Geological Society, London, Special Publications 1993 Bristow 1 11
Geological Society, London, Special Publications 1993 Bristow 1 11
Geological Society, London, Special Publications 1993 Bristow 1 11
From Best, J. L. & Bristow, C. S. (eds), 1993, Braided Rivers, Geological Society
Special Publication No. 75, pp. 1-H.
Channel hierarchies
The presence of a hierarchy of channels within
braided rivers was first suggested by Williams
& Rust (1969), who described three orders of
channel in addition to a series of levels within the
river which represented active and inactive parts
of the channel system. These orders and levels of
bar deposit may also be adjusted to the dominant discharge of the alluvial system (see Thorne
et al. this volume). In the scheme proposed by
Williams & Rust (1969) the entire river and
active channels were termed the 'composite
stream channel' and the 'stream channel' respectively adding two additional levels to the hierarchy. This system was modified by Bristow
(1987a) to a three fold hierarchy and Bridge (this
volume) suggests additional modifications to
this view. If one accepts that the river can
operate as a single entity with channels within
it and that there may be different scales of
channels which depend upon total discharge and
discharge fluctuations, then a threefold hierarchy of channels is required. The first order
comprises the whole river (see fig. 2 of Thorne
c.s.
B R I S T O W & J. L. BEST
(a)
(b)
Fig. 1. Two scales of braided river. (a) Sunwapta River, Alberta, Canada with a one kilometre wide braidplain,
20-50 m wide channels and predominantly gravel bedload. Flow is from left to right. (b) The Congo (Zaire)
River with a braidplain width of approximately 8 km and a sand bed. Flow is from right to left.
Fig. 2. Satellite image of bars in part of a braided section of the Amazon River. Note the paired accretion
topography on the braid bars illustrating that braid bars may accrete laterally on both sides. Braidplain is
approximately 10 km wide.
Fig. 3. Catoons of depositional morphology based on braiding processes derived from scale models of gravel bed
braided rivers (Ashmore 1991) and observations of the Brahmaputra River :Bristow 1987b).
Fig. 4. Scour and fill at the junction of the Ganges and Brahmaputra rivers, Bangladesh. This plot, showing data
collected over a 12 year period, shows the high rates of scour and fill present at this junction. Scour is given as an
actual depth and as a relative depth, ds, through division of the actual scour depth by the mean upstream channel
depths. Double arrows indicate the approximate flood peaks in 1985 and 1986.
aggradation, channel deposits will become reworked, superimposed and stacked to form
multilateral and multistorey sandbodies. Multichannel braided rivers are almost certain to form
multilateral/multistorey sandbodies. Preservation style within the channel will be a function
of the local rate of aggradation and the size and
sequence of bedforms/scour surfaces that affect
any particular spatial location.
Fig. 5. A schematic diagram illustrating the preservation of braided river depositional morphology as a function
of aggradation rate, lateral channel migration and channel-belt avulsion. The preservation of isolated channel
sandbodies preserving morphology requires frequent avulsions as well as rapid aggradation.
References
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