WATER AND ICE - Physical Geography - page 86

Rivers are dynamic in that they often move position around the
landscape through time, change their shape and move sediment,
water and dissolved materials. They are an important agent in
removing weathered material from the land surface and redistribut-
ing it across the landscape or into oceans. This process balances out
the mountain building caused by plate tectonics so that over long
time periods plate tectonics might build mountains but weathering,
erosion and removal by river systems of ice masses (see later in this
chapter) smoothes out the landscape again. Uplift and erosion of
landscapes has traditionally been thought of as cyclical, although
the outcomes in each environment may be very different depend-
ing on rates and types of processes operating there.
Sediment movement in rivers is linked with water flow. If a
piece of sediment is to be picked up from the river bed or bank by
flowing water a critical threshold has to be passed above which the
water velocity or shear stress is sufficiently large to overcome
frictional forces that resist erosion. The transport of materials close
to the bed is known as bedload transport. Particles move by rolling,
sliding or saltating (hopping) along or close to the bed. If the flow
velocity does not change, a particle will only come to rest if it
becomes lodged against an obstruction or falls into an area sheltered
from the main force of the water by a larger particle. With further
increases in the strength of flow, the smaller particles may be
carried upwards into the main body of water and transported in
suspension. Deposition and cessation of movement for an indi-
vidual particle occurs when velocity falls below critical conditions.
This means that finer particles are preferentially moved down-
stream. For suspended sediment within the water body (as opposed
to that transported close to the river bed as bedload transport),
transport is determined not only by water flow but also by the rate
at which it is supplied to the river (e.g. from washflow process).
Where erosion exceeds deposition within a particular section of
a river, there will be lowering of the river bed or widening of the
banks. If erosion and deposition occur at the same rate then the
river will stay at the same level. Eroding channels may undermine
structures such as bridges while depositing channels may submerge
structures such as roads. Stable channels, especially those whose
beds are lined with bedrock, are less likely to be a problem to
engineering structures but fluctuations in river channel dimensions