Fig. 71. Cross Section of a Flood Plain
A cross section of a flood plain ([Fig. 71]) shows that it is highest next the river, sloping gradually thence to the valley sides. These wide natural embankments are due to the fact that the river deposit is heavier near the bank, where the velocity of the silt-laden channel current is first checked by contact with the slower-moving overflow.
Fig. 72. Waste-filled Valley and Braided Channels of the Upper Mississippi
Thus banked off from the stream, the outer portions of a flood plain are often ill-drained and swampy, and here vegetal deposits, such as peat, may be interbedded with river silts.
A map of a wide flood plain, such as that of the Mississippi or the Missouri ([Fig. 77]), shows that the courses of the tributaries on entering it are deflected downstream. Why?