VALLEY DEPOSITS

FLOOD PLAINS are the surfaces of the alluvial deposits which
streams build along their courses at times of flood. A swift
current then sweeps along the channel, while a shallow sheet of
water moves slowly over the flood plain, spreading upon it a thin
layer of sediment. It has been estimated that each inundation of
the Nile leaves a layer of fertilizing silt three hundredths of an
inch thick over the flood plain of Egypt.

Flood plains may consist of a thin spread of alluvium over the
flat rock floor of a valley which is being widened by the lateral
erosion of a graded stream (Fig. 60). Flood-plain deposits of
great thickness may be built by aggrading rivers even in valleys
whose rock floors have never been thus widened.

A cross section of a flood plain 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.

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