Untitled Flashcards Set
The Characteristics & Processes of Rivers
Water on Earth
Only 2.5% of the water on Earth is freshwater
68.7% of freshwater is stored in glaciers and ice sheets and 30% is groundwater
The remaining 1.3% of freshwater is in rivers, soil moisture, lakes and the atmosphere
All water is part of the hydrological cycle
Hydrological cycle
The hydrological cycle is a closed system
Water is constantly recycled through the system
Within the hydrological cycle, there are stores and transfers (flows)
Stores
Stores are those places where water is held for some time. These include:
water in the atmosphere in the form of water vapour or water droplets in clouds
surface stores such as puddles, lakes, rivers and reservoirs
interception is how precipitation is prevented from reaching the ground, usually by being caught on leaves or branches
aquifers are permeable rocks such as limestone and sandstone which can hold water
ice and snow
seas and oceans
Flows (or transfers)
Flows are how water is moved around the hydrological cycle. They include:
Evaporation
The change of water from a liquid to a gas (water vapour) due to heat from the sun
Condensation
When water cools and changes from water vapour into a liquid (water droplets), forming clouds
Transpiration
When plants release water vapour from their leaves
Evapotranspiration
The combined transfer of water vapour from the Earth's surface and plants
Precipitation
The transfer of water from the atmosphere to the Earth's surface in the form of hail, sleet, snow or rain
Overland flow
Any water flowing across the Earth's surface
Infiltration
When water moves down from the surface into the soil
Percolation
The transfer of water down into rocks and aquifers
Through flow
The movement of water through the soil between the groundwater store (water table) and the surface
Groundwater flow
The flow of water through rock
The Drainage Basin Processes
Drainage basin
A major part of the hydrological system, drainage basins drain all the water which lands on the Earth's surface
A drainage basin is an open system
Every drainage basin is unique and is different in shape and size, with different rock types, relief and land use
Drainage basin features
All drainage basins have some features in common:
Watershed
This is the boundary between drainage basins
Source
The furthest point from the mouth where the river starts can be an upland lake, spring or glacier
Confluence
The place where two or more streams/rivers meet
Tributary
A stream or river flowing into a larger stream or river
Mouth
Where the rivers enter the sea/ocean or sometimes a lake
Channel network
Every drainage basin is covered by a network of tributaries that connect to the main river channel. The number of tributaries in a drainage basin is referred to as the drainage density:
Drainage basins with lots of tributaries have a high drainage density
Drainage basins with few tributaries have a low drainage density
River Processes
Erosion
Erosion is the wearing down of surfaces
There are four erosion processes that change the shape of the river channel:
Hydraulic action
The force of the water, which removes material from the bed and banks of the river
Abrasion
When the materials carried by the river scrape away the banks and bed
Attrition
When the material being carried by the river hits each other, the pieces become rounder and smaller
Corrosion (solution)
When rocks are dissolved in slightly acidic water
Erosion can be mainly vertical or lateral:
Vertical erosion is dominant in the upper course of rivers. It increases the depth of the river and valley as the river erodes downwards
Lateral erosion is dominant in the middle and lower courses of rivers. It increases the width of the river and valley as it erodes sideways
Transportation
There are four processes of transportation:
Traction
Occurs when larger rocks and materials are rolled along the riverbed
Saltation
Smaller material, which can be lifted by the water, bounces along the riverbed
Suspension
Lighter material carried within the river flow
Solution
When materials are dissolved in the river water
Deposition
When a river does not have enough energy to carry materials, it drops them
This is deposition
The causes of reduced energy include:
Reduced discharge due to a lack of precipitation or abstraction upstream
Decreased gradient
Slower flow on the inside of a river bend or where the river is shallower
When the river enters a sea, ocean or lake
The heaviest material is deposited first; this is known as the bedload
The lighter materials, gravel, sand and silt, are known as alluvium and they are carried further downstream
The dissolved materials are carried out to sea
As a result of erosion, transportation and deposition the character of a river changes as it moves downstream
These changes are summarised in the Bradshaw model
River characteristics
All rivers have long and cross-profiles
Each river's long and cross profiles are unique but they do have some characteristics in common
These profiles show changes in river characteristics from the source to the mouth
Long profile
The long profile of a river shows the changes in the river gradient from the source to the mouth
Most long profiles have a concave shape with similar characteristics:
The source is usually in an upland area
