RIVERS Flashcards
What is a drainage basin?
The area surrounding the river where the rain is falling on the land that flows into the river, also known as the catchment. The boundary of two drainage basins is called the watershed. They are open systems with inputs and outputs.
What are the inputs to the drainage basin?
Precipitation, including all the ways moisture comes out of the atmosphere – rain, snow, hail, dew and frost
What are the stores in a drainage basin?
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Interception – when precipitation lands on vegetation or other structures before it reaches the soil, this causes a significant store in woodlands
Vegetation storage – water taken up by plants
Surface storage – puddles, ponds and lakes
Groundwater storage – stored in the ground either in soil or rocks, porous rocks that hold water are called aquifers. The water table is the top surface of the zone of saturation.
Channel storage – water held in a river of stream channel
What are the flows in the drainage basin?
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Surface runoff – water flowing overland, common in arid areas where torrential rain falls on hard baked land
Through fall – water dripping from one leaf to another
Stem flow – water running down a plant stem or tree trunk
Throughflow – water moving slowly downhill through the soil, faster through pipes like crack or animal burrows
Infiltration – water soaking into the soil, rates are influenced by soil type, soil structure and how much water is already in the soil.
Percolation – water seeping down through the soil into the water table
Groundwater flow – water flowing slowly below the water table through permeable rock.
Base flow – groundwater flow that feeds into rivers through banks and river beds
Interflow – water flowing downhill through permeable rock above the water table
Channel flow – water flowing into the river or stream – also called river discharge.
What are the outputs of the drainage basin?
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Evaporation – water turning from a liquid to gas
Transpiration – evaporation from plant leaves from water taken up through roots
Evapotranspiration – process of evaporation and transpiration together
River discharge, river flow
What is the water balance?
It shows the difference between the inputs and outputs in a drainage system.
When there is more inputs, precipitation, the soil moisture is either in recharge after a period of evapotranspiration or in surplus. In periods where there is lots of evapotranspiration the soil moisture is in utilisation and then deficit.
What are the factors affecting river discharge and the storm hydrograph? (6)
Characteristics of the drainage basin
- Size of basin, how much precipitation can be caught and distance water has to travel into channel
- Gradient of slopes
- Circular basins have watersheds roughly at the same distance from the point where discharge is measure, lots of water will reach this point at the same time
Antecedent condition
- If the ground is waterlogged then infiltration is reduced and surface runoff increases
Rock type
- Impermeable rocks don’t store water or let it flow through them, reducing infiltration and increases surface runoff, reducing lag time
Soil type
- Sandy soils allow lots of infiltration but clay soils have very low infiltration rates
Precipitation
- Amount of precipitation – more will cause a greater peak discharge
- Type of precipitation – snow can melt and run into the river in spring giving a very long lag time
Temperature
- Hot, dry conditions and cold conditions result in hard round, this reduces infiltration and increases surface runoff – reducing lag time and increasing peak discharge
- High temperatures can increase evapotranspiration so less water will reach the channel
What are the human factors affecting river discharge and the storm hydrographs? (5)
Building of dams and reservoirs
- Traps water. Reduces volume off water in the channel especially in dry seasons
Deforestation
- Removal of vegetation decreases interception and vegetation storage, increasing surface run off, shorter lag time and higher peak discharge
Urbanisation
- New buildings increase the amount of interception which will decrease surface run off
- Tarmac and concrete stop infiltration into the bellow soils which increases surface run off
Climate change
- Humans are affecting global temperatures and more extreme weather
Agricultural
- Irrigation for plants removes water from the drainage system
- Use of machinery compacts the soil making it less able to infiltrate the water thus increasing surface run off
Explain the storm hydrograph
A graph showing:
Peak discharge - greatest recorded discharge
Lag time - delay between peak rainfall and discharge, the time that it takes for the water to flow into the river
Rising limb - the increase in discharge as rainwater flows into the river
Falling limb - decreasing discharge as less water is flowing into the river
What are the 3 types of erosion? What stages of a river do they occur in?
Headward erosion - makes the river longer by eroding back upstream, happens near the source as through flow and surface run-off cause erosion
Vertical erosion - makes the river channel deeper, happens in the upper stage of a river
Lateral erosion - makes the river wider, happens in the middle and lower stages of the river
What are the five main ways that erosion happens?
Hydraulic action – the pressure of the water breaks rock particles away from the bed and banks, strongest in rapids and waterfalls and during flooding
Abrasion – eroded pieces of rock in the water scrape and rub against the bed and banks, removing material
Attrition – eroded rocks smash into each other and break into smaller fragments, the edges get rounded off as they rub together
Cavitation – air bubbled in turbulent stretches of water implode causing shockwaves that break pieces of rock off the bank and beds
Corrosion – dissolving of rock by chemical processes, carbon dioxide dissolves in water to form a weak acid which reacts with rocks like limestone and chalk to break them down
What are the four main ways that eroded material can be carried by the river?
