Geography Reference
In-Depth Information
modified river channels over the past few thousand years although
changes during the last two centuries have been greatest. Activities
such as dam construction, urbanisation, mining, land drainage and
deforestation can all impact river channels. Faster, more peaky flow
from urbanisation or deforestation can accelerate erosion. In the USA,
channel volumes have been found at up to six times the size of those
similar rivers with more natural flows. Dams are having a major
impact on river flows, channels and sediment dynamics. For example,
the Nile now only transports 8 per cent of its natural load of silt below
the Aswan Dam thereby reducing the fertility of the downstream
flood plains and accelerating river bed and coastal erosion as the lack
of sediment entering the sea no longer replenishes the sediment being
eroded by wave action (see section on coastal processes below).
The above discussion has shown that river channels are not static
features in the landscape. Despite this, rivers often act as boundaries
for land ownership which can lead to disputes because the river
course changes through time and so the boundary between prop-
erty, counties, states and countries can change too. Understanding
the processes and dynamism involved in river channels is essential
to good management otherwise great expense might be incurred
when engineering structures fail due to river channel change.
There are many examples over the last century of engineering fail-
ures around rivers. A notable example of this is the Mississippi
River, which regained much of its sinuosity following engineered
channel straightening in the early twentieth century. Indeed, many
rivers that have been subject to engineering features, such as
straightening, became quite sterile environments as the biological
variety within the river was removed so that wildlife suffered.
Many of these rivers are now being rehabilitated to try to restore
features that encourage greater wildlife (such as meander bends
with pools and riffles) and also to work with the erosion and depo-
sition processes within rivers rather than against them.
Water quality and pollution
Water quality is a measure of the various chemicals that are found
within the water. Water quality can vary naturally and be influ-
enced by human action. Water pollution occurs when the water
environment is changed so that the species using the water can no
