V-Shaped Valleys
In the upper course of a river, water flows quickly through a narrow channel with
a steep gradient; as it does so it cuts downwards. This vertical erosion results in
a number of distinctive landforms including the steep sloping v-shaped valley
through which the river flows in its upper course.
So how does a v-shaped valley form?
1. Vertical erosion (in the form of abrasion, hydraulic action and solution)
in the river channel results in the formation of a steep sided valley
2. Over time the sides of this valley are weakened by weathering processes
and continued vertical erosion at the base of the valley
3. Gradually mass movement of materials occurs down the valley sides,
gradually creating the distinctive v-shape.
4. This material is then gradually transported away by the river when there
is enough energy to do so.
As the river flows through the valley it is forced to swing from side to side
around more resistant rock outcrops (spurs). As there is little energy for
lateral erosion, the river continues to cut down vertically flowing between
spurs of higher land creating interlocking spurs.
Visualising V-shaped Valley Formation
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See this excellent animation from www.cleo.net.uk on interlocking spurs
Key Term Check
V-shaped Valley - a valley which resembles a 'v' in cross section. These valleys
have steep sloping sides and narrow bottoms.
Interlocking Spur - spurs are ridges of more resistant rock around which a river
is forced to wind as it passes downstream in the upper course. Interlocking spurs
form where the river is forced to swing from side to side around these more
resistant ridges.
Load - collective term for the material carried by a river
An other feature found in the upper course of a river, where vertical erosion is
dominant, is a waterfall. The highest waterfall in the world is the Angel Falls in
Venezuela (see picture right) which have a drop of 979m. Other particularly
famous examples include Niagara Falls (North America), the Victoria Falls (on
the Zambia / Zimbabwe border) and the Iguazu Falls (South America).
Although much smaller in scale, there are many waterfalls in the upper course of
UK rivers (e.g. Thornton Falls, Yorkshire - above), but how do they form?
The formation of Waterfalls
1.Waterfalls are found in the upper course of a river. They usually occur where
a band of hard rock lies next to soft rock. They may often start as rapids.
2. As the river passes over the hard rock, the soft rock below is eroded (worn
away) more quickly than the hard rock leaving the hard rock elevated above the
stream bed below.
3. The 'step' in the river bed continues to develop as the river flows over the hard
rock step (Cap Rock) as a vertical drop.
4. The drop gets steeper as the river erodes the soft rock beneath by processes
such as abrasion and hydraulic action. A plunge pool forms at the base of the
waterfall.
5. This erosion gradually undercuts the hard rock and the plunge pool gets
bigger due to further hydraulic action and abrasion.Eventually the hard cap rock
is unsupported and collapses. The rocks that fall into the plunge pool will
continue to enlarge it by abrasion as they are swirled around. A steep sided
valley known as a gorge is left behind and as the process continues the
waterfall gradually retreats upstream.
Visualising Waterfall Formation:
The labelled diagram of a cross section through a waterfall below, shows the
formation process (click on diagram for a larger version).
There are also number of excellent animations on the internet which can help
you visualise how a waterfall forms. Try out the following:
1. A good step by step animation of waterfall formation showing all the main
stages involved (Wycombe High School)
2. This simple but excellent animation showing an aerial view of waterfall
formation clearly shows the development of a gorge as the waterfall retreats
upstream! (Cleonet)
3. An animation of waterfall formation from a different 3-dimensional perspective.
Look carefully at how the plunge pool is enlarged during the formation process.
(Classzone)
The Middle Course of a River
Having studied the characteristics of a river in its upper reaches we now need to
follow the river as it enters its middle course. Here the river channel has
become much wider and deeper as the channel has been eroded and the river
has been fed by many tributaries upstream. Consequently, despite the more
gentle gradient the velocity of flow may be as fast as in the uplands. As well as
changes in the river channel, its surrounding valley has also become wider
and flatter in cross-section with a more extensive floodplain. One of the most
distinctive features of the river in the middle course is its increased sinuousity.
Unlike the relatively straight channel of the upper course, in the middle course
there are many meanders (bends) in the river.
Meander Formation
Meanders form due to
the greater volume of water carried by the river in lowland areas which results in
lateral (sideways) erosion being more dominant than vertical erosion, causing the
channel to cut into its banks forming meanders.
