39
Unit 2: Gradational processes
Chapter 12
Rivers and valleys
Drainage basins or watershed
Rivers are born in upland areas and can be classified as
streams when water begins to accumulate and flow following
a definite course. Small streams called tributary streams
carry water downhill and join together with other tributary
streams to create a large river. The large river flows in the
lowest valley as the erosional power of the water wears away
the land, continually deepening the valley. The geographic
area which supplies water to the network of streams above
is called a drainage basin or watershed. A mountain region
may consist of many drainage basins and the separating
ridge between two drainage basins is known as a divide.
divide separating
two major
drainage basins
drainage basin
tributary
steams
river
river
Figure 12.1 Drainage basins and divides
Types of drainage patterns
As rivers erode the landscape a certain drainage pattern begins to take shape. A definite determining factor in the
formation of a drainage pattern is the hardness of rock layers in the region.
• A dendritic drainage pattern is the most common of all drainage patterns and resembles the veins of a leaf. This
pattern type can be found in areas where rock layers are of uniform hardness.
• A trellis drainage pattern is established in areas where folding and tilting of rock layers may confine rivers to parallel
valleys that meet a larger stream at right angles.
• A radial drainage pattern occurs on cone-shaped mountains such as volcanoes. Water will flow away from the high
point equally in all directions.
River valley shapes in wet and dry
climates
In arid regions, river valleys tend to
be deep with steep sides. The lack
of rainfall keeps lateral erosion to
a minimum and very little debris is
knocked from the valley sides into
the river. This allows the river to
erode vertically much faster than it
Radial
Dendritic
Trellis
normally would and a deep canyon
takes shape. In wet regions, heavy
Figure 12.2 Three main drainage patterns
rains make the valley sides unstable
and much sediment ends up in the
river. The erosion occurring along the sides of the valley reduces the gradient to a gentle slope. The river uses its
energy to transport this debris downstream and because of this, vertical erosion (deepening) is not as dominant as it is
in dry regions.
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40
Four stages of river development
Unit 2: Gradational processes
1. Youth stage - A river in youth stage cuts a deep V-shaped valley as the fast moving water transports material
downstream. Rivers in youth stage are dominated by erosion as the turbulent water allows for very little deposition to
occur. Features commonly seen in these rivers include rapids, waterfalls and various sizes of boulders along the river
bed.
2. Mature stage - As the river enters mature stage a definite drainage pattern is evident in the watershed drained by
the river. During this stage of development vertical erosion still continues but downstream lateral erosion of the banks
is evident as meanders and a flood plain start to take shape. Here the velocity of the flow slows considerably and
deposition of river sediment is common.
3. Old age stage - In old age, the flood plain which began to form in mature stage continues to widen and extensive
meandering occurs. The river even cuts across some meanders to create oxbow lakes. Periodic flooding has deposited
rich alluvium throughout the flood plain and natural levees are prominent along the river banks. The river delta is well
developed, and in many cases continues to grow by several meters each year as sediment is flushed down from the
highland region.
4. Rejuvenated stage - In the rejuvenated river stage, the land has undergone a slow uplifting and caused the river to
return to a period of vertical erosion such as was the case in youthful stage and a deep v-shaped valley once again takes
shape. The river cuts through the flood plain created during old age and continues vertical erosion until its elevation
again nears sea level. Once the elevation has been reduced sufficiently, lateral erosion replaces vertical erosion and the
creation of a new flood plain begins.
Youth stage
A) Youth
Mature stage
B) Mature
Old age stage
C) Old age
Figu re
12 .3 of
Stage
of river de velop men t
Figure 12.3
Stages
rivers development
Erosional action of rivers
River erosion involves 4 important processes: • Hydraulic action
As the river flows along, the force of the water has great power. Much material is worked loose from the banks and
river bed and carried downstream and eventually deposited in lower regions where the water flow diminishes. This
erosional process carried out by moving water is called hydraulic action. The greater the velocity (speed) of the river,
the greater the erosion.
• Abrasion
Debris such as sand can have a powerful abrasive effect on the river banks and river bed. Common sights along river
beds are rocks that have been polished completely smooth by the abrasive power of the river’s load.
Copyright © 2008 Green Heron Publishing. No part of this publication may be reproduced without written permission from the publisher.