The upper course of the river includes steep areas with uneven surfaces
In the middle course, the gradient decreases
In the lower section, the gradient decreases further until it becomes almost flat
Cross profiles
The cross profiles of a river are cross-sections from one bank to another
Cross profiles of the upper, middle and lower courses show the changes in the river channel
Upper course characteristics include:
Shallow
Steep valley sides
Narrow
Low velocity
Large bedload
Rough channel bed
High levels of friction
Vertical erosion
Middle course characteristics:
Deeper than upper course channel
Gentle valley sides
Wider than upper course channel
Greater velocity than upper course channel
Material in river decreases in size
Smoother channel bed
Lower levels of friction than upper course channel
Lateral erosion
Lower course characteristics:
Deeper than middle course channel
Flat floodplains
Wider than middle course channel
Greater velocity than the middle course channel (apart from as the river enters the mouth)
Material carried mainly sediment and alluvium
Smooth channel bed
Lowest friction
Deposition is dominant
River Landforms
River landscape characteristics
The changes in river channel characteristics, lead to changes in the river landscape
The upland and lowland areas of rivers have distinctive landforms
Upland:
Waterfalls
Gorges
V-shaped valleys
Interlocking spurs
Waterfalls and gorges
Waterfalls form when there is a drop in the river bed from one level to another
This drop is often due to changes in the hardness of the rock, where hard rock overlies soft rock
Hydraulic action and abrasion are the main erosional processes:
The soft rock erodes quicker, undercutting the hard rock and creating a plunge pool
This leads to the development of an overhang of hard rock which eventually over time, collapses
The overhang falls into the plunge pool increasing abrasion and making the plunge pool deeper
The process then begins again and the waterfall retreats upstream leaving a steep-sided gorge
Vertical erosion is dominant in the upper course of the river
This cuts down into the river bed and deepens the river channel
Weathering and mass movement lead to material from the valley sides collapsing into the river, forming a steep v-shaped valley
Interlocking spurs
In the upper course of the river, the channel starts to meander
Erosion happens on the outside of the bend
In the upland areas this forms interlocking spurs
Potholes
Potholes are round depressions in the riverbed
They are formed by abrasion
Where there are dips in the riverbed, the river flow can cause the sediment to spin
This erodes the dip, forming a circular hollow (pothole)
As the size of the hollow increases, larger material becomes trapped in the pothole
This further increases the erosion of the pothole
Lowland features:
Meanders
Ox-bow lakes
Floodplains
Levees
Meanders
In lowland areas, lateral erosion is dominant
Meanders increase in size
The fastest water flow (thalweg) is on the outside of the river bends, leading to erosion:
The erosion undercuts the riverbank, forming a river cliff
The riverbank collapses and the edge of the meander moves further out
The slowest flow is on the inside of the river bends, leading to deposition:
The deposits form a slip-off slope
Deposition on one side and erosion on the other leads to the meander migrating across the valley
Oxbow lakes
With distance downstream, the size of the meanders increases
The erosion on outside bends can eventually lead to the formation of a meander neck
At a time of the flood, the river may cut through the neck of the meander, forming a straighter course for the water
The flow of water at entry and exit from the meander will be slower, leading to deposition
The meander becomes cut off from the main river channel, forming an oxbow lake
Floodplains and levees
Floodplains are flat expanses of land on either side of the river
The migration of meanders leads to the formation of the floodplain
High discharge may cause the river to overflow the banks
More of the water is in contact with the land surface as the water spreads across the floodplain
Increased friction reduces velocity and material is deposited across the floodplain, gradually increasing the floodplain height
The heaviest material is deposited first, nearest to the river channel, forming natural embankments called levees
Deltas
Deltas are formed when streams flow into standing bodies of water
Rivers must carry a large amount of sediment for deltas to form
Delta formation requires a rapid drop in river velocity
Flocculation, when salt particles stick together, increases deposition
Bioconstruction, when vegetation slows the river, increases deposition
There are a variety of delta formations, such as arcuate and bird’s foot
Causes of River Hazards
The main river hazards are flooding and erosion
Rivers flood when the water in them reaches bankfull discharge and then overspills across the floodplain
Flooding occurs as the result of two main causes:
If there is a period of heavy, torrential rain leading to high levels of overland flow because the water cannot infiltrate
A prolonged period of steady rain means that the ground becomes saturated, leading to high levels of overland flow because the water cannot infiltrate
There are other natural causes of river flooding, which include:
