LC Geography Rivers

Rivers

are large, natural streams which carry water back to the ocean.

Three Stages of a river

1. Youthful Stage/Upper course (steep gradient)
Vertical (downward) erosion Source; Tributaries; V-Shaped valley; Interlocking spurs; Waterfalls; Rapids;

2. Mature Stage/Middle course (gentle gradient) Lateral (sideways) erosion starts; Transportation, River beaches (slip off slopes); Meanders; River cliffs

3. Old Stage/Lower course (very low gradient) Deposition Flood plain; Ox-bow lakes; Levées; Delta; Estuary

Mouth

Where the river meets the sea.

Source

Where the river begins.

Tributary

A small river that flows into a bigger river.

confluence

Where two rivers join/meet.

Factors that influence the speed of a river

1. Discharge - amount of water flowing in the river at a particular time and place (volume)
2. Gradient (Slope)
3. Shape of the channel (rough or smooth)

Processes of Fluvial/River Erosion

1. Hydraulic Erosion
2. Abrasion
3. Solution
4. Attrition

Hydraulic Erosion

The force of the 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 a stream.

Abrasion

The process of a rivers' load crashing and rubbing into a rivers' banks and bed causing pieces to break off. Scrapes the bed and banks of a river.

Solution

The process of water dissolving a rivers' load as well as its bed and banks. Some minerals in rocks more easily dissolved (limestone)

Attrition

Breakdown of a rivers load by the rocks and stones bumping into each other. Corners become rounded.

River Transportation

The river can transport material when it has excess energy.

Processes of Transportation

A river transports its materials in different ways depending on rock type and weight of particles.

1. Suspension - lighter particles such as mud float in the water.
2. Solution - this is the dissolved load.
3. Saltation - this involves particles such as sand and pebbles bouncing a long a river bed.
4. Traction - The heavier load of larger rocks is dragged along the channel.

River Deposition

When a river does not have enough energy, it will start depositing its loads through the process of SORTING - Dropping the heaviest material first with finer particles carrying further. Soluble materials carried all the way out to sea.

Channel

The course (between bed and banks) that a river flows.

Levee - detailed formation - LC***DEPOSITION

*'Wide low ridge of sediment deposited along a river bank formed by the process of river DEPOSTION during flooding'
Man-made: Embankments built next to the river channel.

1. As rivers enter middle/lower course they have a lower velocity - gentle slope/flat gradient and can no longer carry its load. (More prone to deposit sediment)
2. During normal river flow, river is confined to its channel but when flooding occurs the river spills over its banks into its floodplain.
3. The river now has a wider area to flow over and will slow down due to the decrease in depth (loss of energy).
4. The heaviest stones are deposited first / closet to its banks - finer alluvium is carried further away across flood plain.
5. Over repeated floods, deposits of heavy sediment build up on the banks and form ridges known as LEVEES.
6. Levees therefore act as a rivers natural defence against flooding as they keep the river in its channel - crucial in many areas.
7. When river reverts to normal flow deposition on the bed occurs.
8. Eventually deposition on the bed and repeated floods rises the level of the river bed above its flood plain.
9. Levees must be strong enough to prevent flood water reaching the flood plain and are often reinforced by humans.
10. This can lead to problems down stream (altering natural course of a river) and can trigger more extreme events downstream (R.Rhine & R.Mississippi)

Irish example: River Moy, Co. Mayo
Int example: Mississippi, USA.

Floodplain

Areas of flat fertile land either side of a river created by large amounts of alluvial deposits.

meander

Starts to form when there is greater erosion on one side of the channel and deposition on the other. Over time the erosion and deposition will cause the river to bend.

Waterfall - detailed formation - LC***EROSION

*A Waterfall is a vertical fall of water found in the young stage of a river.

1. Waterfalls occur where a river meets a band of hard rock (granite)
2. The erosion processes involved in the formation of waterfalls are hydraulic, abrasion & solution.
3. Through hydraulic forces rocks and soil is broken away from the bank & bed by the force of the water.
4. Eventually a river meets hard rock and the soft rock (shale) down stream is eroded more quickly (DIFFERENTIAL erosion).
5. A small fall forms.
6. Through abrasion (wearing away of riverbed by its load) and the hydraulic force of water the fall deepens and forms a plunge pool.
7. The plunge pool is formed due to erosion and the weight of water and stones carried over the hard rock.
8. Water then splashes against the back wall of the waterfall and erodes it further through the process of solution - cave created.
8. The overhanging piece of the waterfall eventually collapses and waterfall retreats upstream.
9. This is called HEADWARD erosion.
10. This may cause a steep sided gorge (deep channel).

Irish example: Powerscourt Co Wicklow.
Int example: Niagra Falls, USA.

rapid

Formed when you get layers of hard and soft rock. The layers of soft rock erode quicker than the layers of hard rock. This makes the bed of the river uneven creating rough turbulent water.

ox-bow lake

Created when deposition on the outside of the river channel cuts off the old meander and creates an oxbow lake. Often called cut-offs.

