Page 250 - Fundamentals of Geomorphology

FLUVIAL LANDSCAPES 233

Convex floodplains morerapidlythansuspendedload,anddepositionismore
frequent in and near to the channel than it is in overbank
The low-gradient floodplains of most large rivers, includ- sites. In consequence, the channel banks and levees grow
ing those of the Rivers Mississippi, Amazon, and Nile, faster than the flood basins and may stand 1–15 m higher.
are broad and have slightly convex cross-sections, the
land sloping away from the riverbank to the valley sides
(Figure 9.8a). The convexity is primarily a product of Flat floodplains
sedimentation. Bed load and suspended sediment are laid
down in the low-water channel and along its immediate The majority of small floodplains are flat or gently
edges, while only suspended materials are laid down in concave in cross-section (Figure 9.8b). On these flat
the flood basins and backswamps. Bed load accumulates floodplains, natural levees are small or absent and the

Box 9.1

POOLS AND RIFFLES

River channels, even initially straight ones, tend about five times the channel width (Figure 9.6).
to develop deeper and shallower sections. These Continued development sees meanders forming with
are called pools and riffles respectively (Plate 9.6). alternate pools migrating to opposite sides. The mean-
Experiments in flumes, with water fed in at a con- der wavelength is roughly two inter-pool spacings
stant rate, produce pool-and-riffle sequences, in of ten channel widths, as is common in natural
which the spacing from one pool to the next is rivers.

Plate 9.6 Riffles and pools in a straight section of the River Dean, Adlington Hall, Cheshire, England. A pool may
be seen in the foreground, a riffle to the right of the middle-ground bar, with other pools and riffles beyond.
(Photograph by David Knighton)