General Flashcards
What’s the river’s watershed?
The boundary of the drainage basin.
What is a river’s catchment?
The area surrounding where the rain falling on the land flows into that river. It’s also called a river’s drainage basin.
What happens to water falling beyond the watershed?
It enters a different drainage basin.
Are drainage basins open or closed systems?
Open.
What’s an input of a typical drainage basin system?
Precipitation.
Name the storage of a typical drainage basin system.
Interception, vegetation, groundwater, surface and channel.
Name the flows of a typical drainage basin system.
Throughfall, surface runoff, stemflow, infiltration, interflow, channel flow, percolation, groundwater flow, baseflow.
Put these flows in speed order: baseflow, throughflow, groundwater and interflow.
Throughflow (fast)
Interflow (medium)
Baseflow (slow)
Groundwater flow (very slow)
Name the outputs of a typical drainage basin system.
Transpiration, evaporation, evapotranspiration and river discharge.
What is the water balance worked from?
Inputs (precipitation) and outputs (channel discharge and evapotranspiration).
What’s the trend in wet seasons of the water balance?
Precipitation exceeds evapotranspiration creating a water surplus. The ground stores fill with water so there’s more surface runoff and higher discharge, so river levels rise.
What the trend of drier seasons of the water balance?
Precipitation is lower than evapotranspiration. Ground stores are depleted as some water is used and some flows into the river channel but isn’t replaced by precipitation.
What happens at the end of a dry season of the water balance?
There’s a definite of water in the ground.
When are the ground stores recharged in the water balance?
In the next wet season.
Define river discharge.
The volume of water flowing in a river.
Define peak discharge in relation to hydrographs.
The highest point on the graph, when the river discharge is at its greatest.
Define lag time in relation to hydrographs.
The delay between peak rainfall and peak discharge.
Define rising limb in relation to hydrographs.
Part of the graph up to peak discharge, river discharge increases as rainwater flows into the river.
Define falling limb in relation to hydrographs.
Part of the graph after peak discharge. Discharge is decreasing because less water is flowing into the river.
Name the seven physical factors that affect river discharge.
1) Drainage basin characteristics
2) The amount of water already present
3) Rock type
4) Soil type
5) Vegetation
6) Precipitation
7) Temperature
Name the human activities can affect the hydrograph.
1) Urbanisation
2) Man-made drainage systems
Explain how headward erosion affects a river.
Headward erosion makes the river longer. It happens near the river’s source as throughflow and surface runoff cause erosion at the point the water enters the river channel.
Explain how vertical erosion affects the river.
Vertical erosion deepens river channel it happens in the upper stages of a river.
Explain how lateral erosion affects the river.
Lateral erosion makes the river wider happens in the middle and lower stages of a river.
Name the five main ways in which river erosion happens.
1) Hydraulic action - force of water
2) Abrasion - rocks hitting the banks
3) Attrition - rocks hitting together
4) Cavitation - air bubbles implode causing shockwaves that break rock off bed and banks
5) Corrosion - chemical processes
Name the four transportation processes.
1) Traction - rolling
2) Saltation - bouncing
3) Suspension - carried
4) Solution - dissolved
Define deposition.
The process of dropping eroded material.
Why does deposition occur?
It occurs when the river loses energy, dropping some of its load.
Name five ways in which the speed and energy of a river can be reduced.
1) Reduced rainfall
2) Increased evaporation (or abstraction - taking water out of a river for human use)
3) Friction
4) When the river is forced to slow down (e.g. before a narrowing section)
5) When the river meets the sea
What does the Hjulström Curve show the link between?
River velocity and competence.
What does the long profile show?
The river’s gradient from source to mouth.
What’s the base level of a river?
The lowest point the river can erode to, usually sea level.
What are the river’s three stages?
Upper, middle and lower
In the upper stage the gradient is _____ and the river is _____ above sea level, which gives it lots of _____ energy.
In the upper stage the gradient is steep and the river is high above sea level, which gives it lots of potential energy.
As the gradient _____ towards the _____ stage, potential energy is converted to _____ energy - the river ____ velocity.
As the gradient decreases towards the middle stage, potential energy is converted to kinetic energy - the river gains velocity.
In the lower stage, the river has _____ potential energy but lots of _____ energy - it’s flows _____.
