Rivers 5
The long profile of the river
A river uses energy to carry out its ‘work’ of erosion, transport and deposition
of sediment and to move the water in its channel. This energy is produced
when water flows down a slope, so the height of a river above sea level
determines this. The lowest level of a river is called base level and for most
rivers it is sea level. The long profile of a river is a graph drawn along the
course of the river from its source to its mouth. The axes may be calibrated in
conventional units of height and length or alternatively, as percentages of the
river’s total drop (relief) and length. The study of river long profiles shows
that they have a concave shape
with a steeper upper reach and a gentler lower reach. River processes are
related to long profile because every river is trying to achieve a smooth
concave long profile. The ideal long profile is called a graded long profile
and in this ideal situation the available energy and the river processes will be
balanced. Where this is not the case the river will be working to smooth out
its long profile by erosion transportation or deposition. Rivers are trying to
erode to base level (usually sea level). If base level falls, as during tectonic
uplift it sets in train rejuvenation. Adjustment begins at the mouth of the river
and works its way upstream, the point of change being the knickpoint. If
base level rises e.g. during an interglacial period, adjustment is by deposition
rather than erosion.
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Rivers 5
Channel Cross Section
As a river travels downstream the cross section of the river channel also
changes dramatically as a result of river processes.
The most obvious change is that the cross –sectional area changes
downstream – that is the river channel becomes bigger. It also becomes
more efficient. This can be calculated by using the wetted perimeter (the
part of the channel that is in contact with the water at any particular time).
The fuller the channel, the greater the WP and therefore more water exposed
to the frictional drag of the channel sides. The hydraulic radius of a stream
channel is calculated by dividing its cross sectional area by the length of the
WP. It is a measure therefore of how much water there is in relation to the
amount of the channel currently occupied by the stream. The higher the value
of the hydraulic radius the greater the efficiency of the stream or river.
High values are typically associated with streams that have a large discharge
and those that have a cross sectional area that is approximately semi –
circular. Just as streams and rivers seek to achieve the most efficient graded
profile, so in their channels they try to achieve the most efficient cross channel
shape. In contrast, streams with low values are typically wide and shallow.
Patricia Marcus Curran
Rivers 5
River Landforms
The diagram clearly shows that a river and its valley changes downstream.
River landforms can be created by erosion, deposition or a combination of the
two.
Rivers can be divided into three distinct sections, Upper, Middle and Lower
course. The river features differ between each course and you must know
where each type of landform is found.
Upper Course
In the upper course of a river vertical erosion dominates as the stream cuts
downwards. Weathering and erosion of the valley sides removes material
causing the valley sides to retreat forming a v shaped valley.
V Shaped valley
formed by a river
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U shaped valley
formed by glaciation
Rivers 5
This material moves downslope and it may eventually enter the stream
channel where over time the river will erode and transport downstream. If the
river removes the material transported downslope more quickly than material
is provided then a steeper valley is produced. The steepness of the valley
sided also depends on, climate, rock structure and vegetation.
The streams are rarely straight in the upper course but wind round
interlocking spurs of more resistant rock. There is therefore some lateral
erosion taking place as well as vertical erosion. The stream works to achieve
an equilibrium or balance between slope retreat, debris supply and vertical
erosion.
Potholes
Active corrasion along a stream produces potholes especially in fast flowing
rivers with strong eddying. Potholes are cylindrical holes ‘drilled’ into the
bedrock by turbulent high velocity flow. The eddying creates a shallow bowl
that may become occupied with small stones and pebbles. The constant
swirling of the pebbles deepens the depression into a pothole in a process
known as pothole drilling. Adjacent potholes may join together creating
sudden and considerable deepening of channels such as can be seen on the
River Wharfe in North Yorkshire.
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Rivers 5
Waterfalls and Rapids
Waterfalls and rapids occur where the long profile of the river is steep. This is
usually the result of an outcrop of more resistant rock, often called the cap
rock, overlying a softer rock. Erosion especially by the hydraulic power of the
water is concentrated in the plunge pool at the base of the waterfall. The
waterfall becomes undercut and the hard cap rock above periodically
collapses resulting in the headward erosion of the waterfall and the formation
of a gorge of recession.
Formation of a waterfall
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Rivers 5
Rapids
Rapids develop where the gradient of the river increases without a sudden
break of slope ( as in a waterfall) or where the stream flows over a series of
gently dipping bands of harder rock. Rapids increase the turbulence of a river
and hence its erosive power.
Formation of Rapids
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Rivers 5
The Middle course of the river
By the middle course of the river the channel is wider and deeper. There will
be a floodplain with gentle slopes called bluffs. At this point the features
which are most prominent are a product of both erosion and deposition.
Meanders
Meanders are bends in the course of the river. They often begin to appear as
a river reaches its middle course and the gradient of the channel becomes
less steep. They are thought to be a response by the river to the excess
energy it now has having emerged from the steep gradient of the upper
course.
Meanders are characterised by a river cliff on the outside of the bend and a
gently sloping slip off slope, sometimes called a point bar, on the inside of the
meander.
Meanders are a result of helicoidal flow in which the fastest current spirals
downstream in a corkscrew fashion. The movement results in erosion on the
outside bend of the meander to form a river cliff and deposition on the inside
bend forming a slip off slope. The flow moves sediment downstream but not
in a straight line. The material eroded from the outer bank of a meander will
spiral downstream and be deposited on the inner bank building up or
aggrading to form point bar deposits. The flow of fastest water is called the
thalweg. This produces that characteristic asymmetrical shape of the cross
section of a meander.
