Page 151 - Sedimentology and Stratigraphy

138 Rivers and Alluvial Fans

that merge to form a more extensive deposit. The term across the whole floodplain, leaving channel margins
wadi is commonly used for a river or stream in a at the edges of the valley. It is therefore often neces-
desert with ephemeral flow and the resulting deposits sary to use the characteristics of the vertical succes-
are therefore sometimes referred to as wadi gravels. sions deposited within channels (Figs 9.6 & 9.13) as
indicators of fluvial depositional environments.

9.2.4 Channel-filling processes
9.2.5 Trends in fluvial systems
The channel-fill succession in both meandering
and braided rivers described above is built up as a There is normally a general trend of reduction in gra-
result of sideways movement or lateral migration of dient of a river downstream through the depositional
the active part of the channel. Accumulation and tract. The slope of the river and the discharge affect the
possible preservation of river channel deposits can velocity of the flow, which in turn controls the ability of
occur only if the river changes its position in some the river to scour and the size of the material that can be
way, either by shifting sideways, as above, or if the carried as bedload and suspended load. Gravelly braided
channel changes position on the floodplain, a process rivers have the steepest depositional gradient (although
known as avulsion. When a river avulses part of the the angle is typically less than half a degree) and bars of
old river course is completely abandoned and a new pebbles, cobbles and boulders form. Finer debris is
channel is scoured into the land surface (Fig. 9.15). mostly carried through to the lower reaches of the
Oxbow lakes are an example of abandonment of a river. At lower gradients the sandy bedload is deposited
short stretch, but much longer tracts of any type of on bars in braided rivers, the flow having decreased
river channel may be involved. When avulsion occurs sufficiently to deposit most of the gravel upstream. A
the flow in the old river course reduces in volume and meandering pattern tends to develop at very gentle
slows down, and the bedload will be deposited. A gradients (around a hundredth of a degree) in rivers
decrease in the amount of water supplied limits the carrying fine-grained sediment as mixed bedload and
capacity of the channel to carry sediment and the suspended material (Collinson 1986).
water gradually becomes sluggish, depositing its sus- The erosional tracts of rivers exhibit a tributary
pended load. Abandonment of the old river channel drainage pattern as small streams merge into
will leave it with sluggish water containing only sus- the trunk channel (a dendritic pattern; Fig. 9.1).
pended load as all the bedload is diverted into the new This pattern may extend into the depositional tract.
course. Abandoned and empty stretches of river chan- Most rivers flow as a single channel to a lake margin
nel are unlikely to remain empty for very long or the shoreline of a sea, where a delta or estuary
because when the river floods from its new course it may be formed. However, rivers in relatively arid
will carry sediment across the floodplain to the old regions may lose so much water through evap-
channel where sediment will gradually accumulate. oration and soak-away into the dry floodplain that
The final fill of any river channel is therefore most they dry up before reaching a standing water body.
likely to be fine-grained overbank sedimentation In some enclosed (or endorheic) basins (which do
related to a different river course. Channels entirely not have an outlet to the open ocean) with an arid
filled with mud may be very difficult to distinguish climate there may not be a permanent lake (10.4).
from overbank sediments in the stratigraphic record. Due to the loss of water, the channels become smaller
Recognition of channels is one of the key criteria for downstream and end in splays of water and sediment
identifying the deposits of fluvial systems within a called terminal fans (Friend 1978). Rivers that show
sedimentary succession. However, the cut banks of these characteristics may be referred to as fluvial
channel margins are not always easy to recognise. distributary systems (Nichols & Fisher 2007),
The lateral migration of the river channel may result although it should be noted that it is mainly sediment
in a succession of point bar or mid-channel bar depos- that is being distributed. At any time most of the
its that is hundreds of metres across, even though the water flow will be in one principal channel, with
channel itself may be only a few tens of metres wide at other, minor channels splitting off from it (a bifur-
any time. This deposit may be wider than the outcrop cating pattern): a minor channel may subsequently
exposed and in some cases the rivers migrate laterally take over as the main flow route, or a new channel