Page 144 - Sedimentology and Stratigraphy

River Forms 131

and overbank flow occurs on the floodplain adjacent that have depositional bars only on the insides of
to the channel (Fig. 9.2). bends are called meandering.
As water flows in a channel it is slowed down by When considering the deposits of ancient rivers, the
friction with the floor of the channel, the banks and processes of deposition on the mid-channel bars in
the air above. These frictional effects decrease away braided streams and the deposition on the inner
from the edges of the flow to the deepest part of the banks of meandering river bends are found to be
channel where there is the highest velocity flow. The important mechanisms for accumulating sediment.
line of the deepest part of the channel is called the ‘Braided’ and ‘meandering’ are therefore useful ways
thalweg. The existence of the thalweg and its position of categorising ancient fluvial deposits, but consider-
in a channel is important to the scouring of the banks able variations in and combinations of these main
and the sites of deposition in all channels. themes exist both in modern and ancient systems.
Furthermore, not all rivers are filled by deposition
out of flow in the channels themselves (9.2.4).
9.2 RIVER FORMS Anastomosing or anabranching rivers are seen
today mostly in places where the banks are stabilised
Rivers in the depositional tract can have a variety of by vegetation, which inhibits the lateral migration of
forms, with the principal variables being: (a) how channels (Smith & Smith 1980; Smith 1983), but
straight or sinuous the channel is; (b) the presence anastomosing rivers are also known from more arid
or absence of depositonal bars of sand or gravel within regions with sparse vegetation. The positions of chan-
the channel; (c) the number of separate channels that nels tend to remain fairly fixed but new channels may
are present in a stretch of the river. A number of ‘end- develop as a consequence of flooding as the water
member’ river types can be recognised (Miall 1978; makes a new course across the floodplain, leaving
Cant 1982), with all variations and intermediates an old channel abandoned. Recognition of anasto-
between them possible (Fig. 9.3). A straight channel mosing rivers in the stratigraphic record is prob-
without bars is the simplest form but is relatively lematic because the key feature is that there are
uncommon. A braided river contains mid-channel several separate active channels. In ancient deposits
bars that are covered at bank-full flow, in contrast to it is not possible to unequivocally demonstrate that
an anastomosing (also known as anabranching) two or more channels were active at the same time
river, which consists of multiple, interconnected and a similar pattern may form as a result of a single
channels that are separated by areas of floodplain channel repeatedly changing position (9.2.4).
(Makaske 2001). Both braided and anastomosing
river channels can be sinuous, and sinuous rivers
9.2.1 Bedload (braided) rivers

Rivers with a high proportion of sediment carried by
rolling and saltation along the channel floor are
referred to as bedload rivers. Where the bedload is
deposited as bars (4.3.3) of sand or gravel in the
channel the flow is divided to give the river a braided
form (Figs 9.4 & 9.5). The bars in a braided river
channel are exposed at low flow stages, but are cov-
ered when the flow is at bank-full level. Flow is gen-
erally strongest between the bars and the coarsest
material will be transported and deposited on the
channel floor to form an accumulation of larger
clasts, or coarse lag (Fig. 9.6). The bars within the
channel may vary in shape and size: longitudinal
Fig. 9.2 A sandy river channel and adjacent overbank area: bars are elongate along the axis of the channel, and
the river is at low-flow stage exposing areas of sand bars that are wider than they are long, spreading
deposited in the channel. across the channel are called transverse bars and