Transcript Document

The Hydrological (Water) Cycle
The River System –
the drainage basin.
A river system is an open
system. An open system has
inputs, processes and outputs. So
unlike water in the hydrological
cycle, where there is an endless
cycle of rain, rivers, sea and
evaporation, water in the river
system enters the system, and
eventually leaves it again.
A river system operates inside a
drainage basin (or river basin). A
drainage basin is the area of land
drained by a river and its
tributaries. Any water falling
inside the drainage basin will
find its way into the main river
channel (unless it is taken out of
the system - for example by being
piped to a factory). The boundary
of a drainage basin is called the
watershed.
The diagram below shows a
drainage basin. Make sure you
know all the key terms used
when talking about river
systems.
Processes at work
From the source to mouth of a river three processes are
taking place
River
Erosion
River Erosion
River
Transport
River Transport
River
Deposition
River Deposition
River Erosion
Processes:
Corrasion, (abrasion) is the grinding of rock fragments carried by the river against the bed and banks
of the river. This action causes the channel to widen and deepen. This grinding is most powerful in flood
time when large fragments of rock are carried along in the river bed.
Attrition
Attrition is the collision of rock fragments in the water against one another.
The rock particles are broken into smaller pieces and become smoother the
longer the process continues.
Solution
Solution (corrosion) is the process by which river water reacts chemically
with soluble minerals in the rocks and dissolves them. An additional process
is also at work caused by the force of the water itself known as Hydraulic
action. In this rocks are dragged away from the bed and banks by the force
of the running water. When water from a fast moving stream enters
minute cracks in a rock, the pressure exerted weakens and eventually
breaks up the rock
River Transport
A river carries or transports eroded materials such as mud, sand,
boulders and dissolved materials on its journey. These materials are
known as its load. The load is carried along by three processes.
•traction
•suspension
•solution
Traction
Is where boulders or stones are rolled along the stream bed by the force of the
water
Suspension
Fine particles such as clay, silt
and fine sand are carried along in
the river.
Solution
Dissolved materials containing minerals like calcium
and sodium are carried in the water and are invisible to
the naked eye.
River Deposition
A river drops its load when the speed or volume of the river decreases. The load,
which it carries, is deposited. The heavier material is deposited first and the finer
material carried further.
Rivers reduce their speed when they enter flat land, enters a lake or the sea or
reach an arid area. The volume can be reduced during a dry season or when the
river passes over more porous landscapes e.g. sand or limestone.
Video of river processes:
Click on the link below to
see this animated.
http://www.school-portal.co.uk/GroupDownloadFile.asp?file=21393
What is the long river profile?
River Valley Development
A river has a life cycle from source to mouth. The processes at work cause changes to the river valley
and river features in the landscape that it passes over.
Traditionally this cycle has been described in 3 stages, the Upper (Torrent) section, the Middle
(Valley) section and the Lower (Flood Plain) section. The terminology's Youthful, Mature and Old have
also been used to describe the sections.
The stages are not clearly defined but merge from one to the other and rivers show individual
characteristics. There are river features that can be defined within a broad pattern.
A typical long
profile:
upper
middle
lower
Watch videos: from source to mouth
Which stage? How do you know?
1. How much energy do you
think the river has here? How
do you think the river uses
its energy here?
2. How do you think
the channel shape
changes here?
3.In which direction
is the river eroding
here? How do you
know?
4. Do you think
the river is at its
fastest here?
What do you
think happens to
velocity as you
travel
downstream?
5. What do you
think the rivers
load is like here?
Why? Where
does its load
come from?
6. What processes of
erosion do you think
may be happening
here?
7. How do you
think the river
transports here?
8. Do you think
deposition occurs
here? If yes when?
9. What
landforms do you
find in the upper
stage?
Upper Course
Characteristics:
•Deep narrow valley
•(V shaped)
•Interlocking spurs
•Fast flowing river
(erosion predominant)
•Pot holes in the river
bed
•Rapids and
Waterfalls
The main process at work is the downward
or vertical erosion of the stream bed. This
cuts into the landscape deepening the valley
sides.
As the water twists around obstacles such as rocks and boulders the
erosion forces undercut outward bends and a snaking pattern is
produced. From lower down the interlocking of the spurs of land
between the bends blocks the view up the valley.
V-shaped valley with interlocking spurs
Click on the link below to
see this animated.
http://www.school-portal.co.uk/GroupDownloadFile.asp?file=55027&Groupid=12426
Potholes:
Potholes are formed by
corrasion. Pebbles carried
by the river are swirled
around on the riverbed. This
action erodes the rock on
the riverbed forming
potholes. Over time, they
may widen and join with
other potholes to form
larger potholes, and the
whole riverbed is deepened.
Rapids:
Rapids are stretches of fast-flowing water tumbling over a
rocky-shallow riverbed. Different resistance among rocks
cause rapids. They are formed when the water goes from one
hard rock that resists the water's erosion to a softer rock
that is easier eroded.
Waterfalls:
Waterfalls occur when a band of hard rock lies across the river with softer rock downstream
which is more rapidly eroded. At first rapids would form but then develop into a waterfall as the
softer rock erodes further. Variations may be caused by the rock structure.
(1) Rock layer is
horizontal
(2) Rock layer dips
upstream
(3) Rock layer is vertical
Click on the link below to
see this animated.
http://www.school-portal.co.uk/GroupDownloadFile.asp?GroupId=12426&ResourceID=29560
Watch video: waterfall formation
Middle-course features
Meanders
In the middle course the river has more energy and a high volume of water. The
gradient here is gentle and lateral (sideways) erosion has widened the river
channel. The river channel has also deepened. A larger river channel means
there is less friction, so the water flows faster:
•As the river erodes laterally, to the right side then the left side, it forms
large bends, then horseshoe-like loops called meanders.
•The formation of meanders is due to both deposition and erosion.
•The force of the water erodes and undercuts the river bank on the
• outside of the bend where water flow has most energy.
•On the inside of the bend, where the river flow is slower, material is
deposited.
Watch video of meander formation!
Slow, shallow water and deposition on the inside of the bend
Deep, fast water and erosion on the outside of the bend
Click on the link below to see this
animated.
http://www.school-portal.co.uk/GroupDownloadFile.asp?file=21606
LOWER STAGE: FLOODPLAINS
LOWER STAGE: LEVEES