RIVERS

Rivers

-A river is an example of a surface process of erosion, transportation and deposition.

Stages of a riverAs a river travels from its source to sea it passes through 3 stages:

River Feature Process Stage of river Example

V-shaped Valley Erosion Youth Upper River Moy, Ox Mountains, Sligo

Waterfall Erosion Youth Ashleigh Falls, Errif River, Mayo

Interlocking spurs Erosion Youth Upper of most riversMeander Erosion &

DepositionMature/Old Suck River, Galway

Floodplain Deposition Mature/Old River Suir, Clonmel, Tipperary

Oxbow lake Erosion & Deposition

Old Lower River Moy, Mayo

Levees Deposition Old Lower River Moy, MayoDelta Deposition Old The Nile, Egypt

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Fluvial Landforms- Erosion & DepositionFast-flowing streams have lots of energy for erosion. When a river slows down, it will deposit. Factors that influence the speed of a stream’s flow are:

Upper Course / Youthful Stage

Middle Course / Mature

Lower Course/ Old Stage

Discharge- the amount of water flowing in the river at a particular time and place.

Gradient- the slope of the channel. The shape of the channel- rough or smooth.

Processes of fluvial erosionThe effectiveness of these processes is determined by the resistance of the underlying rocks.

Hydraulic Action- is the force of water. A fast-flowing stream has huge energy, so this force is very powerful in dislodging rocks or soil from the bed and banks of the stream.

Abrasion- is the force of the water plus its load. The load is the rocks, pebbles and sand carried by the river. It acts as an abrasive to scrape the bed and banks of the river.

Solution- Is the process of dissolving minerals. Some minerals in rocks are easily dissolved (e.g. limestone)

Attrition- the breakdown of a rivers load. The rocks and stones carried by the river bump into each other and have their corners rounded off.

Types When erosion occurs on the banks of a river, it is called Lateral

Erosion. Downcutting or Vertical Erosion occurs on the riverbed. Headward Erosion refers to the retreat upstream of the river’s

source. This happens where a spring emerges onto the surface and erodes material. Headward erosion also occurs when a waterfall undercuts its channel at a plunge pool.

Processes of Transportation Traction: The heavier load of larger rocks is dragged along the channel

bottom. Saltation: This involves bouncing particles such as sand and pebbles

along the channel bottom. Suspension: Lighter particles such as mud float in the water. Solution: This is the dissolved load. Chemical weathering supplies

dissolved minerals.

Process of Deposition When a river slows down, it loses energy and is unable to carry its load. The river deposits its load by the process of sorting. This is the process of dropping the heaviest material first. The finer particles of mud and silt are carried furthest, with soluble materials carried all the way out to sea.

Deposition occurs when:

Velocity (speed) is reduced: When a river overflows its banks onto its floodplain, the water spreads out and slows down. If the course of the river is interrupted by a lake or reservoir, the river loses velocity when its enters the still body water. The same happens at its mouth, when it flows into the sea.

Discharge is reduced: if the river’s discharge is reduced, then the river will lose energy because it isn’t flowing as quickly anymore. This can occur when the river flows through a desert where there is increased evaporation. It can also occur following a period of reduced precipitation or if water is taken from a river for irrigation or urban usage.

Load is increased: This may be caused when there is increased soil erosion following heavy rainfall. It can also occur when a fast-flowing tributary joins the main river and adds new load to it.

Feature of Erosion – Waterfall

The landforms of river erosion are mainly found in a river’s youthful stage. In this stage, the river is near its source, is fast-flowing and is down cutting.

A waterfall is a steep fall of water along a river. It forms in the upper course of a river in its youthful stage. It occurs at a change in the slope of the channel. This change in slope

occurs where a layer of hard rock runs across the bed of the stream. The channel is often narrow and deep.

The river erodes the less resistant rock more rapidly than the resistant rock. Differential erosion means that the river lowers the section of softer rock rapidly.

By the process of hydraulic action (the force of the water) and abrasion (using its load of sand and pebbles as a tool), it lowers its bed of softer rock.

The river now tumbles down onto the lowered section to form a waterfall.

The steep gradient of the falls gives the river energy. The river plunges into a plunge pool and erodes the rock.

Splashback undermines the overhanging section of harder rock. It collapses and the waterfall retreats upstream.

