Geoscience Reference
In-Depth Information
3.1.2 Significance of fluvial sedimentary
environments
(
0.0625 mm) moved in suspension. The sus-
pended load forms most (generally
<
90%) of
a river's sediment and, because its settling velo-
cities are generally low, is transported at the
same speed as the river's flow. During floods,
the size of the suspended load can increase dra-
matically due to increased stream power. The
suspended load is supplied from the physical
erosion of river banks and surface materials,
and resuspension of fine channel-bed material.
Fluvial sediment also can be classified accord-
ing to either its physical or chemical charac-
teristics (Goudie 1990). Physically, sediment is
classified mainly according to its size and particle
shape. Estimating the relative proportions of
the different size fractions in a river sedimentary
unit is important because the size is related to the
source and abrasion during transport, and size
also plays a major role in the distribution of con-
taminants. Particle shape is also determined to
a large part by the abrasion and corrosion that
occurs during transport, but also depends on
the original weathering that took place to form
the grain, and on post-depositional chemical
processes such as dissolution and precipitation.
Particle shape is described in three ways: (i) form,
(ii) sphericity and (iii) roundness (see Chapter 1).
River gravels, for example, often have high to
medium sphericity and are subrounded. In gen-
eral, the further a river sediment particle travels
and the more mechanical abrasion it undergoes,
the more well-rounded, higher in sphericity and
finer-grained it will become. Rounding occurs
more readily in cobbles and pebbles than in sands
or smaller-sized particles.
The physical and chemical characteristics of
fluvial sediments are determined by the sediment
provenance, that is, the origin (source) of the sedi-
ment and the physical, chemical and biological
processes that have operated on it during trans-
port to the site of deposition in the river system.
Most fluvial sediment is composed of geogenic
materials such as bedrock, soil and vegetation that
are released to river systems through both natural
(e.g. erosion, forest fire, volcanic eruption) and
anthropogenic activities (damming, deforesta-
tion, mining), many of which are discussed in
section 3.3. The variability of these geogenic
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In effect, many of the issues and problems related
to river basin disturbance are in turn related to
differences between the Developed world, with
its largely temperate rivers, and the Develop-
ing world, with its largely arid, semi-arid and
tropical rivers. Issues for river development and
management can be quite different for these two
worlds. Many of the largest river basins in the
world occur in the arid and semi-arid Develop-
ing world (Fig. 3.1), and here the major problems
include drought, irrigation, poverty, salinization,
pesticide contamination and desertification, all of
which have an impact on river systems (Newson
1992; see also Chapter 5). By contrast, the
Developed world is coming to terms with man-
aging and restoring river systems that have been
under pressure from impacts such as contamina-
tion. Both the Developed and Developing worlds
will have, increasingly, to deal with the issue of
climate change related to global warming.
River systems are not country-selective: they
cross international boundaries. This fact raises
important issues relating to sediment transport
and deposition in rivers, especially with respect
to contamination, erosion and flooding. Newson
(1992) has called for workers in all related dis-
ciplines to consider the impact of contemporary
decisions about river management in both the
short- and long-term, consider basins as integrated
water, sediment and contaminant entities, and for
science to inform, rather than solve, problems of
river basin (including sediment) management.
3.2
SEDIMENT SOURCES AND ACCUMULATION PROCESSES
3.2.1 Characteristics and provenance of
fluvial sediments
3.2.1.1 Characteristics of fluvial sediment
Transported fluvial sediment can be clas-
sified as either bed load, the coarse sediment
(
0.0625 mm) carried along the river bot-
tom, or suspended load, the finer sediment
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