As a river flows from its source to its mouth, changes take place in its morphology and landforms. In the upper course, waterfalls and rapids form where the gradient suddenly changes. Potholes are carved by turbulent water. Meanders develop from sediment bars and cause lateral stream migration. Oxbow lakes are left when meanders are cut off. In lowlands, flooding leads to levees, floodplains and deltas where deposition exceeds removal of sediment at the river's mouth. A variety of landforms result from erosion and depositional processes along a river's course.

2. The long profile of a river illustrates the
changes in altitude of the course of the
river from its source, along the entire
length of its channel, to the river mouth.
In general, the long profile is smoothly
concave, with the gradient being steeper
in the upper course and becoming
progressively gentler towards the mouth.

3. Irregularities in the gradient frequently
occur and may be represented by rapids,
waterfalls or lakes.
There may also be marked breaks or
changes in slope, known as knick points,
which generally are the product of
rejuvenation.
Rejuvenation occurs either when sea level
falls or when the land surface rises.

4. Either situation allows the river to
revive its erosion activity in a vertical
direction.
The river adjust to the new base level,
at first in its lowest reaches, and then
progressively inland.
The processes of erosion and
deposition and transportation along the
long profile of a typical river are
summarised in Figure 1.7

6. The valley cross profile is the view of
the valley from one side to another.
For example, the valley cross profile of
a river in an upland area typically has a
V-shape, with steep sides and a narrow
bottom.

7. Variations in the cross
profile can be described and
explained as follows (also
look at Figure 1.8):
In the upper course - a
narrow steep-sided valley
where rivers occupies the
entire valley floor. This is a
result of dominant vertical
erosion by the river.
In the middle course - a
wider valley with distinct
valley bluffs, and a flat
floodplain. This is as a
result of lateral
erosion, which widens the
valley floor.
In the lower course - a
very wide, flat floodplain in
which the valley sides are
difficult to locate. Here
there is lack of erosion, and
reduced competence of the
river, which results in large-
scale deposition.

8.  Over a long period of time a river may display
an even and progressive decrease in
gradient down the valley, creating the typical
smooth concave shape which has adjusted to
the discharge and the load of the river.
 This may also be referred to as the profile of
dynamic equilibrium, where a balance has
been achieved between the processes of
erosion and deposition.

9. Recently it has been accepted that the
channel may still be graded if it exhibits
some irregularities in its long profile.
Some geographers define the graded river
as that which has been attained when the
river uses up all the energy in the
movement of water and sediment so that
no free energy is left to undertake further
erosion.

10. In this situation the gradient at each point
along the river is sufficient to discharge the
water and load but there is little excess
energy available for further erosion.
If the volume and load of the river change
over the long term, then the river’s channel
and its long profile will also adjust to the
new conditions.

11. Theoretically, river systems
should reach an equilibrium when
the inputs and outputs are
balanced, but changes in the
system bring adjustments to the
profile as the river attempts to
counter the change. In this way it
regulates the system.

12. In relation to rivers, potential or (stored)
energy is fixed by the altitude of the
source of the stream in relation to base
level.
Kinetic energy, or energy due to
movement, is generated by the flow of the
river which converts potential energy into
moving energy.

13. The amount of kinetic energy is
determined by the volume of flowing water
(discharge), the slope or channel gradient
down which it is flowing and its average
velocity.
An increase in velocity and/or discharge
results in an increase in kinetic energy.
All channel processes are dependent on
the amount of energy available. This is a
delicate balance.

14. If there is excess after
transportation of load the river will
erode, but if energy is insufficient to
move the load, deposition will
occur.
The river channel adjusts in
shape and size to accommodate
changes in the volume of water and
sediment.

16. • The channel cross profile (or section) is the
view of the river bed and banks from one side
to the other side at any one point on its
course.
• As a river flows from its source to its mouth, a
number of typical changes take place in the
channel. In the upper course, the channel is
narrow and uneven, because of deposited
boulders.