V-shaped valley - detailed formation - LC***EROSION

V-Shapped valleys form by the river cutting downwards into its bed.

1. The discharge of a river in its youthful stage can be low due to lack of tributaries in upper stages (low volume)
2. Where the slope is steeper & bed smoother - velocity increases.
-VSVs usually found in these areas (mountains/highlands)
3. Volume also increases due to heavy rainfall or melting of winter snow and VERTICAL erosion takes place (cuts into bed)
4. Hydraulic forces loosens the rocks and sediments from the BED by the force of the water and carries them downstream.
5. Through ABRASION (Water & Load), the bedrock of the channel is cut even deeper forming steeper sides to the channel forming a valley.
6. Valley sides are then exposed to weathering which weakens the rock and soil.
7. The depth of valley is controlled by:
- Speed of erosion (fast = deep Valleys)
-Type of Rock (Hard = shallower valleys)
-if weathering and mass movement is taking place.
8. Rainfall and snow melts may trigger landslides of material making the valley even deeper.

Irish example: River Moy, Co. Mayo
Int example: Grand Canyon, USA.

Gorge

A deep sided valley left behind when a waterfall retreats.

interlocking spurs

Areas of the valley (hills) that stick out into the river forcing it to meander around them.

delta

When a river meets the sea its velocity suddenly reduces. This reduction in velocity means that much of the rivers load is deposited at the mouth of the river. If the river deposits quicker than the sea can erode then this feature starts to develop.

distributary

A small river or stream that breaks off from the main river in deltas.

bankfull discharge

When the river channel is full and can not hold any more water.

Alluvium

Mineral rich load that is deposited on floodplains in times of flood. It is essential to keep farmland fertile.

LC***Human interaction with river processes - The River Rhine & Leeves.

1. The River Rhine is one of Europe's main transport arteries and the need to save costs and time has lead to the development of the river.
- Mouth in Rotterdam 'Gateway to Europe'
2. Building Levees along the Rhine has has prevented natural flood processes from occurring.
3. These are naturally occurring raised river banks but are also raised or built by humans to stop a river flooding its flood plain.
4. Flood plains are flat fertile areas which are often densely populated(floods cause major problems).
5. Duisburg, Germany is the largest inland port in the world and is built on the flood plain of The Rhine.
6. The building of Levees here has stopped damage to large factories and infrastructural links (economic benefit).
- Holland spent 1.3 billion on Levees in 80s
- Polderlands protected by Levees 10 high
7. The Levees of the Rhine have got so high that it can no longer flood resulting in increased deposits on the bed - river rises.
8. Higher stronger Levees are required - Expensive & constant maintenance needed.
9. Floodwater is now trapped in the river and can not escape naturally which can lead to increased problems downstream.
10. Man made Levees confine the flow of the river in narrower channels resulting in The Rhine flowing 30% faster than before construction.

Drainage Patterns

TBC

LC***River Rejuvenation = make young again (Two landforms created by Isostatic Processes)

2 landforms: Knickpoints and River Terraces.

Intro (what isostsatic/eustatic processes are)
1. Involves the change in sea level (eustatic changes) due to uplift (isostacy) or sinking of earths lithosphere caused by removal/ addition of ice sheets during ice ages.
2. Weight of ice sheets on the land pushes it down into mantle/ removal causes land to rise. Process known as Isotacy.
- isostatic uplift may also occur due to tectonic activity.
3. In Ireland, isostatic processes affect the base/sea level of any rivers flowing over land.
4. Land uplifted = rivers base level lowered and has renewed ability to erode.
5. This is termed River Rejuvenation.

Formation of Knickpoints
1. They are rapids/small waterfalls found in mature/lower stage of a river (drop).
2. They are the point where the river previously entered the sea (before isostatic change).
3. Due to isostatic uplift - sea level lowered = renewed ability to erode.
4. Rejuvenated river cuts a new profile (level) through vertical erosion.
5. Point where new profile meets old is known as a Knickpoint.

E.g South East of Ireland, River Barrow in KK - knickpoints show sea level was previously much higher in Ireland.

Formation of River Terraces
1. Rejuvenated river (more power as a result of isostatic change) begins to cut down into existing flood plain.
2. This creates a deeper, narrower channel eventually creating a new flood plain (at a lower level)
3. Original valley floor can now be seen high above the original floodplain forming steps of valleys around the rejuvenated river.
4. Theses steps known as river terraces - often multiple steps can form as river is rejuvenated more than once (isostatic change occurs more than once)
5. Each step/terrace may be matched with a Knickpoint.

E.g River Barrow Co KK.

Isostatic Processes

Involve the change in sea level due to uplift or sinking of the earths lithosphere. (Lithosphere = upper mante, moho & crust)