1. Water flows fastest on the outer bend of the river where the channel is
deeper and there is less friction. This is due to water being flung towards the
outer bend as it flows around the meander, this causes greater erosion which
deepens the channel, in turn the reduction in friction and increase in energy
results in greater erosion. This lateral erosion results in undercutting of the river
bank and the formation of a steep sided river cliff.
2. In contrast, on the inner bend water is slow flowing, due to it being a low
energy zone, deposition occurs resulting in a shallower channel. This
increased friction further reduces the velocity (thus further reducing energy),
encouraging further deposition. Over time a small beach of material builds up on
the inner bend; this is called a slip-off slope.
Remember - a
meander is asymmetrical in cross-section (see diagram). It is deeper on the
outer bend (due to greater erosion) and shallower on the inside bend (an area
of deposition).
Over time meanders gradually change shape and migrate across the floodplain.
As they do so meander bends becomes pronounced due to further lateral erosion
and eventually an ox-bow lake may form.
Ox-Bow Lake formation
1. As the outer banks of a meander continue to be eroded through processes
such as hydraulic action the neck of the meander becomes narrow and
narrower.
2. Eventually due to the narrowing of the neck, the two outer bends meet and
the river cuts through the neck of the meander. The water now takes its
shortest route rather than flowing around the bend.
3. Deposition gradually seals off the old meander bend forming a new
straighter river channel.
4. Due to deposition the old meander bend is left isolated from the main
channel as an ox-bow lake.
5. Over time this feature may fill up with sediment and may gradually dry up
(except for periods of heavy rain). When the water dries up, the feature left
behind is knwon as a meander scar.
Visualising Meander formation:
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This excellent animation (from Cleo Net) looks at the process of meander
formation and how meander develop overtime becoming more sinuous,
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resulting in the narrowing of the meander neck and the formation of an oxbow lake.
Another excellent animation - this one focuses particularly on the
development of an ox-bow lake as a meander continues to grow.
Key Terms Check:
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Meander - a bend in a river
River Cliff - a small cliff formed on the outside of a meander bend due to
erosion in this high energy zone.
Slip off Slope - a small beach found on the inside of a meander bend
where deposition has occured in the low energy zone.
Ox-bow lake - a lake formed when the continued narrowing of a meander
neck results in the eventual cut through of the neck as two outer bends
join. This result in the straightening of the river channel and the old
meander bend becomes cut off forming an ox-bow lake.
Meander scar - feature left behind when the water in an ox-bow lake dries
up.
Cap Rock - layer of hard resistant rock forming the 'step' over which the
'falls' occur in a waterfall.
Waterfall - a cascade of water over a hard rock step in the upper course
of a river
Plunge Pool - a deep pool beneath
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Gorge - a steep sided valley left behind as a waterfall retreats upstream
Abrasion - where rocks and boulders scrape away at the river bed and
banks
Hydraulic Action - where the force of water compresses air in cracks
resulting in mini-explosions as the increased pressure in the cracks is then
released.
Moving between the Middle and Lower Course of the River
As a river continues its journey towards the sea, the valley cross section
continues to become wider and flatter with an extensive floodplain either side of
the channel. The river erodes laterally and deposition also becomes important.
By the time it reaches the lower course the river is wider and deeper and may
contain a large amount of suspended sediment.
When the river floods over the surrounding land it loses energy and deposition of
its suspended load occurs. Regular flooding results in the building up of layers of
nutrient rich alluvium which forms a flat and fertile floodplain.
When the river water bursts its bank, the shallower depth of water flowing over
the surface results in frictional drag and a consequent reduction in velocity
(speed) of flow. This results in the loss of energy and therefore deposition
occurs. The heaviest materials are deposited first as these require the most
energy to be transported and therefore build up around the sides of the river
forming raised banks known as Levées (click on diagram above). Finer material
such as silt and fine clays continuing to flow further over the floodplain before
they are deposited.
Find out more:
See these Wikipedia articles on floodplains and on natural and artificial levées
This article called "Raising the Bar for Levees" looks at the role of Levees in flood
protection an idea we will come back to in a few lessons time when we look at
the causes, effects and management of river floods.
Visualising
Floodplain and Levée formation
A nice animation showing the development of a floodplain and Levées Floodplain
animation
Key Term Check
Floodplain - the area of land around a river channel which is formed during
times of flood when the amount of water in a river exceeds its channel capacity
and deposition of rich silt occurs.
Levées - a raised river bank (can be natural features formed by deposition or
artificial structures built to increase channel capacity and reduce flood risk)