Solution – substances that can dissolve are carried along in the water, for example limestone is dissolved into river water that is slightly acidic
Suspension – very fine material, silt and clay, is whipped up by turbulence and carried along by the water, this is the way most eroded material is transported
Saltation – larger particles, pebbles or gravel, are too heavy to be carried by suspension, instead the force of the water causes them to bounce along the river bed
Traction – very large particles, boulders, are pushed along the river bed by the force of water
*Material transported by saltation or traction is called bedload
Why does deposition in a river occur?
Deposition occurs when the river loses energy, it slows down and drops some of its load. The speed and energy can be reduced in many ways:
• Reduced rainfall causes lower discharge, reduces velocity of the river
• Increased evaporation or abstraction causes lower discharge
• Friction reduces the speed of a river
• The see absorbs the rivers energy when they meet
What is the capacity of a river?
The total load that a river can transport at a given point, load can be divided into different categories according to particle size which can range from fine silt and clay to big boulders. The competence is the maximum particle size that a river can transport at a given point.
Explain the Hjulstrom curve
A graph showing the link between river velocity and competence
The critical erosion velocity curve shows the minimum velocity needed for the river to pick up and transport particles of different sizes.
The mean settling velocity curve show the velocities at which particles of different sizes are deposited.
How does the process of erosion change from the source to the mouth?
Upper stage
- Mainly vertical and by abrasion
- Erosion occurs where there are high-energy conditions.
- The rough channel causes turbulence and the large angular bedload is dragged along the river bed, causing intense vertical erosion
Middle stage
- Mainly lateral and by abrasion
- Attrition of larger particles in this stage ,means that sediment size decreases from source to mouth
Lower stage
- Less erosion because turbulence is lower and sediment particle size has been reduced
- Some lateral erosion occurs during the formation of meanders
How does the process of transportation change from source to mouth?
Upper stage
- Mainly large particles such as boulders carried by traction or saltation during high-energy condition
Middle stage
- More material carried in suspension as particle size decreases
- Larger particles moved by saltation
Lower stage
- Mainly smaller particles such as silt and clay are carried by suspension or in solution
How does the process of deposition change from source to mouth?
Upper stage
- Mainly largest particles deposited in the river bed as energy levels drop
Middle stage
- Sand and gravel are deposited across the flood plain as the river floods and friction reduces the river energy
Lower stage
- Smaller particles such as sand and silt are deposited on the flood plain where the river floods and in the river mouth as the sea absorbs the rivers energy
What is the long profile? How does it change downstream?
A long profile shows how the gradient of a river changes from the source to its mouth by showing the height of the river bed above the base level for the length of the river. The base level is the lowest part that a river can erode to, usually sea level.
The upper stages have a steep gradient and the river is high above sea level so has lots of potential energy.
As the gradient decreases towards the middle of the river the energy is converted to kinetic energy giving the river more velocity.
In the lower stages the river has lots of kinetic energy so it flows faster.
What is a graded profile?
In a long profile he total amount of erosion and deposition is balance but the rates change along the river causing landforms such as waterfalls. Over time the long profile will change to a smooth curve called a graded profile.
What is the cross profile? How does the cross profile change downstream?
The cross profile shows what a cross section of the river channel or valley looks like, it has different shapes during different stages of a rivers long profile.
The upper valley is a steep v shape, vertical erosion creates narrow valley floors and steeply sloped sides.
In the middle stage the valley are wider due to lateral erosion, deposition at this stage creates a flood plain on the valley floor.
In the lower stages of the river the valley is wide with a much wider flood plain caused by further deposition.
How and why does velocity and discharge change downstream?
Velocity and discharge increase downstream from source to mouth. Discharge increases as tributaries and more surface runoff join the channel, as there is more water in the channel the water flows faster and a smaller proportion of the water is in contact with the sides and beds there is less friction so the velocity is increased.
What is hydraulic radius and how does it change downstream?
The hydraulic radius is a measure of a rivers efficiency. It is calculated by dividing the cross sectional area by the wetted perimeter. The higher the hydraulic radius the less water is in contact with the wetted perimeter, less friction, which reduces energy loss increasing efficiency, velocity and discharge. The wetted perimeter is increased by a rough channel, protruding banks and boulders, this decreases the hydraulic radius.
How does channel roughness change downstream?
Channel roughness causes turbulence which is most effective at picking up particles from the river bed so causes greater erosion. The channel is roughest in the upper stages so the river loses a lot of energy to friction which causes discharge and velocity to be lowest here.
Erosional landform
What is a potholes and how is it formed?
Small circular hollows in a river bed
Formed by abrasion as turbulence swirls a rivers bedload round in a circular motion causing it to rub and scrape out holes.
Erosional landform
What are rapids and how are they formed?
Relatively steep sections of river with turbulent flow where there are several sections of hard rock.
Formed like waterfalls but produced by more gently inclined resistant rock that steepen