ 185.—Diagram illustrating scour and fill in the Missouri River. A record of soundings at Blair Bridge (near Omaha), 1883. Shows also the cross-sections of the river at various rates. (Todd, Bull. 158, U. S. Geol. Surv.)] =Materials of the flood-plain.=—As a result of its varying velocities in flood and low water, a stream may deposit coarse material at one time and fine at another. A similar sequence of deposits takes place in the flood-plain of a meandering stream, irrespective of floods. Flood-plain deposits are often therefore very heterogeneous, as shown in Fig. 186, which represents the constitution of the alluvium of the Missouri River at Omaha. The deposits of the streams range from the finest clay, through sand to gravel, and even bowlders. In general they become finer down-stream. In a given plain, they are usually coarser below and finer above. [Illustration: +Fig.+ 186.—Diagram to show the heterogeneous character of alluvial deposits. (Todd, Bull. 158, U. S. Geol. Surv.)]