17. • As the river enters its middle course and starts
to meander, the channel becomes
asymmetrical on the river bends but mainly
smooth and symmetrical on the straight
stretches.
• In the lower course, the river widens and
deepens further, but banks of deposition and
eyots (islands of deposition) can disrupt the
shape of the channel cross section, leading to
a braided channel. Sometimes embankments
called levees can be seen on either side of the
channel. Levees can also be man-made.

18. • The shape of the channel influences the
velocity of the river.
• In the upper course, where the channel is
narrow and uneven due to the presence of
large boulders, there is a large wetted
perimeter.
• The wetted perimeter is the total length of
the river bed and banks in cross section that
are in contact with the water in the channel.

19. • River levels only rise after heavy rain or
snowmelt and in the upper course the river is
relatively shallow.
• When there is a large wetted perimeter in
relation to the amount of water in the river,
there is more friction.
• Friction results in energy loss, and
consequently, the velocity of the river is
slowed.

20. • As channels become larger and smoother, in the
middle and lower course of the river, they tend to
be more efficient.
• The wetted perimeter is proportionally smaller
than the volume of water flowing in the channel.
Therefore, there is less friction to reduce velocity.
Although the turbulent flow of mountain streams
might appear faster than that of the gently
meandering downstream channel, average
velocity is actually slower.
• This is because so much energy is expended
overcoming friction on the uneven channel bed in
the upper course, whereas in the lower course
there is little to disrupt water flow.

22. • A high hydraulic radius means that the river is
efficient. This is because the moving water
loses proportionally less energy in overcoming
friction than when the ratio between the
cross-sectional area and the wetted perimeter
is low.
• Larger channels tend to be more efficient;
area increases to a greater degree than
wetted perimeter.

25. As a river flows from its source to its mouth a
number of changes take place in its
morphology. These changes affect the shape
and size of the channel and result in
distinctive landforms along its course. Some
of these landforms are the result of erosion,
some are the results of deposition and some
are the consequence of both.

26. Waterfalls and rapids occur
when there is a sudden change
in the gradient of the river as it
flows downstream. Waterfalls
are more dramatic features
than rapids and may be the
result of:
A resistant band of rock occurring
across the course of the river
The edge of a plateau
The rejuvenation of the area, giving
the river renewed erosional power as
sea level falls.

27.  The river falls over a rock edge into a deep
plunge pool at the foot of the fall, where the
layers of weak rock are eroded more quickly
than the overlying resistant rock.
 The force of the swirling water around the
rocks and boulders enlarges and deepens the
plunge pool by hydraulic action and abrasion.
 This undercuts the resistant rock above.
Eventually the overhanging cap rock
collapses and the waterfall retreats
upstream, leaving a gorge ahead of it.

29. In upper Teesdale an outcrop of an igneous rock called Whin Sill
causes the formation of the High Force waterfall. The Whin Sill is
the resistant cap rock which overlies softer sandstone,
limestone, shales and coal seams. These are eroded more
quickly, leaving the overhang of High Force. The waterfall
created is 22 meters high- the tallest in England. Ahead of it lies
a gorge stretching over 500 meters down stream.

30. • Potholes are cylindrical holes drilled into the rocky bed of a
river by turbulent high-velocity water loaded with pebbles.
The pebbles become trapped in slight hollows and vertical
eddies in the water are strong enough to allow the
sediment to grind a hole into the rock by abrasion
(corrasion). Attrition rounds and smoothes the pebbles
caught in the hole and helps to reduce the size of the
bedload.
• Potholes can vary in width from a few centimetres to
several metres. They are generally found in the upper or
early-middle course of the river. This is where the valley lies
well above base level, giving more potential for
downcutting, and where the river bed is more likely to be
rocky in nature.

