THE MIDDLE COURSE
 In the middle course, rivers aren’t as high above sea level as they are in the
upper course.
 There is more energy because the discharge is greater.
 Discharge has increased because there is more water from tributaries which
have joined, and because more water has arrived from throughflow and surface
run-off.
 Less energy is lost to friction since less water is in contact with the bed and
banks and the load size is smaller.
 So they erode sideways (laterally) as well as vertically. As this happens it twists
from side to side (meanders)
 This means that the channels are wider and deeper than upstream.
 A meander is a large bend found along a river’s course.
 Meanders tend to occur in series. Thus, meanders are
generally found in groups.
 Normally found in the middle and lower course of a
river. They are more pronounced in the latter and may
be associated with ox-bow lakes.
 Generally created by a combination of erosion and
deposition.
 Water in a river channel does not flow uniformly.
There is always an area within the water body where
the flow is the fastest.
 This area of fastest flow tends to occur because of
variations in the channel caused by:
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Different types of sediment
Changes in channel slope/gradient
Changes in bed roughness
Variations in depth
 As a result water in the channel will twist and turn
around stones and other obstructions resulting in
areas of slower and faster water movement and
creating deeper areas along the bed.
 The result of this is turbulent flow
and the
development of a thalweg .
 The Thalweg is a line connecting the lowest points
along the stream bed. It will therefore have the least
resistance and the fastest flow.
 The thalweg
accentuates the variations in velocity
across the river channel and bends from side to side
naturally and due to variations in the channel itself.
 The end result is that water in river channels does not
flow in a straight line, even in an apparently straight
river channel since the thalweg meanders across the
channel, from one side to the next.
 Such meandering is actually the normal behaviour of
fluids and gases in motion.
 The river starts to flow from side to side in a winding
course but still in a relatively straight channel.
 As said before, turbulent flow within the channel causes the
development of a thalweg. It also causes riffles and pools.
 Pools are areas of calmer and apparently slower moving water
found in deeper parts of the river filled with fine sediments.
 Riffles are areas of faster and more turbulent flow are found in
shallower parts of the river over larger stones and gravel .
 The creation of pools and riffles causes more erosion to occur on
one side and more deposition on the other, which eventually
causes the river channel to bend from side to side and meanders
to develop.
 Over time, the pool tends to move to opposite side of
the channel and this is where a meander will develop.
 Water moving faster has energy to erode. This occurs
on the outside of the pool.
 Water moving slowly tends to deposit material as it has
little energy left to erode or transport material. This
occurs on the inside of the pool.
 Erosion,
transportation and deposition are all
processes that create the characteristic features of
meanders.
 There are several stages involved in the creation of
meanders.
 Stage 1
 In low flow conditions straight river channels have bars of
sediment on their beds.
 Flowing water weaves around these bars of sediment.
 This creates deeper pathways where most of the water
flows (thalweg) called pools and shallow areas where less
water flows called riffles.
 This causes the river flow to swing from side to side.
 Stage 2
 Where the river swings towards the bank erosion
causes undercutting.
 On the opposite side of the channel where the velocity
is lower material is deposited.
 Therefore the river does not get any wider. Instead the
shape or form shifts or migrates across the landscape
and the sinuosity increases
 Stage 3
 Continued erosion along the outer bank, as the result of hydraulic
action and abrasion, creates a river cliff or bluff.
 A slip-off slope and point bar forms on the inner bank. This is a
gently sloping deposit of sand, gravel and pebbles.
 A point bar is a depositional feature made of alluvium that
accumulates on the inside bend of streams and rivers below the slip-off
slope. Point bars are found in abundance in mature
or meandering streams. They are crescent-shaped and located on the
inside of a stream bend
 The following image shows a the top of a slip-off slope.
 Stage 4
 Meanders are perpetuated through a process called
helicoidal / helical flow.
 As the surface flow of water hits the outer bank it
corkscrews, flows along the river bed then deposits
eroded material on the inner bank.
 Step 5
 Erosion is greatest beyond the middle of the bend in
the meander. This causes the meander to migrate
downstream over time.
Meanders
On the side where the
fast water is, there is
more erosion.
This makes the river
deeper and cuts into
the bank, to make a
river cliff.
On the other side,
where the slower water
is, there is deposition.
This makes a shallower
area of sediment called
a point bar.
The gentle slope down
to the point bar is
called the slip-off
slope.