Page 89 - Fundamentals of Geomorphology

72 INTRODUCING LANDFORMS AND LANDSCAPES

to be suspended for very long under normal flow con- the maximum amount of debris that it can carry in
ditions). In detail, the three components of stream load traction as bed load.
are as follows:
In addition to these three loads, the suspended load
1 The dissolved load or solute load comprises ions and the bed load are sometimes collectively called the
and molecules derived from chemical weathering solid-debris load or the particulate load. And the wash
plus some dissolved organic substances. Its compo- load, a term used by some hydrologists, refers to that
sition depends upon several environmental factors, part of the sediment load comprising grains finer than
including climate, geology, topography, and vege- those on the channel bed. It consists of very small
tation. Rivers fed by water that has passed though clay-sized particles that stay in more or less permanent
swamps, bogs, and marshes are especially rich in dis- suspension.
solved organic substances. River waters draining large
basins tend to have a similar chemical composition, Stream erosion and transport
with bicarbonate, sulphate, chloride, calcium, and
sodium being the dominant ions (but see p. 43 for Streams may attack their channels and beds by corro-
continental differences). Water in smaller streams sion,corrasion,andcavitation.Corrosionisthechemical
is more likely to mirror the composition of the weathering of bed and bank materials in contact with the
underlying rocks. stream water. Corrasion or abrasion is the wearing away
2 The suspended load consists of solid particles, of surfaces over which the water flows by the impact
mostly silts and clays, that are small enough and or grinding action of particles moving with the water
light enough to be supported by turbulence and body. Evorsion is a form of corrasion in which the sheer
vortices in the water. Sand is lifted by strong cur- force of water smashes bedrock without the aid of parti-
rents, and small gravel can be suspended for a cles. In alluvial channels, hydraulicking is the removal of
short while during floods. The suspended load loose material by the impact of water alone. Cavitation
reduces the inner turbulence of the stream water, occurs only when flow velocities are high, as at the
so diminishing frictional losses and making the bottom of waterfalls, in rapids, and in some artificial
stream more efficient. Most of the suspended load conduits. It involves shockwaves released by implod-
is carried near the stream bed, and the concentra- ing bubbles, which are produced by pressure changes
tions become lower in moving towards the water in fast-flowing streams, smashing into the channel walls,
surface. hammer-like, and causing rapid erosion. The three main
3 The bed load or traction load consists of gravel, erosive processes are abetted by vortices that may develop
cobbles, and boulders, which are rolled or dragged in the stream and that may suck material from the
along the channel bed by traction. If the current is streambed.
very strong, they may be bounced along in short Streams may erode their channels downwards or
jumps by saltation. Sand may be part of the bed sideways. Vertical erosion in an alluvial channel bed
load or part of the suspended load, depending on the (a bed formed in fluvial sediments) takes place when
flow conditions. The bed load moves more slowly there is a net removal of sands and gravels. In bedrock
than the water flows as the grains are moved fit- channels (channels cut into bedrock), vertical erosion
fully. The particles may move singly or in groups is caused by the channel’s bed load abrading the bed.
by rolling and sliding. Once in motion, large grains Lateral erosion occurs when the channel banks are worn
move more easily and faster than small ones, and away, usually by being undercut, which leads to slumping
rounder particles move more readily than flat or and bank collapse.
angular ones. A stream’s competence is defined as The ability of flowing water to erode and transport
the biggest size of grain that a stream can move rocks and sediment is a function of a stream’s kinetic
in traction as bed load. Its capacity is defined as energy (the energy of motion). Kinetic energy, E k ,is