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Summary IEB Grade 12 Geography-Fluvial Processes
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IEB Geography notes for Grade 12 These notes are on Fluvial Processes
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F l u v i a l P r o c e s s e sRiver profiles – transverse profiles and longitudinal profiles,
and their relationship to different stages of a river
The longitudinal profile shows the course of a river from its source to its mouth. The transverse
profile is drawn transversely across the valley at right angles to the river.
The longitudinal profile of a river
o The longitudinal profile of a river is the visual representation of the gradient down
which the river flows from its source to its mouth.
o The stages into which a river may be divided are evident on the longitudinal profile.
1. The upper course is the steeper upstream (towards the source) section of the
profile near the source of the river.
2. The middle course is downstream (towards the mouth) of the upper course, where
the river has left the mountains and the gradient is more gentle.
3. The lower course is the flatter, final stage of the river as it gets closer to sea level.
o The gradient of a river is usually steeper where it rises in the mountains and gets
progressively flatter as the river flows towards the sea.
Permanent base level and temporary base level
o In reality, there are many changes in gradient along the course of a river, so a river will
never have a smooth concave profile.
o The changes in gradient along the course of a river are referred to as temporary base
levels because the vertical erosion (erosion downwards) of the river is slowed down
temporarily.
o Rapids, waterfalls, lakes and dams are all temporary base levels.
o Sea level is known as the permanent base level or ultimate base level, as this is the
lowest level to which a river will erode the land.
, The transverse profile of a river
The transverse profile of a river is also known as the cross-profile of a river. This is drawn
transversely across the valley at right angles to the river and shows the shape of the valley.
o Generally, the cross-profile of a river valley is related to its gradient and longitudinal
profile.
o Where the gradient is steep and there is greater vertical erosion, the cross-profile will
be steep, deep and narrow.
o Where the gradient is gradual, the cross-profile will be wider and shallower.
o The cross-profile of a river in the upper course will have a deep, narrow V-shape, the
cross-profile of a river in the middle course will have a more open V-shape, and the
cross-profile of a river in the lower course will have a wide, open V-shape.
o Upstream of a temporary base level, such as a waterfall, the cross-profile is wider and
more open in shape, as there is less vertical erosion.
o Downstream of the waterfall is a steep-sided gorge or ravine (deep, steep-sided
valley), as the velocity of the water is high and there is much vertical erosion.
o Vertical erosion (erosion downwards) by the river deepens the channel.
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