41
Unit 2: Gradational processes
• Attrition
When the water moves fast, as during times of flood, rocks of varying sizes may be moved along by the river water. As
they move, they repeatedly strike other rocks, causing breakup into smaller and smaller pieces. This process is known
as attrition.
• Corrosion
All water is mildly acidic and some rocks such as limestone are readily dissolved in water and transported away. This
process is known as corrosion.
Ways that rivers transport sediments
• Solution
Rivers carry enormous amount of material in solution.
Transport by solution refers to anything that has been
dissolved by river water and carried away in this
manner. It is important to realize that material carried
in solution is invisible and the water can appear crystal
clear yet have in it many soluble minerals. As much as
half of the sediment load of a river may be carried in
solution.
suspension
traction
saltation
Figure 12.4 Sediment transport by rivers
• Suspension
Very light material will carry in suspension so that it does not touch the bottom of the river. Clay particles are so light
that they will carry in suspension even in slow moving water.
• Saltation
Particles that are too heavy to travel in suspension may move along the bottom by bouncing and rolling. This method
of transport is referred to as saltation. With suspension and saltation, it is the speed of the river’s flow that plays a key
role in how material is moved down stream.
• Traction
Very heavy rock particles may only drag along the river bottom in a process called traction. Some heavy rocks may
only move during time of spring runoff when river velocity is the greatest.
Landforms associated with upland rivers
In upland areas where gradients are steep, water flows swiftly over
boulders, rapids, and waterfalls, as vertical erosion dominates. In
mountainous regions with rock layers of varying hardness, some layers
wear away far quicker than others, resulting in many waterfalls and
rapids. Regions with a volcanic origin may have waterfalls that cascade
over dikes and sills of igneous rock which is very hard and takes a
longer time to wear away than many other rock types. As the water
moves along, a pothole may form along a river bed where pebbles
become trapped in a small depression. The power of the water spins
these pebbles around and abrasive action deepens the hole, creating
a circular hole with smooth sides and bottom. When vertical erosion
is rapidly occurring, a river may form a deep, narrow slot through the
landscape, known as a gorge or canyon.
pebbles enter depression and are spun
around by force of water
abrasion
makes hole
deeper
Figure 12.5 Formation of a pothole
Copyright © 2008 Green Heron Publishing. No part of this publication may be reproduced without written permission from the publisher.
42
Unit 2: Gradational processes
Features associated with lowland rivers
The formation of a flood plain takes place in lowland areas when the river has reached an elevation which is close to
sea level. At this point, vertical erosion is no longer important and the river uses its energy laterally creating a broad
flood plain as it nears the ocean. The river seasonally overflows its banks and with the flood waters come rich alluvial
deposits which make flood plains ideal for farming.
undercut bank
(erosion)
A
river erodes banks
point bar or slip-off slope
(deposition)
B
more lateral erosion widens flood plain
flood plain
oxbow lakes
Figure 12.6 Meander with undercut bank and point bar
As elevation nears sea level and the river begins to slow, meanders
begin to form. Erosion is prominent along the outer edge of a meander
where the water moves the fastest. This area is called the cut or
undercut bank as severe undercutting creates a steep slope on this
side of the river. Along the inner edge of the meander, the water moves
slower and deposition of sediment occurs. This inner bank is known
as the point bar or slip-off slope. Here gravel, sand, and silt deposits
create a gradual slope and shallow water close to the shoreline.
When a river floods its banks, water covers the entire flood plain.
Deposits of alluvium are added to the flood plain with each flood.
Some of the lightest sediment returns with the receding water into the
river channel. As the water returns slowly back into the river channel,
additional sediment is deposited directly on the river bank. Each flood
adds a layer of silt along the bank and slowly a small ridge begins to
build. These mounds of sediment along river banks are called levees
and help hold flood water in the river channel. Farmers create artificial
levees called dikes by adding sediment to the natural levees aimed at
minimizing floods.
When seasonal floods are restricted by natural and especially artificial
levees, the river begins to deposit its load along the river bed. Over
time, the river flows above the level of the surrounding flood plain
and in situations such as this, when a river overruns its banks, levees
are often breached with catastrophic results.
