Changes in Channel Characteristics
The velocity of a river increases as you go from source to
mouth. This is because rivers at the mouth have highest
hydraulic radius and greatest cross sectional area.
Hydraulic radius is a measure of efficiency – the larger the
hydraulic radius, the greater the efficiency. The more efficient
a channel is, the higher its velocity and discharge. This is
because with a large hydraulic radius (cross sectional area /
wetted perimeter) a smaller proportion of the water is in
contact with the bed and banks. This means that friction is
lower, so less energy is lost. As we move towards the mouth,
the channel bed roughness decreases; this means that there is
much less friction, so less energy loss and a greater velocity. In
the upper course, lots of boulders increase the wetted
perimeter and so the hydraulic radius is smaller, meaning more energy is lost
through friction.
Discharge increases downstream because more tributaries and surface run off joins
the main channel.
Landforms of Fluvial Erosion
Waterfalls eg Thornton Force, River Twiss
A waterfall is a place where flowing water rapidly drops in elevation as it flows
over a steep region or a cliff. Waterfalls are found in a river’s upper course, or in a
rejuvenated lower course, where the river has large amounts of potential energy.
Waterfalls are often ‘knick points’ marking a change in the bed gradient due to
differing rates of erosion. A waterfall can be formed where:
a) A river has been rejuvenated and therefore has new erosion power
b) The river flows over the edge of a hard cap rock with a softer rock underneath
c) The river flows over alternating bands of hard and soft rock
Formation of a
waterfall: Water flows
over hard limestone cap
rock. Below this, there is
a softer slate rock,
known as greywacke.
The planes of parting are
in the same orientation
as the flow of the water,
and so this is eroded
more easily. A plunge
pool is created by
hydraulic action, and
undercutting of the soft
rock eventually causes the overhang to collapse. The fallen rock can be used to
erode the plunge pool further by abrasion, and also creating potholes. The waterfall
retreats upstream, headward erosion, leaving a gorge.
Potholes
Potholes are found in the upper and middle courses of a
river, where the channel is high above base level and
therefore has lots of potential energy. The bed load is
angular and the flow turbulent due to high velocities.
Pebbles become trapped in hollows and due to strong
vertical eddies, the load drills holes in the rock through
the process of abrasion. The edges are smoothed by the
action of the swirling, eddying water.
Rapids
Rapids are areas along the rivers course where water
becomes more turbulent often creating white water. It
is caused by a localised increase in gradient along the
rivers gradient or where the river flows over alternating
bands of harder and softer rocks. These are often linked
in with pool and riffle sequences (the rapids form the
riffles). The pools are areas of deeper water whereas the
riffles are areas of shallower water. The pool is an area of
greater erosion as the water is deeper and therefore
flows faster, whereas the riffles encourage deposition
because they are shallower.
Landforms of Fluvial Deposition
Braiding
Braiding is when load is deposited in the middle of a
channel, forcing the water to diverge into a number of
distributaries which seek to find the easiest route past the
obstructing sediment. Braiding occurs in areas where there
are frequent significant changes in discharge;
Glacial streams have a high discharge in spring due to melt
water. This increases the river’s competence, and so more
load can be transported. However, as temperatures decrease and there is no longer
melt water adding to the river’s discharge, the river is no longer competent for the
transportation of the load and so deposits it as braids in the middle of the channel.
Braided channels occur where the gradient is steep, there is a large sediment load
and the river discharge fluctuates daily or annually. Braids are mobile landforms;
they change their positions rapidly.
Levees
A levee is a natural raised embankment formed as a river
overflows its banks. Larger material is deposited by the river
closest to the channel; this is the heaviest load and so is
dropped first. This heavier load settles first while the finer
material travels further over the floodplain. Over time, these
deposits build up, creating the natural embankments.
Flood plains
A flood plain is an area at the side of a river channel,
built up from alluvial deposition. Flood plains develop
when rivers at bank full spill over onto the surrounding
land. This results in a sudden increase in the wetted
perimeter (the area of land with which the river is in
contact), and so a decrease in hydraulic radius (a
measure of the efficiency of the channel). These
factors increase the amount of friction that the water
must flow against, which decreases the water’s energy. It is no longer competent to
carry its load, and so any material that is being carried in suspension is deposited.
Over time, the layers of deposition build up to create the floodplain. A flood plain
can be widened by meander migration. As the river becomes more sinuous, as more
erosion and deposition occurs, the flood plain is extended as eventually areas of high
land (such as interlocking spurs) are removed.