landslides
snow and ice melt
storm surges push water up the river channel
Although the root cause of flooding is precipitation, the risk of flooding can be increased by human activities such as
urbanisation
deforestation
building of bridges and dams
human-induced climate change
agriculture
Flooding often goes hand in hand with erosion of the banks, as both result from increased discharge and velocity
Hazards of flooding and erosion
Flood waters may increase the spread of water-related diseases;
The water may act as a breeding ground for the animals that spread disease, for example, the mosquito
The water may be contaminated by bacteria, which can spread diseases such as cholera
Deaths and injuries as floodplains are often densely populated due to the fertile soils
Bridges and transport routes may be damaged or destroyed by the flood waters
Erosion of the river banks leads to the loss of farmland, housing and transport routes
Destruction of crops
Increased insurance costs
Lower house prices
Opportunities
As well as the hazards, there are many opportunities provided by rivers:
The silt deposited during flooding is often rich in minerals and nutrients
This makes it ideal for growing crops
Rivers are a source of food
The floodplains are flat land
Construction and building of transport networks easier
Water can be used to irrigate farmland
Leisure and tourism
Generating electricity
Transporting goods and people
Managing the Impacts of River Flooding
Flood prediction
Prediction of flooding means that steps can be taken to manage flooding
Flood hydrographs are used to predict the reaction of a river's discharge to a rainfall event
A flood hydrograph shows the changes in river discharge after a storm event
The graph shows a short period of time, usually 24 hours
The flood hydrograph has a number of features:
Base flow
Peak rainfall
Rising limb
Peak discharge
Lag time
Recessional limb
The key factor in assessing the flood risk is time taken for the precipitation to reach the river from where it falls—the lag time
Rivers with a short lag time and steep rising limbs have a much greater risk of flooding
The water reaches the river rapidly and the river may not have the capacity to cope with the influx of water
Rivers with a long lag time and gentle rising limbs have a lower flood risk
The water reaches the river more slowly, causing a gradual increase in discharge
The lag time depends on some human and physical factors
These lead to increased overland flow, which shortens the lag time
Human factors which increase the risk of flooding
Deforestation
Lack of trees reduces interception and infiltration, increasing overland flow
Urbanisation
Impermeable concrete and tarmac increase the overland flow
Water flows into the drains, reaching the river rapidly
Agriculture
Bare soil and ploughing increase overland flow
Climate change
Rising global temperatures may increase storm frequency and intensity
Physical factors which increase the risk of flooding
Relief
Steep slopes reduce infiltration and increase overland flow
Rock type
Impermeable rocks reduce percolation and increase overland flow
Soil
Frozen, saturated or compacted soil reduces infiltration and increases overland flow
Some soil types, such as clay, reduce infiltration and increase overland flow
Weather
Heavy or prolonged rainfall means that the rate at which water reaches the surface exceeds the infiltration rate, leading to increased overland flow
After a period of snow, rising temperatures can cause rapid melting, which increases overland flow
Seasonal variations
Flooding in Northern Europe tends to occur in the autumn and winter when rainfall is more frequent
In areas affected by monsoons, much of the annual rainfall occurs in a few weeks, saturating the ground and increasing overland flow
Higher temperatures in spring lead to snow melt in mountainous areas, increasing overland flow
Drainage density
Where drainage density is high, there are many tributaries taking water to the main channel, causing a rapid increase in discharge
Vegetation
Where there is little natural vegetation, there is reduced interception leading to increased overland flow
Flood management
The key cause of flooding is the amount and duration of precipitation; this cannot be altered
There are a number of methods of managing floods and reducing the severity and/or impact
The two main categories of flood management are hard and soft engineering:
Hard engineering involves building structures or changing the river channel
Soft engineering works with natural processes of the river and surrounding environment
Soft engineering is increasingly popular
Soft engineering is an example of mitigation where schemes aim to minimise damage rather than trying to prevent the flooding
Types of hard engineering
Dams and reservoirs which enable the amount of discharge downstream to be controlled
Embankments or levées increase the capacity of the river
Straightened channels mean that the river flows more quickly past vulnerable areas, reducing the risk of flooding
Flood relief channels allow some water to flow out of the main channel, reducing the discharge
Spillways or overflow channels: these take excess water away from the main channel
Types of soft engineering