As the waterfall retreats, a gorge replaces the former positon of the falls. The rate of retreat can be 1.5m per year. Ashleigh Falls on the Errif River

in Co.Mayo is a well-known waterfall. A waterfall is a temporary feature. Headward erosion will eventually

undermine all the resistant rock, with the river flowing over a series of rapids. Eventually the river will once again develop a regular profile.

Feature of Deposition- Floodplain

A flood plain is a fairly level surface on the banks of a river and is covered with deposits of alluvium. (Alluvium: The general term for the sorted sediments, sand, clay, silt, that are deposited in layers (strata) by rivers).

It is the area that us periodically flooded by a stream. Floodplains occur alongside meanders in the middle and old age stages of

a river. Heavy rainfall causes the river level to rise. As a river in flood has a higher

discharge. It can erode and transport a heavy load. When it rises to the limit of its bank, the river overflows and spreads over

the floodplain. Friction with the bed slows the river, increasing the rate of deposition. Repeated flooding builds up layers of alluvium on the floodplain. This mineral-rich soil has long attracted settlement, e.g. the north Italian

Plain. Floodplains often show meanders and oxbow scars. The frequency of flooding has increased as more housing is built on

floodplains. This impedes the natural fluvial processes. The water has nowhere to go except into the river system. Floodplains are found found on the River Suir, Clonmel, Co. Tipperary. The use of dams, levees and dikes usually allows people to control the

flooding of rivers.

Drainage Patterns

Dendritic: Tributaries join the main river at acute angles. Dendritic (tree-like) drainage patterns are found in areas where

the rock type is uniform, e.g. the River Shannon.

Trellised: Tributaries join the main river at right angles. Trellised drainage patterns are found in areas where the rocks are of unequal hardness. This results in an equal resistance to erosion, leaving a ridge and valley landscape e.g. the River Lee

Radial: Surface rivers flow out in all directions from a central area. They all share a common watershed at their source, e.g. the rivers on the uplands of the Kerry Mountains.

Deranged: The main rivers in an area flow in a disorganised manner. Deranged patterns are found in areas of. glacial deposits, e.g. Drumlin belts in Monoghan.

Formation of Waterfalls| sample answerQ. ‘Explain with the aid of a labelled diagram(s) the formation of one landform of erosion that you havestudied’ (2013 Q2 b (i))

A waterfall is a cascade of falling water where there is a vertical or almost vertical drop in the river's course, waterfalls are found in the youthful stage of a river because the water is flowing the fastest and with themost power. Gravity gives water this power and speed as it forced the river to flow down the mountain.

Most waterfalls develop where a river meets a band of softer, less resistant rock like limestone, after flowingover a harder, more resistant rock like granite

As a result of differential erosion, the soft rock on the downstream side is eroded at a faster rate and a fall(drop) develops as the river bed is steepened where the 2 rocks meet.

When the water approaches the fall, it increases in speed because the water in front of it is freefalling, this

Different drainage patterns develop due to variations in rock type and structure in the area over

makes hydraulic action more effective. The underlying softer rock is worn away as the water drops onto it. Over a

period of time the hard rock isundercut, becomes unstable and collapses, this is called an overhang.

The rock that has collapsed to the bottom of the fall is swirled around in an eddying action and uses abrasionto widen and deepen the food of the waterfall

The force of falling water increases the size and depth of the hollow at the bottom of the fall. Also the fallingwater is unhindered by friction and therefore its velocity and hence the power to erode are increased.

This carves out a deep hollow called a plunge pool. The falling water splashed against the back of the waterdissolving some of it by solution. The splash back creates a cave behind the waterfall, leaving an overhangof hard rock above it.

As the underlying rock erodes, the overhang collapses. This process of undercutting and collapse arerepeated many times, causing the waterfall starts to retreat upstream, leaving steep sided gorge downstreamthe waterfall.

An Irish example is Torc waterfall in Killarney, Co. Kerry. Waterfalls are temporary features and eventually they become rapids and

disappear from the landscape. Forexample the Niagara falls retreats by 1 meter per year.

