The causes of flooding

Weather

Rainfall (volume) 

A lot of rain causes higher river levels, whereas less rainfall or prolonged dry periods will result in lower river levels due to both the volume of rain and the fact that it will take longer for the rainwater to reach the river. 

Rainfall (type)

The type of rainfall can also have an influence as long periods of heavy rain can saturate the soil and underlying rock, meaning further rainfall will flow as surface run off towards the river. Light rain or drizzle has time to infiltrate the soil freeing up space for more rain. 


Snow

Melting snow can cause large volumes of water to saturate the land and run off towards the river. 

Physical Factors

Relief

Steep slopes encourage fast run off as the water spills rapidly downwards due to gravity. On gentle slopes, absorption into the land reduces the volume of water reaching the river. 


Land use 


Urbanisation (including the building of roads, houses and hard standing)

Creates an impermeable surface preventing rainwater soaking into the ground. Man made drains are very effective ways of transporting water away from the built up areas, and into rivers. 

Deforestation

Allows water to reach the surface quicker as the trees do not intercept it and the trees also do not extract water from the ground. 

Factors affecting river discharge

Weather

Rainfall (volume) 

A lot of rain causes higher river levels, whereas less rainfall or prolonged dry periods will result in lower river levels due to both the volume of rain and the fact that it will take longer for the rainwater to reach the river. 

Rainfall (type)

The type of rainfall can also have an influence as long periods of heavy rain can saturate the soil and underlying rock, meaning further rainfall will flow as surface run off towards the river. Light rain or drizzle has time to infiltrate the soil freeing up space for more rain. 

Temperature

When the temperature is higher, river levels go down due to evaporation (water lost from the surface) and transpiration (water lost from pores in vegetation). Freezing temperatures can lead to frozen ground so water cannot soak in. 

Physical Factors

Relief

Steep slopes encourage fast run off as the water spills rapidly downwards due to gravity. On gentle slopes, absorption into the land reduces the volume of water reaching the river. 

Rock Type

Important in determining how much water infiltrates and how much stays on the surface. 
Impermeable rock, e.g granite and clay have more surface river water cannot infiltrate so it reaches rivers more quickly than permeable rock, e.g chalk and sandy soils which absorb the water. 

Land Use

Farming

The ploughing of fields can create channels assisting rainwater to reach the rivers during wet periods, increasing the water levels. However, farmers extract water from rivers during dry periods which can reduce the water levels. 

Levees and Floodplains

Floodplain: the flat area next to the river channel, especially in the lower part of the course. This is a natural area for the water to spill onto when the river reaches the top of its banks. 

Levees: raised banks along the course of a river in its lower course. They are formed naturally but can be artificially increased in height. 

How are levees formed? 

• Rivers flood on a regular basis. The area over which they flood is known as the floodplain and this often coincides with regions where meanders form.
• When rivers flood in the middle valley the cover an area of land known as the flood plain.
• When they flood velocity is slowed and deposition of any rocks being transported is encouraged.
• This deposition leaves a layer of sediment across the whole floodplain.
• After several floods there are several layers of sediment (rocks) deep on the flood plain.
• In addition, the largest rocks and most deposition occurs next to the river channel.  This leaves a ridge of higher material next to the river channel on both banks of the river known as a levee.

Other information: 

• The largest material us deposited next to the channel as this is where the river starts to lose energy
• Smaller material is carried further away and then deposited

Meanders and Oxbow lakes

Meander: a bend or curve in the river channel  

Oxbow lake: a horseshoe or semi circular area that used to be a meander. Oxbow lakes are cut off from a supply of water and so will eventually become dry. 

How do oxbow lakes form? 

• As water travels down the length of the river, it carries material with it
• Larger loads carried by the river hit the bed and banks, causing bits to break off. 
• As the river begins to meander, the outside bend is eroded via lateral erosion. 
• Material is deposited on the inside bend. This continues until the neck of both meanders meet. 
• The semi circular meander is eventually cut off when the river floods across the neck. 
• A oxbow lake is formed as it is cut off from the main river. 

