Rivers

Question 1

What are the 4 types of erosion?

Answer 1

Hydraulic action-Water smashes into the river bank and air gets trapped and breaks the rocks apart
Abrasion-Pebbles grind along the river bed and bank
Attrition-River rocks collide in with each and break
Solution-The water dissolving certain chemicals in rocks

Question 2

What are the 4 types of transportation

Answer 2

Traction - large, heavy pebbles are rolled along the river bed. This is most common near the source of a river, as here the load is larger
Saltation - pebbles are bounced along the river bed, most commonly near the source
Suspension - lighter sediment is suspended (carried) within the water, most commonly near the mouth of the river.
Solution - the transport of dissolved chemicals. This varies along the river depending on the presence of soluble rocks

Question 3

Define deposition.

Answer 3

When the river loses energy, it drops any of the material it has been carrying.

Question 4

What are the type of features in a drainage basin?

Answer 4

Watershed - the area of high land forming the edge of a river basin
Source - where a river begins
Mouth - where a river meets the sea
Confluence - the point at which two rivers meet
Tributary - a small river or stream that joins a larger river
Channel - this is where the river flows

Question 5

Explain upper course and lower course

Answer 5

Upper course - in the upper course, where the river starts, there is often an upland area. The river’s load is large in the upper course, as it hasn’t been broken down by erosion yet. The river erodes vertically.
Lower course - in the lower course, the land is a lot flatter. The river’s load is fine sediment, as erosion has broken down the rocks. The river is much more deeper and the river erodes laterally.

Question 6

What is a waterfall?

Answer 6

A waterfall is a sudden drop along the river course. It forms when there are horizontal bands of resistant rock (hard rock) positioned over exposed, less resistant rock (soft rock).

Question 7

Explain the process of making a waterfall.

Answer 7

The soft rock is eroded quicker than the hard rock and this creates a step.
As erosion continues, the hard rock is undercut forming an overhang.
Abrasion and hydraulic action erode to create a plunge pool.
Over time this gets bigger, increasing the size of the overhang until the hard rock is no longer supported and it collapses.
This process continues and the waterfall retreats upstream.
A steep-sided valley is left where the waterfall once was. This is called a gorge.

Question 8

What is an interlocking spur?

Answer 8

In the upper course there is more vertical erosion. The river cuts down into the valley. If there are areas of hard rock which are harder to erode, the river will bend around it. This creates interlocking spurs of land which link together like the teeth of a zip.

Question 9

What is a meander?

Answer 9

As the river makes its way to the middle course, it gains more water and therefore more energy. Lateral erosion starts to widen the river. When the river flows over flatter land they develop large bends called meanders.

Question 10

Explain the features of a meander.

Answer 10

As a river goes around a bend, most of the water is pushed towards the outside. This causes increased speed and therefore increased erosion (through hydraulic action and abrasion).
The lateral erosion on the outside bend causes undercutting of the bank to form a river cliff.
Water on the inner bend is slower, causing the water to slow down and deposit the eroded material, creating a gentle slope of sand and shingle.
The build-up of deposited sediment is known as a slip-off slope (or sometimes river beach).

Question 11

How is an oxbow lake formed?

Answer 11

Due to erosion on the outside of a bend and deposition on the inside, the shape of a meander will change over a period of time. Erosion narrows the neck of the land within the meander and as the process continues, the meanders move closer together. When there is a very high discharge (usually during a flood), the river cuts across the neck, taking a new, straighter and shorter route. Deposition will occur to cut off the original meander, leaving a horseshoe-shaped oxbow lake.

Question 12

What is a floodplain?

Answer 12

A floodplain is an area of land which is covered in water when a river bursts its banks

Question 13

What are floodplains used for?

Answer 13

Floodplains are often agricultural land, as the area is very fertile because it’s made up of alluvium (deposited silt from a river flood). The floodplain is often a wide, flat area caused by meanders shifting along the valley.

Question 14

How are levees made?

Answer 14

Levees occur in the lower course of a river when there is an increase in the volume of water flowing downstream and flooding occurs.
Sediment that has been eroded further upstream is transported downstream.
When the river floods, the sediment spreads out across the floodplain.
When a flood occurs, the river loses energy. The largest material is deposited first on the sides of the river banks and smaller material further away.
After many floods, the sediment builds up to increase the height of the river banks, meaning that the channel can carry more water (a greater discharge) and flooding is less likely to occur in the future.

Question 15

What is an estuary?

Answer 15

An estuary is where the river meets the sea. The river here is tidal and when the sea retreats the volume of the water in the estuary is reduced. When there is less water, the river deposits silt to form mudflats which are an important habitat for wildlife.

Question 16

What are the causes of flooding?

Answer 16

Prolonged rainfall - if it rains for a long time, the land around a river can become saturated (it’s holding as much water or moisture as can be absorbed). If there is more rainfall it cannot be soaked up, so it runs along the surface - this is known as surface run-off.
Heavy rainfall - if there is heavy rainfall there is less chance of it being soaked up by the soil (infiltration) so it runs off into the river. The faster the water reaches the river, the more likely it will flood.
Relief - a steep valley is more likely to flood than a flatter valley because the rainfall will run off into the river more quickly.
Geology - permeable rocks allow water to pass through pores and cracks, whereas impermeable rocks do not. If a valley is made up of impermeable rocks, there is a higher chance of flooding as there is an increase in surface run-off.
Vegetation - trees and plants absorb water, this is known as interception. Lots of vegetation reduces flood risk. Sometimes people cut down trees (deforestation). This will increase the flood risk, as the water will not be intercepted and flow into the river.
Urban land use - when an area surrounding a river is built on, there is an increase in the amount of tarmac and concrete, which are impermeable surfaces. Drains and sewers take water directly to the river which increases flood risk.

