Geoscience Reference
In-Depth Information
channel width. Channel adjustment, however,
is not easy to model, because it depends on
several factors including slope, and the pre-
existing channel geometry, planform and bed
configuration. Further examples of the types
and causes of channel change are discussed later
in this chapter.
Fluvial (also known as alluvial) deposits are
characterized by a huge variety of bed forms and
sedimentary structures, which are well-described
in Collinson (1986) and Bridge (2003, chapters 4
and 5). These include longitudinal, bank-attached,
linguoid, side, diagonal, point and transverse bars,
islands, riffles, ripples, pebble/cobble clusters,
sand ribbons, cross-stratification, dunes, sand
flats, plane beds and chutes-and-pools (largely in
channels), lateral deposits (in channel margins),
alluvial fans (in piedmont areas), crevasse splays,
levees and vertical accretion deposits (in flood-
plains) (Fig. 3.5). These features are used by
geomorphologists to deduce the history of river
sedimentation, to evaluate contamination and
to record flood histories.
No study or topic on environmental sediment-
ology would be complete without consideration
of fluvial sedimentary environments. Because all
humans essentially reside in river basins, they
have, since pre-history, had a significant impact on
them and the functioning of their sedimentary
processes. In many cases, these impacts outstrip
those of their natural counterparts. In other
cases, natural (section 3.3) and anthropogenic
processes (section 3.4) act in tandem to produce
large-scale modifications to fluvial sediment trans-
port and deposition.
Straight
Braided
Meandering
Anastomosed
Fig. 3.4
Continuum between principal planform types of rivers,
illustrating the diversity of river channel types. Between the
straight, meandering, anastomosed and braided types of
planform are rivers that exhibit characteristics of at least two of
these end-members. (After Bristow 1996.)
that occur during floods can cause moderate to
sometimes drastic channel change (Schumm &
Lichty 1963). Attempts have been made to model
channel changes that occur in response to changes
in water and sediment discharge. Schumm (1969),
for example, suggests that a decrease in stream-
flow and concomitant increase in sediment supply
should produce a decrease in channel sinuosity
and depth, and either an increase or decrease in
Abandoned
channel-fill
Floodplain
Point-bar
deposit
Levee
Earlier deposit
Channel lag
Crevasse-splay
Fig. 3.5
Classic point-bar model for a meandering
stream, showing various types of fluvial sedimentary
deposit. (After Allen 1964, 1970; Collinson 1986.)
Decreasing
grain size
Cross-
bedding
Cross-
lamination
Flat-bedding