Waterfalls and gorges

Waterfall: the sudden, and often vertical, drop of a river along its course. 

Gorge: a narrow, steep sided valley.

Waterfalls provide some of the most spectacular scenery in mountainous areas. In their wake, they leave gorges as they retreat back up the valley. 

Features of a waterfall: (you may be given a photograph)

• Steep/ vertical drop
• Descends in two steps
• Falls
• Plunge pool visible at base
• White water
• Covers width of valley
• Gorge visible in foreground
• Steep sides

How are they formed?

Waterfalls begin where a band of hard rock (e.g. granite) overlies a softer rock (e.g. sandstone).

Erosion processes such as Hydraulic Action (the force of the water) and Abrasion (where the river rubs stones that are being transported against the bed of a river thereby breaking it down) dominate.

The softer rock is eroded quicker than the harder rock and gradually washes away downstream.

This creates a plunge pool where water is swilled around, potholing can occur here and any rocks and debris swept into the plunge pool by the river will be swirled around and rub against the bed and banks of the plunge pool (called corrasion), deepening it further.

Over time, the softer rock is eroded further creating an over hang of hard rock.  This overhang is unstable as it's weight is unsupported.

Eventually, this hard rock collapses because it is unsupported and the waterfall moves back upstream.

This creates Gorges, which are steep sided deep river valleys.  This process will repeat continually, with the location of the waterfall moving back upstream.

Long and Changing Cross profiles

What does a 'long profile show?

A long profile shows how the height of a river changes along its course typically from its source, to where it meets the sea. 

How does a theoretical long profile change from the source to the mouth of the river?

There is generally a steep reduction in the height of a river from its source to a more gradual reduction in height further downstream to the mouth of the river. Traced on a diagram, the profile would have a concave appearance. 

However, long profiles are rare as there are a number of geographical factors that can interfere with this. For example, the land can be uplifted along the course of the river and can be affected by different bands of hard and soft rock crossing the path of the river. Also the sea level can change. 

How does a river change downstream? 

Upper: Shallow narrow channel

In the upper channel, the river carries a large load causing abrasion. At the high flow, there is some saltation and traction. Large boulders are transported by the river and some are in suspension. There is little solution. The large load is deposited. Mostly vertical erosion occurs with Hydraulic action and attrition. 

Middle: Wider, deeper channel

The channel is deeper and wider. Vertical erosion decreasing in importance. More lateral erosion and deposition occurs. Suspension is the main transportation type. Saltation and traction are still present. The land becomes smaller and less regular. 

Lower: Widest, Deepest channel

The channel is at it's widest and deepest, and may be tidal. Fine Material is deposited. There is only a little lateral erosion - deposition is more important than erosion. There is a large amount of load, but the size is very small and rounded. Suspension is the dominant process. 

What is the cross profile of a river? 

The cross profile of a river shows the shape of a river valley which will change and generally become wider and flatter at the mouth of the river. 

Erosion Dictionary

  

Lateral erosion:

Sideways erosion, for example in a river channel at the outside bend of a meander.

Vertical erosion:

Downwards erosion, for example when a river gouges out a deep valley. 

Deposition:

The dumping of sediment that has been transported by a river. This occurs when a river loses energy.

Abrasion:

Large loads carried by the river hit the bed and banks causing bits to break off. This wears down the river bed and banks. 

Attrition:

Stones and boulders carried by the river knock together making them gradually smaller and smoother. 

Solution:

Rocks and minerals in the river are dissolved by rainwater. 

Hydraulic action:

The power of the volume of water against the banks can cause air to be trapped in cracks and crevices. The pressure weakens the banks and wears it away. 

Suspension:

Fine, light material is carried along by the river. It floats within the water and is moved as it flows. 

Traction:

Large boulders and rocks are rolled along as they are too heavy to lose contact with the river bed. 

Saltation:

Small pebbles and stones are bounced along the river bed. This lighter load leaves the river bed in a hopping motion.