C
Figure 12.7 Formation of a flood plain
deposits made with every flood
increase height of levees
river flows above flood plain
as deposition on river bed continues
Figure 12.8 Formation of a levee
Copyright © 2008 Green Heron Publishing. No part of this publication may be reproduced without written permission from the publisher.
43
Unit 2: Gradational processes
The Mississippi River floods of 1993 caused hundreds of millions of dollars in property damage. For decades the
U.S. army corps of engineers had designed and built artificial levees to curb the flood waters of the Mississippi with
favorable results. The series of levees minimized flooding along the river but sediment that normally would have been
deposited on flood plains with each flood ended up on the bottom of the river channel. Over many years this river
bed deposition caused the river to flow higher and higher so that the normal water level in the river was far above
the elevation of the flood plain. In the spring and summer of 1993, weeks of abnormally heavy rain throughout North
America caused the waters of the Mississippi River to rise many meters above the surrounding landscape. Thousands
of volunteers worked throughout the summer helping to stabilize the levee systems but even then in many areas the
levees collapsed and the waters of the Mississippi covered one community after another. Satellite images from space
showed that an area almost the size of the Great Lakes was under water. The collapsed levee systems in many areas
will not be rebuilt and the river will be allowed to flood and flow without restrictions.
undercut bank
point bar
(or
slip-off slope)
A
stream cuts
through
meander
oxbow lake
deposition
B
Meanders can become oxbow lakes
As the meanders of the aging rivers become more
pronounced, the river may straighten its path by cutting
through some meanders to create oxbow lakes. Rivers
with oxbow lakes have a very broad flood plain.
land uplifts
river creates new flood plain
C
Figure 12.9 Stages of oxbow lake formation
terraces
Formation of river terraces
Lowland regions may sometimes go through a period of uplifting
and as the elevation of the land rises the meandering river again
begins a stage of vertical erosion forming a brand new gorge
or canyon through the original flood plain. After uplifting
stops and the river has sufficiently reduced its elevation, lateral
erosion again is dominant and a new flood plain begins to form.
The old uplifted flood plain now sits high above the river banks
and these ledges are referred to as river terraces.
Figure 12.10 Formation of river terraces
Braided river channel
Not all rivers have a single meandering channel in lowland regions. Some rivers take on a braided appearance with
many sand and gravel islands splitting up the water flow. Braided streams or rivers all have one thing in common, they
transport large volumes of debris and much of it
is deposited along the river channel to create sand multiple channels
bars and islands. In some locations islands can
become large enough that vegetation will have
an opportunity to establish itself and this helps in
stabilizing the entire island.
River deltas
A river carries sediment from its drainage basin
toward the sea and much of it ends up deposited on
the flood plain when the stream flow slows down.
Some deposits are light enough that the current
will carry them into the ocean before they
continually changing
sandbars
Figure 12.11 Braided Stream
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44
Unit 2: Gradational processes
are deposited. These deposits form the river delta. Sand is the first
to be deposited close to shore as it is the heaviest, followed by silt
and clay. Currents from larger rivers can be detected as much as
fifty kilometers from shore. This would be the final resting place
for the lightest river sediment. Rivers with large sediment loads
have constructive river deltas, meaning that they increase in size
each year. Large rivers tend to have broad deltas with shallow water
extending far out to sea. Deltas of this type are not easily eroded by
ocean waves.
3 main types of deltas
vy
hea
increasing
distance from
shoreline
nt
s
me
edi
direction of
sediment
transport
e
s
ht
lig
t
en
dim
Figure 12.12 Sediment transport and deposition
1. Arcuate delta
The arcuate delta has many distributaries that carry water and sediment across a very symmetrical delta that has the
shape of an inverted cone.
Example: Nile River Delta
2. Bird’s foot delta
The bird’s foot delta has many distributary channels that branch out from the main river channel which gives this delta
the appearance of a bird’s foot.
Example: Mississippi River Delta.
3. Estuarine delta
The estuarine delta forms when river sediment is deposited in a submerged river mouth. The delta grows in the shape
of the estuary.
Direction of flow
Arcuate delta
Bird’s foot delta
Estuarine delta
Figure 12.13 River deltas
Copyright © 2008 Green Heron Publishing. No part of this publication may be reproduced without written permission from the publisher.