Question 17

Why can lag time be different?

Answer 17

The lag time can be short or long depending on different factors. For example, if there is no vegetation in an area, the water runs off into the river quicker, therefore it would have a short lag time. Alternatively, if there is plenty of vegetation in the area, the lag time would be longer as the plants would intercept the rainfall. A short lag time means water is reaching the river quickly, so there is a greater chance of a flood.

Question 18

What are the factors affecting lag time?

Answer 18

Factors influencing lag time include:

Size of drainage basin
Vegetation
Valley side steepness
Soil type

Question 19

What is hard and soft engineering?

Answer 19

Hard engineering involves building artificial structures which try to control rivers. They tend to be more expensive.
Soft engineering does not involve building artificial structures, but takes a more sustainable and natural approach to managing the potential for river flooding.

Question 20

Give the advantages and disadvantages of hard engineering methods?

Answer 20

The dam traps water, which builds up behind it, forming a reservoir. Water can be released in a controlled way.an be used to produce electricity by passing the water through a turbine within the dam.
Reservoirs can attract tourists.
Disadvantages
Very expensive.
Dams trap sediment which means the reservoir can hold less water.
Habitats are flooded often leading to rotting vegetation. This releases methane which is a greenhouse gas.
Settlements are lost leading to the displacement of people. In developing countries locals are not always consulted and have little say in where they are relocated.
Embankments
Raising the banks of a river means that it can hold more water.
Advantages
Cheap with a one-off cost
Allows for flood water to be contained within the river.
Disadvantages
Looks unnatural.
Water speeds up and can increase flood risk downstream.
Flood Relief channels
The floodwater flows into the relief channel and is taken either to an area where it can be absorbed, or re-enters the river further down its course.
Advantages
Removes excess water from the river channel to reduce flooding.
Disadvantages
Expensive to build.
If water levels continue to rise, the relief channel may also flood.

Question 21

Explain and give advantages and disadvantages of soft engineering methods

Answer 21

Flood warnings and preparation
The environmental agency monitors rivers and issues warnings via newspapers, TV, radio and the internet when they are likely to flood so people can prepare.Advantages
People have time to protect their properties, eg with sandbags.
Many possessions can be saved, resulting in fewer insurance claims.
Disadvantages
Some people may not be able to access the warnings.
Flash floods may happen too quickly for a warning to be effective.
They do not stop land from flooding - they just warn people that a flood is likely.

Floodplain zoning
Allowing only certain land uses on the floodplain reduces the risk of flooding to houses and important buildings.Advantages
More expensive buildings and land uses are further away from the river, so have a reduced flood risk.
Less damage is caused, leading to fewer insurance claims.
Disadvantages
Not always possible to change existing land uses.
Planners have to decide what type of flood to plan for.

Question 22

Why does the velocity change downstream?

Answer 22

he speed the water travels at is dependent on which course of the river it is travelling through. Friction occurs between the water and the riverbed which slows the water down. The more contact the water has with the floor, the more friction occurs which means the water travels slower. In the upper course, the channel is shallow and so most of the water passes the riverbed, slowing the rate of flow due to friction. As the river channel gets bigger less water is in contact with the riverbed which means the velocity of the water increases. In the lower course of the river, the water travels a lot faster than the upper course as there is less friction from the river bed. The river is wider and deeper due to the increased erosion that’s occurred from the fast water.

Question 23

What are the causes of flooding?

Answer 23

Physical causes of flooding:

heavy rainfall
long periods of rain
snowmelt
steep slopes
impermeable rock (doesn’t allow water through)
very wet, saturated soils
compacted or dry soil

Human factors increasing flood risk:

urbanisation, because towns and cities have more impermeable surfaces

deforestation, because removing trees reduces the amount of water
intercepted and increases
run-off

Question 24

How human activities affect river landscapes:

Answer 24

An increasing amount of land is built on as urban areas have grown in the UK. This includes construction on flood plains. Surfaces that were formally permeable have become impermeable due to the use of materials such as concrete and tarmac. Water cannot infiltrate these surfaces, leading to an increase in surface run-off. Gutters and drains quickly transfer water to rivers, leading to a rapid increase in river discharge.

Due to more water entering the river channel in a shorter period of time, the river has more energy increasing the amount of erosion and transportation. Additionally, rivers are more likely to flood due to the increased volume of water. As a result of this humans are more likely to manage rivers to reduce the risk of flooding. The construction of river defences can have a significant impact on rivers and river landscapes.

Farming influences rivers and river landscapes in a number of ways. Water is extracted from rivers for irrigation (watering crops), reducing the volume of water within the channel. This decreases its capacity to erode and transport materials. Deposition is likely to occur with a decrease in the volume of water in the river.

As farms have become more intensive vegetation cover such as hedgerows have been removed. This decreases interception, leading to higher levels of surface run-off. Also, ploughing land leads to more soil being eroded when it rains, increasing sediment load in rivers. Deposition will increase further downstream.

Many farms have draining systems to remove water from the soil to rivers. This leads to a rapid transfer of water to rivers, increasing discharge and erosion and transportation.

Question 25

What is discharge?

Answer 25

Discharge = velocity x cross-sectional area

Question 26

What is a long profile?

Answer 26

A section of the course of a river drawn from source to mouth.

Question 27

Explain a long profile graph A-Upper course
B-Middle course
C-Lower Course

Answer 27

A - as the river flows downhill there is an increase in vertical erosion. The channel is shallow and narrow because there is not a lot of water in the channel.
B – as the river flows into the middle course, there is some vertical erosion but more lateral erosion. The channel is wider and deeper as a result.
C - in the lower course there is a lot less erosion, with only some lateral erosion. The channel is at its widest and deepest.