Deltas
A delta is a flat, low lying area of land at sea
level. Deltas are found at the mouth of a
river, where it reaches the sea or a lake.
The velocity of the river is greatly
reduced, and so its competence and
capacity decreases. The wetted perimeter
increases so the hydraulic radius
decreases, making the river less efficient.
The bed load and material being carried in
suspension is deposited. The topset beds are the large and heavy particles which are
deposited first. The foreset beds are the medium particles, deposited as angled
wedges. The bottom set beds are the finest particles which travel furthest into the
sea or lake before being deposited. If sediment is coarse grained arcuate deltas
form. If it is fine grained then bird’s foot deltas can be created (e.g. Mississippi).
ARCUATE DELTA
CUSPATE DELTA
BIRD’S FOOT DELTA
Conditions necessary for formation:
Large sediment load of river
Weak sea currents that are incapable of removing sediment after its deposition
and which have a small tidal range.
Landforms of fluvial erosion and deposition
Meanders
A meander is a large, sweeping curve in a river’s lower and middle course. They are
formed by a combination of erosion and deposition.
Meanders form where alternating areas of deep water (pools) and
shallow water (riffles) are evenly spaced along a stretch of river.
Where the water is deeper, there is a larger hydraulic radius,
meaning that there is less friction and so more energy and a greater
erosive power. At riffles, there is more friction, so a loss of energy
occurs. The fastest flow (“Thalweg”) becomes uneven, and goes
from one side of a river to the other. Turbulence increases around
pools as the water speeds up, causing corkscrew like currents, called
helicoidal flow, to spiral from bank to bank between pools. The
helicoidal flow causes the material eroded at the outer bank to be
deposited at the inner bank, creating the asymmetric cross section.
Continual undercutting of the outside bank river cliff and deposition
on the inside bank slip off slope results in the exaggeration of the
meander, making it more sinuous, causing the river to migrate
laterally, widening the valley floor and its floodplain
Faster moving water on the outer bank caused
by centrifugal force makes erosion occur.
Water has more energy and erodes the bed
and banks through processes of lateral
hydraulic action and abrasion.
Slow moving water loses energy due to friction
with he bed and banks. Deposition occurs
because the river no longer has enough energy
to carry its load.
Ox Box Lakes
An oxbow lake is formed when the neck of a meander is broken through and
deposition dams off to the loop, leaving a lake. As rivers become more sinuous, more
erosion occurs at the outer bank, and more deposition at the inner bank, meaning
that the meander neck gets thinner and thinner. At times of high discharge, the river
has enough energy to erode through the meander neck, giving the channel a
straighter, shorter route downstream. Deposition of a point bar seals off the
meander loop, creating a cut off loop. This can be colonised by vegetation, and
eventually, if all of the water evaporates, it will become land once more, leaving a
‘meander scar’ on the landscape.
Rejuvenation; Knickpoints, Incised meanders and River Terraces
Rejuvenation is the renewal of the river’s energy as a result of a relative fall in base
level. This can be caused by the land rising relative to the sea as a result of crustal
movements (ISOSTATIC) or a fall in sea level (EUSTATIC). The drop in base level gives
the river greater potential energy, increasing its vertical erosion potential.
A knick point relates to the extent to which the river has created a newly graded
profile to adjust to the new base level. It is identified by a break in slope (sharp
change in gradient) and is usually marked by a waterfall. This is the junction between
the original long profile and the new long profile.
An incised meander is formed when the change in base level
continues for a long period of time. A river will erode
vertically forming one of 2 types of incised meander:
Entrenched meanders (eg. River Wear, Durham) have
symmetrical cross sections and result from a very rapid
incision of the valley sides or due to the resistance of the
valley sides to erosion. Ingrown Meanders occur when the
incision by the river is less rapid allowing the river time to shift laterally, producing
an asymmetrical cross section (eg. River Wye, Tintern Abbey)
River Terraces are remnants of former flood plains which, following
vertical erosion caused by rejuvenation, have been left high above
the maximum level of present day flooding. If a river cuts rapidly
into its plain, a pair of terraces of equal height may be seen at each
side of the river, creating a valley in valley feature. More often, the
river cuts down relatively slowly enabling it to meander at the same
time. The result is unpaired terraces, present at different points at
different levels.