Waterfalls may also be formed by river rejuvenation and after mark the knick point of intersection between the old and new river profile

Formation of Meanders- sample answer‘Explain with the aid of a labelled diagram(s) the formation of one landform of deposition that you have studied’ (2012 Q2 b (i))

Meanders are a series of gently curving bend in the course of a river. They reflect both the processes of erosion and deposition. Examples can be found along the Suck River Co. Galway and the Amazon River, South America.

Meanders are the pronounced in mature and old stage because they do not have enough energy to go through raised bumps in the landscape. The bumps in the terrain cause the river's flow to gently swing side to side.

Pools and riffles develop along a straight stretch of river. The pools are areas of deep water and erosion. While the riffles are areas of shallow water and deposition, they affect the pattern of currents in the river. The uneven depths of water cause further swinging of the current. Within the main flow of a river there are secondary flows. The primary flows are predominantly straight while the secondary flows move in a corkscrew pattern. These spiralling flows are called helical flows.

The flows help to move the material along the river bed from the outside of a meander bend and deposit it on the inside of the next bend.

This secondary flow helps to develop and increase the curving nature of the meander. The current is travelling quickly at the outside bend of a meander, and lateral erosion occurs here due to hydraulic action and abrasion.

The bank of the river becomes undercut, it collapses and a small river cliff is formed. On the convex bend, sand and shingle are deposited as the current is travelling slowly and friction is at its greatest. As a result, a point bar is formed on the inner bend.

With erosion on the outer bend and deposition on the inner bank the meanders begin to migrate, shifting position from side to side and migrating downstream.

If one meander is migrating at a faster rate than its neighbours, a narrow neck develops between them because the faster the water flows the faster it will erode.

During times of flood, the river will cut through the narrow neck and form a new, straighter course. The river maintains this straight, narrow course and the meander is bypassed as the river abandons part of its original channel

Deposition occurs alongside the bank and the old bend of the river is now cut off. This is called an oxbow lake. The still water may evaporate and this as a result forms a meander scar or a mort.

A: Meander (fluvial)

B: Corrie/Cirque (glacial)

C: Arête (glacial)

D: V-Shaped Valley (fluvial)

A:Flood plain

B: Meander

C: Ox bow lake

D: River cliff/ Cut bank

E: Point Bar/ River Beach

Short Questions

Cavitation- is a form of hydraulic action. It is an interesting method of erosion. Air bubbles trapped in the water get compressed into small spaces like cracks in the river's banks. These bubbles eventually implode creating a small shockwave that weakens the rocks.

A: Arête

B: Tarn/ Corrie/ Cirque

C: Ox bow lake/ Mort lake

i) A: Youthful / Young / Upper

B: Mature / Middle

C: Senile / Old Age / Lower

ii) A- V-shaped valley B- Meander C- Oxbow Lake

iii) Peneplain- an area reduced almost to a plain by erosion.

iv) The base level of a stream can be defined as the lowest level to which running water can flow. At the base level, the water in the stream has less velocity, which means the water flow has less energy, so its ability to erode or chip away at the land surrounding it is decreased.

A: Meander (fluvial)

C: V-Shaped valley (fluvial)

D: U-Shaped valley (glacial)

Long Questions

2019 1.B- Examine the role of the process of deposition on the formation of one fluvial landform.

2019 2.C- The impact of human activity on river processes. 2018 1.B- Describe & explain how isostasy on the fluvial landscape. 2017 1.C- The impact of flood control measures on river processes. 2017 3.B- Impact of the processes of erosion on the formation of one

fluvial landform. 2016 2.B- Examine the role of deposition on the formation of any one

landform. 2016 2.C- Examine how changes in base level impact on geomorphic

processes and landforms in fluvial enviro. 2015 1.B- Impact of the processes of erosion on the formation of one

fluvial landform. 2014 1.C- Impact of dams on river processes. 2014 2.B- Formation of one landform of erosion & one landform of

deposition. 2013 1.C- How rivers adjust in base level, with references. 2013 2.B- Explain with diagram formation of one landform.

i) A: Youthful / Young / Upper

B: Mature / Middle

C: Senile / Old Age / Lower

ii) A- V-shaped valley B- Meander C- Oxbow Lake

iii) Peneplain- an area reduced almost to a plain by erosion.

iv) The base level of a stream can be defined as the lowest level to which running water can flow. At the base level, the water in the stream has less velocity, which means the water flow has less energy, so its ability to erode or chip away at the land surrounding it is decreased.