River restoration supports the river by restoring it to its original regime—putting meanders back in, stabilising banks and connecting to flood plains
Wetland conservation these areas provide somewhere for excess water to go and slow the flow of floodwater
Catchment management plans assess the risk of flooding in an area and outline how this will be managed
Floodplain zoning means that only certain land uses are allowed on the floodplain, reducing the risk
Afforestation involves the planting of vegetation and trees to increase interception and infiltration
Additional flood control methods
Other methods that can be used to decrease the risk of flooding are:
Leaving the stubble on the fields after the crop is harvested helps to stabilise the soil and increase infiltration
Contour ploughing involves ploughing fields across the slope rather than up and down. This gives the water more time to infiltrate and stops the ploughed furrows becoming channels for water
Improved forecasting and flood warnings
Dredging the rivers to increase capacity, however, often leads to the need for concrete reinforcement of the banks
Case Study: Ganges/Brahmaputra
Case Study
The Ganges is 2,510 km long
It flows through India and Bangladesh, where it becomes the River Padma and joins with the Brahmaputra River
The drainage basin covers 1.2 million km2
The population living within the area drained by the Ganges is over 650 million people
The Brahmaputra River is 3,969 km long
It flows through Tibet, India and Bangladesh, where it joins with the River Padma
The drainage basin covers 651,334 km2
Both rivers;
Have their source in the Himalayan Mountains
Waters enter the sea in the Bay of Bengal
The Ganges and Brahmaputra Basin
Opportunities
Water supply
There are many cities, including New Delhi and Kolkata, along the Ganges/Brahmaputra drainage basins which take their water supplies from the river
Agriculture and fishing
The regular flooding in the drainage basin leaves deposits of alluvium, which are rich in nutrients and ideal for growing crops such as rice and jute
The rivers provide water for irrigation and for the flooding of rice fields
Fish from the river provide food and jobs for local people
Culture
The Ganges is sacred to Hindus and is worshipped as the goddess Ganga
Tourism
Three sites which are holy to Hindus lie on the banks of the Ganges (Haridwar, Allahabad and Varansi). This leads millions of pilgrims to visit each year
Rafting and river cruises are also increasingly popular
Flat land
The flat floodplains mean that construction is easy, and the floodplains have one of the highest density populations in the world
Energy
There are a number of dams along both rivers, including the Tehri Dam on the Ganges, which is the biggest hydroelectric power plant in India
Hazards
The Ganges/Brahmaputra drainage basin regularly experiences floods, including most recently in May 2022
In 1998, 75% of Bangladesh was flooded over 30 million people were made homeless
Over 1000 people died
700,000 hectares of crops were destroyed
'Normal' floods are vital to provide fertile soil and irrigation, but increasingly flooding is becoming more unpredictable and extensive
Human causes of flooding
Deforestation particularly in the upland areas
This leads to less interception and infiltration, increasing overland flow
Human-induced climate change has led to increased melting of Himalayan snow and ice, which increases discharge
It may also have affected climate patterns, leading to increased frequency and severity of tropical cyclones
Urbanisation, as the population increases and there is more rural-urban migration, leads to increased overland flow due to impermeable surfaces
Agriculture increases overland flow and soil erosion, which reduces the capacity of the rivers
Natural causes of flooding
Low-lying land in the Ganges delta in Bangladesh which is at or just above sea level, meaning that it floods more easily
Monsoon climate which means that there are heavy and prolonged rains for some months
Tropical cyclones bring heavy rainfall
Melting snow and ice from the Himalayas in spring leads to a rapid increase in river discharge
Management of flooding
Bangladesh is an LEDC
The country lacks the money for large schemes to reduce the impact of flooding
Flood Action Plan (FAP) was funded by the World Bank and a number of MEDCs. Measures that were proposed include:
Monitoring of flood levels
Construction of levées/embankments
Building 5000 flood shelters
Creating floodwater storage systems
A more effective flood warning system
Building of dams to store water
Reducing deforestation
The FAP was not considered a success because;
Many parts of the project were never completed, including the dams and floodwater storage areas, due to inadequate funding and corruption
There was later a recognition that some flooding was necessary to maintain agriculture in many areas
8 million people were forced to move to accommodate the FAP construction
Changing the channel upstream meant that areas downstream suffered more
The government cannot afford the maintenance costs
New management suggestions include;
Better flood forecasting and warning systems
More well-stocked flood shelters
These are less damaging to the environment and cheaper to maintain than hard engineering such as embankments, dams and floodwater storage areas