In the lower stage, the river has little potential energy but lots of kinetic energy - it’s flows faster.
Does velocity increase or decrease downstream?
Increase.
Efficiency is measured by what?
Hydraulic radius.
Hydraulic radius = ? / ?
Hydraulic radius = channel’s cross sectional area / wetted perimeter.
How does friction effect river efficiency?
It increases energy loss and slows the river down.
A ______ hydraulic radius means a smaller proportion of water is in contact with the wetted perimeter. So friction is ______ which reduces energy loss, increasing ______ and ______.
A larger hydraulic radius means a smaller proportion of water is in contact with the wetted perimeter. So friction is lower which reduces energy loss, increasing velocity and discharge.
How does channel roughness effect efficiency?
It reduces it.
Where is channel roughness greatest? (In relation to the stages of the long profile)
In the upper stages.
Explain erosion in the upper course.
Mainly vertical and through hydraulic action.
Explain transportation in the upper course.
Mainly large particles (boulders) carried by traction or saltation.
Explain deposition in the upper course.
Little deposition - mainly largest particles dropped when energy levels drop.
Explain erosion in the middle course.
Vertical and lateral by mostly abrasion.
Explain transportation in the middle course.
Particle size is decreasing down the course so more material carried by suspension. Some larger particles moved by saltation.
Explain deposition in the middle course.
Sand and gravel deposited across the flood plain.
Explain erosion in the lower course.
Lateral erosion (formation of meanders)
Explain transportation in the lower course.
Mostly particles are carried by suspension and solution.
Explain deposition in the lower course.
Smaller particles are deposited on the floodplain.
Cross profile of upper course?
V
Cross profile of middle course?
\ /
U
Cross profile for lower course?
__ __
u
Explain potholes.
Potholes are small circular hollows in the river bed. They are formed by abrasion as turbulence swirls the river’s bed load round in a circular motion, cause it to rub and scrape out holes.
Explain rapids.
Rapids are relatively steep sections of the river with turbulent flow where there are several sections of hard rock. They’re a bit like mini waterfalls.
How are meanders formed?
Meanders form where riffles and pools develop and equally spaced intervals. The river flows efficiently over pools and has friction over riffles. This causes the river’s flow to be uneven and maximum flow to be concentrated to one side of the river. Turbulence increases around pools and speeds up the flow in a corkscrew currents called helicoidal flow. Helicoidal flow causes more erosion and deepens pools, leaving more material deposited on the opposite side too.
Name the four landforms formed from deposition.
Braids, levees, deltas and flood plains.
Explain braiding.
Braiding occurs when rivers are carrying vast amounts of eroded sediment. When velocity drops this material is deposited in the channel forcing the river to divide into many small, winding channels that eventually rejoin to form a single channel.
Explain levees.
Levees are natural, raises embankments formed from the river flooding. When the river overflows material is deposited across the whole floodplain dropping the heaviest material first.
Explain deltas.
When a river reaches the sea the energy of the river is absorbed by the slower moving water of the sea. This caused the river to deposited it’s load and these deposits built up on the sea bed until it rises above sea level, partially blocking the mouth of the river. The river has to braid into several distributaries in order to reach the sea, forming a delta.
What are the physical factors that increase the risk of flooding?
Prolonged rainfall Heavy rainfall Steep slopes High drainage density Circular drainage basin Impermeable ground Low infiltration or intercepting trees
What are the human factors that increase the risk of flooding?
Urbanisation
Deforestation
Flood management strategies
Agriculture
How can flood frequency be calculated?
By keeping records of the magnitude (size) of floods and frequency (how often) floods occur we can predict return periods. Large floods happen less often.
Explain rejuvenation.
A river is said to be rejuvenated when the base level that it is flowing down to is lowered (eustatic fall or isostatic rise).
Landforms formed from rejuvenation?
Incised meanders
River terraces
Knickpoint
Explain incised meanders.
Ingrown meanders result from slow rejuvenation and steady downcutting. Entrenched meanders result from rapid incision.
Explain river terraces.
As the river incises and cuts down to its new level the former floodplain is left suspended above. (River Thames)
Explain knickpoints.
A waterfall in the lower course that works its way back up the long profile as the river regrades itself to its new level.