32. o Braiding occurs when the river is forced to spilt into
several channels separated by islands.
o It is a feature of rivers that are supplied with large
loads of sand and gravel.
o It is most likely to occur when a river has variable
discharge. The banks formed from sand and gravel are
generally unstable and easily eroded.
o As a consequence, the channel becomes very wide in
relation to its depth. The river can become choked,
with several sandbars and channels that are constantly
changing their locations.

33. Braiding also occurs in environments in which there
are rapidly fluctuating discharges:
• Semi-arid areas of low relief that receive rivers
from mountainous areas
• Glacial streams with variable annual discharge.
• In spring, meltwater causes river discharge and
competence to increase, therefore the river can
transport more particles.
• As the temperature drops and the river level falls,
the load is deposited as islands of deposition in
the channel.

35. Meanders are sinuous bends in a river.
In low flow conditions straight channels are seen
to have alternating bars of sediment on their
beds and the moving water is forced to weave
around these bars.
This creates alternating shallow sections (riffles)
and deeper sections (pools).
The swing of the flow that has been induced by
the riffles directs the maximum velocity towards
one of the banks, and results in erosion by
undercutting on that side.

36. An outer concave bank is therefore created.
Deposition takes place on the inside of the
bend, the convex bank.
Consequently, although the river does not get
any wider, its sinuosity increases.
The cross section of a meander is asymmetrical.
The outer bank forms a river cliff or bluff with
a deep pool close to the bank. This bank is
undercut by erosion, particularly abrasion and
hydraulic action. The inner bank is a gently
sloping deposit of sand and gravel called a
point bar.

38. Once they have been created, meanders are
perpetuated by a surface flow of water across
to the concave outer bank with a
compensatory subsurface return flow back to
the convex inner bank.
This corkscrew-like movement of water is
called helicoidal flow.

40. Eroded material from the outer bank is
transported away and deposited on the inner
bank.
Modern research suggests that the flow is
rarely strong enough for the river to transport
material across to the point bar on the oppisite
bank.
Point bars are most likely to be maintained by
sediment from erosion at the bluff of the
meander upstream on the same side of the
channel.

41. The zone of greatest erosion is downstream of
the midpoint in the meander bend, because of
the strongest current does not exactly match
the shape of the meander.
As erosion continues on the outer bank, the
whole feature begins to migrate slowly, both
laterally and downstream.

42. • Oxbow lakes are features of both erosion and deposition.
• An oxbow lake is a horseshoe-shaped lake separated from
an adjacent river. The water is stagnant (still), and in time
the lake gradually silts up, becoming a crescent-shaped
stretch of marsh called a meander scar.
• An oxbow lake is formed by the increasing sinuosity of a
river meander. Erosion is greatest on the outer bank, and
with deposition on the inner bank, the neck of the
meander becomes progressively narrower.
• During times of high discharge, such as floods, the river
cuts through this neck, and the new cut eventually
becomes the main channel. The former channel is sealed
off by deposition.

44. • In its middle and lower courses, a river is at risk
from flooding during times of high discharge.
• If it floods, the velocity of the water falls as its
overflows the banks.
• This results in deposition, because the
competence of the river is suddenly reduced. It is
usual for the coarsest material to be deposited
first, forming small raised banks (levees) along
the sides of the channel.

45. • Subsequent floods increase the size of these
banks and further deposition of the bed of the
river also occurs. This means that the river,
with the channel sediment build up, now
flows at a higher level than the floodplain.
• For this reason, the authorities sometimes
strengthen levees and increase their heights.

47. • Floodplains are created as a result of both
erosion and deposition, although they
accumulation of river deposits suggests that
they are predominantly depositional features.
• They are the relatively flat areas of land either
side of the river, which form the valley floor in
the middle and lower courses of the river.

48. • They are composed of alluvium - river-deposited
silts and clays. Over time, a floodplain becomes
wider and the depth of sediment accretions
increases.
• The width of the floodplain is determined by the
amount of meander migration and lateral erosion
that has taken place.
• Lateral erosion is most powerful just downstream
of the apex (tip/point) of the meander bend.
• Over time, this results in the migration of
meanders, leaving their scars clearly visible on
the floodplain.