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Rivers 5
Cross section of a meander
Plan view of meanders / pools and riffles
Meanders are linked with the development of pools and riffles in the channel
bed. The riffles are areas of deposition of coarse material that creates areas
of shallow water. The pools are areas of deeper water between the riffles.
The pools and riffles develop in sections along a river channel creating
differences in the channel gradient. The coarser pebbles create a steeper
gradient than the eroded pools. The pools and riffles are usually equally
spaced in both natural and artificial channels. The water increases its velocity
as it passes over the riffled surface while it flows more sluggishly out of the
deeper pools. The riffles are not static but pebbles and gravel are added to
them from upstream at about the same rate as material is lost downstream.
The sequence of pools and riffles develops until the average spacing is
between 5 and 7 channel widths and the entire channel is meandering
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Rivers 5
Meander migration:
Meanders change their location over time hence the term ‘migrate’. The
meanders move in two directions:
Migrating Meanders
Sideways because of lateral erosion. This broadens the floodplain and
erodes away the ends of the interlocking spurs.
Downstream due to the pattern of erosion in relation to the thalweg (the
zone of fastest velocity in the channel flow). The greatest erosion is
downstream of the midpoint in the meander bend because the flow of
the strongest current does not perfectly match the meandershape.
Incised
meanders, a
product of
vertical erosion
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Rivers 5
Meander wavelength is the distance from the tip of one bend to the next –
usually 10 tomes the channel width
Meander wavelength
Pool
Riffle
Ox-bow lakes
As the meander moves downstream, it is possible for one side of the meander
to ‘catch up’ with the river channel downstream. Eventually the river may
break through the neck of the meander causing a major river diversion. The
river abandons the original meander channel in favour of the shorter, steeper
‘new’ channel. The formation of the cut off is usually accomplished at times of
high energy of the river, such as a bankfull, peak discharge or flood
conditions. Reduced velocity at the entrance to the former meander ,
especially as floodwaters subside, results in deposition which seals off the
meander to leave an ox-bow lake. The water in the ox-bow lake becomes
calm resulting in deposition of any sediment and over time the water in the
lake may disappear through infiltration and evaporation and leave a meander
scar.
Patricia Marcus Curran
Rivers 5
Patricia Marcus Curran
Rivers 5
Depositional Landforms
In the lower course of the river, where the gradient is less steep, it is
depositional landforms that dominate.
Levees
Levees are high banks of silt close to the river channel. They are formed by
repeated river flooding which is most common in the lower course of the river
where there is a floodplain. Rivers with well developed levees generally carry
a large load of sand and silt, indicating active erosion in the upper course, and
they fold repeatedly.
When a river overflows its banks there is an increase in friction between the
water that leaves the river channel and the floodplain. The water on the river
banks and valley floor is shallower and the velocity falls. This results in
deposition of the load. The coarsest material is deposited first building up
natural embankments along the channel called levees. In times of low flow
such as during a dry season the river may also deposit sand and silt
aggrading (building up) the river bed. This raises the river and in some cases
may lead to the river level being above the level of the floodplain.
The levees along the banks of the Mississippi are very high; some have been
artificially heightened and strengthened to act as flood defences. The river
bed has also been raised by deposition or aggradation of material so that the
river now flows above the level of its floodplain. This is disastrous when
flooding occurs (as in Hurricane Katrina) or a levee collapses because the
water can no longer naturally drain back into the river. Large stretches of the
Mississippi are dredged to increase the capacity of the river channel and to
reduce the level of the river.
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Rivers 5
Formation of levees
Floodplains
A floodplain is the most common depositional feature of a river. It is the
relatively flat area of land either side of the river forming the valley floor.
Floodplains are most well developed in the lower course of a river as it nears
the sea. They are composed of alluvium – river deposited material – and form
some of the most fertile soil for agriculture to be found in the world.
The width of the floodplain is determined by meander migration and lateral
erosion, while the depth of the alluvium on the floodplain is the result of
flooding. As meanders migrate downstream, eroding laterally, the valley floor
is widened. Point bar deposits and meander scars are incorporated into the
floodplain. The they are stabilized vegetation as the meanders migrate and
abandon their former courses. Coarser sands and gravel are incorporated
into the floodplain in this way.
When the river floods as the water overflows the channel velocity falls and
deposition takes place, contributing in particular finer sands and clays to the
alluvium on the floodplain. Rivers which flood their lower courses frequently
and carry large load tend to build up great thicknesses of alluvium on the
floodplain, such as the Ganges valley in Bangladesh.
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Rivers 5
Braided Streams
A braided stream has islands, or eyots, of deposited material within the
channel.
The overall channel is relatively straight although the eyots and smaller
channels may rapidly and frequently change their position. Braiding tends to
occur in streams where the load contains a high proportion of coarser sands
and gravels.
Braiding is a characteristic of streams and rivers with very variable discharges
common in semi- arid environments or glacier fed streams. In semi-arid
environments torrential downpours lead to overland flow creating streams with
high velocity and large loads. Rapid evaporation and infiltration following the
storm rapidly reduces the volume and velocity resulting in the deposition of
the load. Streams and rivers fed by glaciers have high discharges when there
is rapid melting of the ice during the day in summer but low discharges at
night and in winter.
At times of high discharge the streams are capable of transporting a large
load. However when the velocity falls the stream’s competence and capacity
are reduced. The large load is deposited forming the eyots and causing the
stream to divide into a series of smaller channels.
Patricia Marcus Curran
Rivers 5
Patricia Marcus Curran