Geoscience Reference
In-Depth Information
landforms. Rivers result in the formation of a number of important landforms that are
described below.
The most important landforms resulting from river processes are the channels of
rivers and streams (Fig. 11). When rain falls onto a land surface some of it soaks into
the land (forming groundwater), whilst the remainder runs along the surface, collecting
in topographical lows and producing stream and river channels. Today, many of South-
ern England's river channels tend to be relatively narrow and shallow - only metres or
tens of metres in width and less in depth - so they occupy an extremely small percent-
age of the area that they drain. However, they are still the dominant agents of landscape
change, causing downwards and/or sideways erosion as well as acting as conduits to
transport the eroded material out of their catchments.
Most river channels develop a sinuous course, becoming curved (or meandering )
to varying degrees, or developing a number of channels separated by islands of sedi-
ment (becoming braided ). The positions of the curves or islands change with time as
sediment is shifted downstream, and the position of a river channel will change with
time correspondingly.
Because of their ability to erode material and remove the resulting debris, river
channels create valleys. The sides of a river valley are referred to as slopes. When a
channel cuts downwards the valley sides generally become steeper and slope material
(generated by ongoing weathering processes) moves down-slope towards the channel.
The material is transported either as small individual fragments or as larger mass flows.
Where down-slope movements involve the collapse of large areas of material, the
terms landslip or slump are often used. Slope material is then deposited in the channel
and removed downstream by the river.
The simplest valleys result from down-cutting by a river or stream to yield a V-
shaped profile in cross-section. The gradient of the valley sides depends on the strength
of the material that the slopes are composed of in the face of erosion. Stronger materi-
als are more difficult to erode and remove, and so can form steeper slopes than weaker
materials. In some areas, the river channel is unable to form valley slopes as the mater-
ial is too weak to form a noticeable gradient. In the Areas we will be investigating, it is
clear that some of the slopes are largely the result of a particularly strong layer in the
bedrock resisting erosion as the landscape has developed.
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