GEOMORPHIC MATERIALS AND PROCESSES - Fundamentals of Geomorphology - page 72

to be suspended for very long under normal flow con-
ditions). In detail, the three components of stream load
are as follows:

the maximum amount of debris that it can carry in
traction as bed load.

In addition to these three loads, the suspended load
and the bed load are sometimes collectively called the
solid-debris load or the particulate load . And the wash
load , a term used by some hydrologists, refers to that
part of the sediment load comprising grains finer than
those on the channel bed. It consists of very small
clay-sized particles that stay in more or less permanent
suspension.

1 The dissolved load or solute load comprises ions

and molecules derived from chemical weathering
plus some dissolved organic substances. Its compo-
sition depends upon several environmental factors,
including climate, geology, topography, and vege-
tation. Rivers fed by water that has passed though
swamps, bogs, and marshes are especially rich in dis-
solved organic substances. River waters draining large
basins tend to have a similar chemical composition,
with bicarbonate, sulphate, chloride, calcium, and
sodium being the dominant ions (but see p. 43 for
continental differences). Water in smaller streams
is more likely to mirror the composition of the
underlying rocks.

Stream erosion and transport

Streams may attack their channels and beds by corro-
sion, corrasion, and cavitation. Corrosion is the chemical
weathering of bed and bank materials in contact with the
stream water. Corrasion or abrasion is the wearing away
of surfaces over which the water flows by the impact
or grinding action of particles moving with the water
body. Evorsion is a form of corrasion in which the sheer
force of water smashes bedrock without the aid of parti-
cles. In alluvial channels, hydraulicking is the removal of
loose material by the impact of water alone. Cavitation
occurs only when flow velocities are high, as at the
bottom of waterfalls, in rapids, and in some artificial
conduits. It involves shockwaves released by implod-
ing bubbles, which are produced by pressure changes
in fast-flowing streams, smashing into the channel walls,
hammer-like, and causing rapid erosion. The three main
erosive processes are abetted by vortices that may develop
in the stream and that may suck material from the
streambed.
Streams may erode their channels downwards or
sideways. Vertical erosion in an alluvial channel bed
(a bed formed in fluvial sediments) takes place when
there is a net removal of sands and gravels. In bedrock
channels (channels cut into bedrock), vertical erosion
is caused by the channel's bed load abrading the bed.
Lateral erosion occurs when the channel banks are worn
away, usually by being undercut, which leads to slumping
and bank collapse.
The ability of flowing water to erode and transport
rocks and sediment is a function of a stream's kinetic
energy (the energy of motion). Kinetic energy, E k ,is

2 The suspended load consists of solid particles,

mostly silts and clays, that are small enough and
light enough to be supported by turbulence and
vortices in the water. Sand is lifted by strong cur-
rents, and small gravel can be suspended for a
short while during floods. The suspended load
reduces the inner turbulence of the stream water,
so diminishing frictional losses and making the
stream more efficient. Most of the suspended load
is carried near the stream bed, and the concentra-
tions become lower in moving towards the water
surface.

3 The bed load or traction load consists of gravel,

cobbles, and boulders, which are rolled or dragged
along the channel bed by traction. If the current is
very strong, they may be bounced along in short
jumps by saltation. Sand may be part of the bed
load or part of the suspended load, depending on the
flow conditions. The bed load moves more slowly
than the water flows as the grains are moved fit-
fully. The particles may move singly or in groups
by rolling and sliding. Once in motion, large grains
move more easily and faster than small ones, and
rounder particles move more readily than flat or
angular ones. A stream's competence is defined as
the biggest size of grain that a stream can move
in traction as bed load. Its capacity is defined as