49. • Interlocking spurs are eventually removed by
lateral erosion leaving behind a bluff line and
widening the valley.

50. • Feature of deposition
• located at the mouth of a river
• Deposition occurs as the velocity and sediment-
carrying capacity of the river decrease on
entering the lake or sea, and bedload and
suspended material are dumped.
• Flocculation occurs as fresh water mixes with sea
water and clay particles coagulate(mix/join) due
to chemical reactions. The clay settles on the
river bed.

51. Deltas form only when the rate of deposition exceeds
the rate of sediment removal. In order for a delta to
form the following conditions are likely to be met:
• The sediment load of the river is very large.
• The coastal area into which the river empties
its load has a small tidal range and weak
currents. This means that there is limited
wave action and therefore, little
transportation of sediment after deposition
has taken place.

52. 3 Types Of Deposit
• 1- The larger and heavier particles are the first to
be deposited as the river loses its energy. These
form the topset beds.
• 2- Medium graded particles travel a little further
before they are deposited as steep-angled
wedges of sediment, forming a foreset beds.
• 3- The very finest particles travel furthest into the
lake before deposition and form the bottomset
beds.

54. Deltas can be described according to thier
shape:
• The most commonly recognised is the
characteristic arcuate delta, for example the
Nile Delta, which has a curving shoreline and a
dendritic pattern of drainage.
• Many distributaries break away from the main
channel as deposition within the channel itself
occurs, causing the river to braid. Longshore
drift keeps the seaward edge of the delta
relatively smooth in shape.

55. • The Mississippi has a birds foots delta. Fingers
of deposition build out into the sea along the
distributaries channels, giving the appearance
from the air, of a birds claw.
• A cuspate delta is pointed like a cup or tooth
and is shaped by gentle, regular, but opposing,
sea current or longshore drift.

57. • Rejuvenation occurs when there is either a
fall in sea level relative to the level of the
land or a rise of the land relative to the sea.
• This enables a river to renew its capacity to
erode as its potential energy is increased.
• The river adjusts to its new base level, at
first in its lower reaches and then
progressively inland. In doing so, a number
of landforms may be created: knick points,
waterfalls & rapids, river terraces and
incised meanders.

58. A knick point is a sudden
break or irregularity in
the gradient along the
long profile of a river.

59. • The river gains renewed cutting power (in the
form of vertical erosion), which encourages it
to adjust its long profile.
• In this sense the knick point is where the old
long profile joins the new.
• The knick point recedes upstream at a rate
which is dependent on the resistance of the
rocks, and may linger at a relatively hard
outcrop.

60. A river terrace is a remnant of a former floodplain,
which has been left at a higher level after
rejuvenation of the river.
• Where a river renews its downcutting, it sinks its
new channel into the former floodplain, leaving
the old floodplain above the level of the present
river.
• The terraces are cut back as the new valley is
widened by lateral erosion.
• The river Thames has created terraces in its lower
course by several stages of rejuvenation.

62. • If a rejuvenated river occupies a valley with well developed
meanders, renewed energy results in them becoming incised
or deepened.
• The nature of the landforms created is largely a result of the
rate at which vertical erosion has taken place.
• When incision is slow and lateral erosion is occurring, an
ingrown meander may be produced.
• The valley becomes asymmetrical, with steep cliffs on the
outer bends and more gentle slip-off slopes on the inner
bends.
• With rapid incision, where downcutting or vertical erosion
dominates, the valley is more symmetrical, with steep sides
and a gorge-like appearance. These are described as
entrenched meanders.

63. • The valley becomes asymmetrical, with steep
cliffs on the outer bends and more gentle slip-
off slopes on the inner bends.
• With rapid incision, where downcutting or
vertical erosion dominates, the valley is more
symmetrical, with steep sides and a gorge-like
appearance. These are described as
entrenched meanders.