Rivers Flashcards
what is a drainage basin?
the area of land drained by a river and its tributaries
what is the confluence
a point where two rivers or streams meet
what is a tributary
a stream or small river that joins a larger stream or river
what is the mouth
the point where a river leaves its drainage basin and flows into the sea
what is the source
the starting point of a stream or river
what is the hydrological cycle
the continuous movement of water on, above and below the surface of the Earth
what is a watershed
the boundary of a drainage basin. It separates one drainage basin from another and is usually high land, such as hills and ridges
how does the water cycle work
- the sun evaporates the water
- the air rises, it cools and condenses
- the water particles join together, become too heavy and fall as rain
- the rain returns to the sea via rivers
- the process repeats
what is evaporation
water (liquid) changes to water vapour (gas) through energy from the sun
what is evapotranspiration
water is removed from vegetation through energy from the sun
what is groundwater flow (base flow)
water percolates into permeable sedimentary rock layers and flows at a slow steady rate through the cycle
what is stemflow
water runs down stems and branches and infiltrates the soil
what is throughfall
water drips off leaves and infiltrates the soil.
what is throughflow
water infiltrates into soil and flows down slope due to gravity.
what is surface runoff/overland flow
saturated soils limit infiltration enabling water to travel across the surface of the land.
what is interception
water is prevented from travelling any further because of vegetation.
what is river runoff into the sea
water leaves the river by the coast and enters the sea.
what is precipitation
any form of water reaching the earth’s surface: rain, snow, sleet or hail.
what are rivers
water is stored in the river channel .
what are glaciers
water is stored in the mountains in glaciers.
what is condensation
water vapour cools and condenses forming clouds.
7 key characteristics of a river and do they increase or decrease as it moves downstream?
- Width: the distance from one bank to the other - increases
- Depth: the distance from the surface of the water to the bed - increases
- Velocity: how fast the water is flowing - increases
- Discharge: the rate at which the water is moved through the river channel - increases
- Gradient: the steepness of the river bed - decreases
- Wetted Perimeter: Length of the wet part of the channel cross section containing water.
- Cross-section area: area of flowing water when measured from bank to bank.
what is a cross profile
a line that represents what it would be like to walk from one side of a valley, across the channel and up the other side
what is a long profile
shows how the river changes in height along its course
what is discharge
the amount of water that reaches the channel by surface runoff, throughflow and groundflow. The amount of water in the river.
what is lagtime
the time between peak rainfall and peak discharge.
what is run off
the volume of water that flows over the land to reach the river.
what is base flow
the average discharge of a river.
what is rising limb
the line that represents the increasing discharge – it is steep if run off is high.
what is receding/falling limb
the section of the line graph that represents a river returning to base flow.
what is bank full discharge
This is when the river’s water level reaches the top of its channel, any further discharge will result in flooding.
what is peak flow
the maximum discharge of the river as a result of the storm
6 factors influencing a hydrograph
- Size
- Shape
- Rock Type
- Slope
- Vegetation
- Land Use
4 types of erosion
hydraulic action, attrition, abrasion, corrosion
what is percolation
the transfer of water down into the rocks and aquifers
what is hydraulic action
the force of water which removes material from the bed and banks of a river
what is abrasion
what the material carried away by river scrape away the banks and bed
what is attrition
when the material being carried by the river hit each other and the pieces become smaller
what is corrosion
when rocks are dissolved by slightly acidic water
what is vertical erosion
dominant in the upper course of rivers. It increases the depth of the river and valley, as the river erodes downwards
what is lateral erosion
dominant in the middle and lower course of rivers. It increases the width of the river and valley as it erodes sideways
4 processes of transportation
- traction
- saltation
- suspension
- solution
what is traction
when large sediment/rocks is rolled along the river bed
what is saltation
when smaller material is bounced along the river bed
what is suspension
lighter material carried by river flow
what is solution
materials dissolved in river water
what is deposition
set down of material when the river doesn’t have enough energy
how are waterfalls formed
Waterfalls form where 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
- The soft rock erodes quicker by hydraulic action and abrasion, 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
how are potholes created
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
how are meanders formed
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
how are oxbow lakes formed
- With distance downstream the size of the meanders increase
- 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
how are floodplains and levees formed
- Floodplains are flat expanses of land 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
how are deltas formed
- Deltas are formed when streams flow into standing bodies of water
- Rivers must carry a large amount of sediment for deltas to form
- Flocculation increases deposition
- Delta formation must have a rapid drop in stream velocity
2 causes of river hazards
- erosion
- flooding
3 physical causes of flooding
Landslides
Snow and ice melt
Storm surges pushing water up the river channel
5 human causes of flooding
Urbanisation
Deforestation
Building of bridges and dams
Human induced climate change
Agriculture
6 hazards of flooding and erosion
- Flood waters may increase the spread of water related diseases e.g. cholera or breeding ground for mosquitoes
- 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
6 opportunities of rivers
- The silt deposited during flooding is often rich in minerals and nutrients, making it ideal for growing crops
- Rivers are a source of food
- The floodplains are flat land which makes the construction and building of transport networks easier
- Leisure and tourism
- Generating electricity
- Transporting goods and people
how do dams and reservoirs manage floods
enable the amount of discharge downstream to be controlled
how do embankments or levees manage floods
increase the capacity of the river
how does channelisation manage floods
the river channel may be widened or deepened allowing it to carry more water.
the river channel may also be straightened so water can travel faster along the course
this increases capacity and velocity of channel, allowing water to drain from the area faster
how do flood walls manage floods
vertical barrier increases capacity of river channel and reducing flood risk.
how does floodplain zoning manage floods
means that only certain land uses are allowed on the flood plain, reducing the risk
how do wash lands manage floods
low value areas either side of river are allowed to flood if river levels rise to protect areas downstream.
how do warning systems manage floods
network of sirens warn people of flooding, giving residents and early warming and allows them to prepare for evacuation.
how does afforestation manage floods
involves the planting of vegetation and trees to increase interception and infiltration
Bangladesh Flooding Case Study 5 Causes
- heavy monsoon rains - extremely heavy rainfall, 600mm in a few days, overwhelmed river systems and drainage capacities = widespread flooding
- meltwater from Himalayas - rose river levels, combination of rainfall + meltwater = higher flow of major rivers e.g. Ganges
- cyclones - cyclone Amphan in May 2020, remnants of cyclone added further rainfall and exacerbated flooding conditions
- low lying terrains - land prone to abundance of people + infrastructure, flood prone region.
- urbanisation - rapid expansion in cities e.g. Dhaka increased run off due to prevalence of concrete surfaces, reducing absorption
- deforestation - diminishes natures ability to absorb excess rainfall = increased run off and risk
Bangladesh Flooding Case Study 4 impacts
- displacement - 5.5 million affected, 550 dead
- agricultural damage - extensive crop loss impacted food security, destruction of rice, etc. threatened livelihoods
- infrastructure damage - roads, bridges, houses destroyed, disrupted transport + access to essential services
- health risks - standing in water increased risk of cholera, displacement lead to overcrowding - unsanitary
Bangladesh Flooding Case Study 5 Short Term Responses
- government mobilized food relief efforts
- emergency shelters put up for displaced people
- water purification tablets + clean water distributed to prevent disease outbreak
- volunteers distributed relief materials
- mobile health clinics deployed to affected areas
Bangladesh Flooding Case Study 3 Long-Term Responses
- Government made plans for enhancing flood resilience through improved infrastructure
- investments raised for flood shelters to provide safe havens for future flooding events
- flood warning systems for alerts on rising water levels
