Page 148 - Sedimentology and Stratigraphy

River Forms 135

carried in shallower parts of the flow and is deposited
along the inner bend of a meander loop where friction

reduces the flow velocity. The deposits of a meander
bend have a characteristic profile of coarser material
at the base, becoming progressively finer-grained up
the inner bank (Fig. 9.11). The faster flow in the
Fig. 9.10 Flow in a river follows the sinuous thalweg deeper parts of the channel forms subaqueous dunes
resulting in erosion of the bank in places. in the sediment that develop trough or planar cross-
bedding as the sand accumulates. Higher up on the
while the opposite bank has slower flowing water inner bank where the flow is slower, ripples form in
alongside. Meanders develop by the erosion of the the finer sand, producing cross-lamination. A channel
bank closest to the thalweg, accompanied by deposi- moving sideways by erosion on the outer bank and
tion on the opposite side of the channel where the deposition on the inner bank is undergoing lateral
flow is sluggish and the bedload can no longer be migration, and the deposit on the inner bank is
carried. With continued erosion of the outer bank referred to as a point bar. A point bar deposit will
and deposition of bedload on the inner bank the show a fining-up from coarser material at the base to
channel develops a bend and meander loops are finer at the top (Fig. 9.13) and it may also show larger
formed (Figs 9.11 & 9.12). A distinction between scale cross-bedding at the base and smaller sets of
river sinuosity and meandering form should be recog- cross-lamination nearer to the top. As the channel
nised: a river is considered to be sinuous if the dis- migrates the top of the point bar becomes the edge
tance measured along a stretch of channel divided by of the floodplain and the fining-upward succession of
the direct distance between those points is greater the point bar will be capped by overbank deposits.
than 1.5; a river is considered to be meandering if Stages in the lateral migration of the point bar of
there is accumulation of sediment on the inside of a meandering river can sometimes be recognised as
bends, as described below. inclined surfaces within the channel-fill succes-
Meandering rivers transport and deposit a mixture sion (Fig. 9.11). These lateral accretion surfaces
of suspended and bedload (mixed load) (Schumm are most distinct when there has been an episode of
1981). The bedload is carried by the flow in the low discharge allowing a layer of finer sediment to be
channel, with the coarsest material carried in the deposited on the point-bar surface (Allen 1965;
deepest parts of the channel. Finer bedload is also Bridge 2003; Collinson et al. 2006). These surfaces

Fig. 9.11 Main morphological
features of a meandering river.
Deposition occurs on the point bar

on the inner side of a bend while

erosion occurs on the opposite cut
bank. Levees form when flood waters
rapidly deposit sediment close to the
bank and crevasse splays are created
when the levee is breached.