Transcript rivers - BioGeoWiki-4ESO

RIVERS
When they are young, rivers ideally start
out relatively straight in map view,
entrenched in V-shaped valleys. You’ll
also find plenty of waterfalls and rapids at
this “Youth” stage.
As time goes by, the river loses the
gravitational impetus to incise any
deeper. The river now begins to meander
side to side, and as it does so, enlarges
the size of its valley by lateral erosion.
It is “Mature.” As time goes by, the valley
walls get further and further apart.
If enough time goes by, the river can enlarge
the size of its valley so much that you
can’t really tell it’s a valley any more.
At this stage, meandering can get
pronounced enough to fold back on itself and
create oxbow lakes (visible in the map view of
the “Old Age” stage).
The story could end here. However, if base level
were to drop anew, the river will begin to incise
again, producing a valley profile (cross-section)
that looks pretty much identical to the “Youth”
stage. It has been made young again, or
“Rejuvenated.” In map view, however, you can
see from the meandering shape of the re-incised
valley that the river must once have been at the
“Old Age” stage.
Outline
Here, we will learn on these processes:
- River Erosion
River Erosion
- River Transportation
- River Deposition
River Erosion is the river erodes away its
channel vertically and laterally.
Vertical erosion is the downward erosion
which deepens the river channel.
Lateral erosion is sideward erosion which
widens the river channel.
The work of erosion depends on the
energy of the river.
River Transportation
A river uses its energy to carry or transport
eroded materials such as mud, sand, boulders
and dissolved materials. These materials are
called its load.
River transport their load by 4 processes:
Traction (rodadura)
Saltation
Suspension
Solution
River Landforms

Potholes

Rapids

Waterfalls

Gorges
Potholes (marmitas de gigante) are holes in the
river bed.
They vary in size from a few centimetres to
several metres in diameter.
Pothole formation:
Pebbles carried by the river are swirled
around on the river bed. This action erodes
the rock on the river bed forming potholes.
Over time, they may widen and join with other
potholes to form larger potholes, and the
whole river bed 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.
As result, the rapids form a series of steps.
Waterfalls
A waterfall is formed where there is the
sudden fall in height. The river plunges and
hits the bottom of the river bed with great
force.
Rocks are swirled around to form a deep
pool at the river bed known as a plunge pool.
Gorges
Gorges are formed when rivers erode vertically
through resistant rocks.
Since these rocks are not easily eroded, the
sides of the valley remain steep.
Gorges is a deep valley, narrow and steep
sided.
River Deposition
There are many types of River Deposition:
Floodplain
Meanders
Delta
Ox-Bow Lake
Floodplain
A floodplain is a low-lying plain on both sides of a
river that has repeatedly overflowed its banks and
flooded the surrounding areas.
When the floods subside, alluvium is deposited on
the floodplain.
The larger materials, being heavier, are deposited
at the river banks while the finer materials are
carried and deposited further away from the river.
The larger materials at the river banks build up into
embankment called levees.
Meanders
Meanders are loop-like bends in a river.
The erosion takes place on the outer bank and
deposition on the inner bank.
Gradually, a steep river cliff is formed on the
outer bank, making it concave in shape.
On the inner bank, deposition of alluvial
materials produces a gentle slip-off slope and
the bank takes on a convex shape.
Ox-Bow Lake
An oxbow lake is a crescent-shaped lake formed on a river when a meander has been cut
through and abandoned.
When a river meanders in very big loops, the outer bank is so rapidly eroded that the river
cuts through the narrow neck of the meander.
The river then flows straight through the channel. When deposition seals off the cut-off from
the river channel, an oxbow lake is formed. It may silt up and eventually dry up.
1) The river is starting
to meander. Erosion
is greater on the
outside of the bend,
deposition more on
the inside.
2) Large meanders
have formed
3) The river cuts
through the meander,
leaving a straighter
section and an oxbow lake.
Delta
A delta is a flat piece of land built-up from layers
of sediment deposited by a river where it enters
a lake or calm sea.
The river may have to branch into smaller
distributaries to carry the water to the sea.
The Ideal Stream Cycle (Davis, 1880)
Youth
Maturity
Old Age
Rejuvenation
Youth
V-Shaped Valley
Rapids
Waterfalls
No Flood Plain
Valley Being Deepened
How Do We Know Rivers Cut Their Valleys? How Rivers Widen Valleys?
The river only erodes a slot. Other processes carry material from the valley walls to the
river to be carried away. So rivers only deepen their valleys; other processes widen
them.
Maturity (Late)
Valley has flat bottom
Narrow Flood Plain
Divides begin to round off
Relief diminishes
Sediment builds up, flood plain widens
River begins to meander
Old Age
Very Wide Flood Plain
Land worn down to flat surface (Peneplain)
Resistant rocks form residual hills
(Monadnocks)
Pronounced River Meanders
Cut-off Meanders (Ox-bow lakes)
Rejuvenation
Uplift of Land (Lowering of Sea Level,
sometimes greater stream flow) causes
stream to speed up and cut deeper.
Stream valley takes on youthful
characteristics but retains features of older
stages as well.
Can happen at any point in the cycle.
Evolution of the River Valley
River valleys can also be categorized into
maturity stages based upon their rate of
water flow as well as the formation of
certain features within the valley.
Youthful river valleys typically have fast
flowing rivers and steep valley walls which
rapidly erode.
When a river valley reaches maturity,
water velocity slows and the river
broadens out to meander from side to side
within the valley.
Finally in old age the river valley has
broadened out to form wide flood plains
with extensive meandering. Features such
as secondary flow channels and oxbow
lakes can been seen in these valleys.
When the river evolution is interrupted
due to uplift, change in sealevel, or
climate change, the erosive power
of the rivers may increase again,
and they begin downcutting
through their previously deposited
floodplain deposits. They develop a
new floodplain at a lower level. The
river valley has been terraced.
This process can occur repeatedly,
and we can get a series of river
terraces (or fluvial terraces) in a
river valley.
Terraced river valleys are quite
common, due to the strong changes
of climate and sealevel in recent
Earth history.
Because the underlying processes
are global in nature, episodes of river
terraces can be used to correlate
recent Earth history over large areas
and even from continent to continent