MEANDERS.
Meanders are formed in the mature to old age stage of the river. They are
pronounced curves or bends in a river. Geographers are uncertain as to why
meanders start.
One theory suggests that the pool and riffle effect causes turbulence in the
water. Pools are deeper parts of the river and riffles are deposits of bed load
making the river shallower. The water begins to swing from side to side as the
water tries to avoid the slowing effect of the riffles. The water flows faster on the
outer concave side of the river, leading to undermining. This is known as bank
caving. River cliffs are formed.
The river erodes by means of hydraulic action. This is the sheer force of
moving water, rushing into cracks, widening them, sweeping away any loose
material and breaking up solid rock. When the river is in flood the water may be
forced into cracks with sufficient force to prise out slabs of bedrock. With
continued erosion on the outer concave bank, the river cliff will collapse which
enlarges the bend. Erosion also occurs because of cavitation. This is where
bubbles of air collapse and form shock waves against the banks. The concave
bend is widened.
On the inner convex bend the water is travelling much slower. Deposition
occurs here forming a point bar. The point bar builds up to above the level of the
water on the convex bend and the river is forced to go around it. Thus meanders
become more pronounced over time.
e.g. The Shannon river near Limerick.
WATERFALL:
A waterfall is a feature of river erosion. It occurs in the youthful stage of a river
where a band of hard rock, slightly inclined or horizontal, is lying across the
river valley which interrupts it’s attempt towards a graded profile.
The river erodes by means of abrasion. This is where the river uses it’s load of
sand, gravel and pebbles to bang and bounce and scrape against the bed of the
river, knocking out pieces of rock and wearing away the softer rock. The load is
used in a scouring action to deepen the river bed. The greater the speed and
volume of a river, the greater the load it can carry. Therefore large, fast flowing
rivers erode quicker than smaller, slower rivers.
The river also attacks the bed of the river chemically. This process is known as
solution. River water tends to be slightly acidic and this reacts chemically with
certain elements in rocks such as calcium carbonate in limestone and chalk.
These elements are dissolved, weakening the structure of the rock, making it
easier to erode by other physical processes and carrying away parts of the rock
in solution.
Both these processes attack the less resistant rock in the bed of the river and
erode it quicker than the hard band of rock, leading to a vertical drop.
Once a vertical drop has been formed, the water, falling from a height, swirls
and armed with boulders starts to dig out a plunge pool at the base of the
waterfall. The face of the falls also comes under attack from the swirling water
and is undercut, especially if the face is composed of softer rock than the hard
band of cap rock. After a while the overhanging hard rock is unsupported and
collapses. The falls retreat upstream and this is called headward erosion. The
Niagra Falls retreats at the rate of 1 mt. per year. A steep sided channel is
created downstream and this is called a gorge.
Ex. Ashleagh Falls Co. Mayo.
Waterfall:
A waterfall is a feature of river erosion. It occurs in a youthful stage of a river
where a band of hard rock, slightly inclined or horizontal, is lying across the
river valley which interrupts it’s attempt towards a graded profile. As the less
resistant rock around this band is eroded quicker, a vertical fall in the river
results. The river erodes by means of hydraulic action.
This is the sheer force of moving water, rushing into cracks, sweeping away
any loose material and breaking up solid rock. Turbulent water and eddying
may undermine the banks at the bend of a river. This is known as bank caving.
Erosion also occurs because of cavitation. This is where bubbles of air collapse
and form shock waves against the banks. If the banks are composed of loose
sands and gravels, erosion will occur more rapidly.
Once a vertical drop has been formed, the water, falling from a height, swirls
and armed with boulders starts to undercut the base of the waterfall forming a
plunge pool. Undercutting occurs and the overhanging hard rock is unsupported
and collapses. The falls retreat upstream and this is called headward erosion.
The Niagra Falls retreats at a rate of 1mt. per year. A steep sided channel is
created downstream and this is called a gorge.
Ex. Ashleagh Falls Co. Mayo.
The river also erodes by abrasion. This is where the river uses it’s load of sand,
gravel and pebbles to bang and bounce and scrape against the bed of the river,
knocking out pieces of rock and wearing away the softer rock. The load is used
in a scouring action to deepen the river bed. The greater the speed and volume of
a river, the greater the load it can carry. Therefore large, fast flowing rivers
erode quicker than smaller, slower rivers.
Waterfall:
A waterfall is a feature of river erosion. It occurs in a youthful stage of a river
where a band of hard rock, slightly inclined or horizontal, is lying across the
river valley which interrupts it’s attempt towards a graded profile. As the less
resistant rock around this band is eroded quicker, a vertical fall in the river
results. The river erodes by means of hydraulic action.
This is the sheer force of moving water, rushing into cracks, sweeping away
any loose material and breaking up solid rock. Turbulent water and eddying
may undermine the banks at the bend of a river. This is known as bank caving.
Erosion also occurs because of cavitation. This is where bubbles of air collapse
and form shock waves against the banks. If the banks are composed of loose
sands and gravels, erosion will occur more rapidly.
Once a vertical drop has been formed, the water, falling from a height, swirls
and armed with boulders starts to undercut the base of the waterfall forming a
plunge pool. Undercutting occurs and the overhanging hard rock is unsupported
and collapses. The falls retreat upstream and this is called headward erosion.
The Niagra Falls retreats at a rate of 1mt. per year. A steep sided channel is
created downstream and this is called a gorge.
Ex. Ashleagh Falls Co. Mayo.
DELTAS:
When a river carries a heavy load of sand, mud and pebbles into an area of calm
water, such as an enclosed or sheltered sea area or lake, it’s speed is checked by
the near stationary body of water. It slows down and as it has less energy and no
longer has the competence or capacity to carry all of it’s load, it begins to deposit
the largest particles first. If the tides and currents are strong enough, most of
these deposits will be swept out to sea. However, if the load is very large and the
sea water is shallow with a small tidal range, the material builds up in layers
called beds.
1. Fine particles are carried out to sea and deposited to form bottomset
beds.
2. Coarser particles are deposited in sloping layers over the bottomset beds
to form foreset beds. This causes the delta to advance seaward.
3. Fine particles of clay, silts and muds are deposited on top of the foreset
beds to form the topset beds.
The deposits build up to form islands which begin to grow in size. The main river
channel becomes clogged up and this causes the estuary to split up into many
smaller streams called distributaries. These streams flow from the main channel
of the river over the delta deposits to the sea.
Sometimes part of the sea becomes enclosed by deposits forming lagoons. These
will fill in with sediment later
Not all rivers form deltas. Most Irish rivers are too short to form deltas. If the
river has many lakes on it’s course, these will filter out much of it’s load.
There are a number of different types of deltas.
The classic type is called Arcuate. This is triangular in shape and consists of
coarse material. It has many distributaries. e.g. Nile Delta.
Another type is called Bird’s Foot. This type is mainly made up of finer material
and there are only a few large distributaries extending out into the sea like
fingers. e.g. Mississippi Delta.
An example of a lacustrine delta would be Glendalough Co. Wicklow.
Flood Plains:
Flood plains are those areas on either side of a river in it’s mature/old age stage
that are occasionally covered when rivers in flood overflow their banks. Flood
plains generally occur in wide and flat valley floors.
In a mature/old age stages of a river, deposition is more common than erosion. A
river in flood has a large amount of energy which it uses to transport huge
amounts of material in suspension. As the river spreads over the flood plain, the
flood waters encounter increased friction leading to a corresponding decrease in
velocity resulting in deposition of material previously held in suspension. A great
deal of it’s load of sand, pebbles and silt is also deposited on the bed of the river
when the flood subsides. This deposition has the effect of raising the level of the
river water. In this way the river is raised above the level of the surrounding
plain.
After heavy rainfall, the volume of the river increases, causing it to overflow it’s
banks. The flood waters cover the plains on either side of the river as far as the
bluff lines.
The coarse, heavier materials are deposited first. These build up to form natural
ridges known as Levees on either bank of the river. The flood waters then wash
over these banks into the adjacent flood plains, carrying with them fine sand and
silt known as alluvium. This process is called aggradation.
e.g. The Shannon flowing through the Central Plain of Ireland.
The river also erodes by abrasion. This is where the river uses it’s load of sand,
gravel and pebbles to bang and bounce and scrape against the bed of the river,
knocking out pieces of rock and wearing away the softer rock. The load is used
in a scouring action to deepen the river bed. The greater the speed and volume of
a river, the greater the load it can carry. Therefore large, fast flowing rivers
erode quicker than smaller, slower rivers.