River Processes Flashcards
effect of headward erosion
makes the river longer.
happens near a rivers source as throughflow and surface runoff causes erosion at the point the water enters the river channel (the valley head)
effect of vertical erosion
deepens river channels
happens in the upper stages of a river
effect of lateral erosion
makes the river wider
happens in the middle and lower stages of a river
5 ways in which erosion can occur
hydraulic action abrasion/corrasion attrition cavitation corrosion/solution
what is hydraulic action
pressure of water breaks rock particles away from the bed and banks - its strongest in rapids and waterfalls and during floods
what is abrasion
eroded pieces of rock in the water scrape and rub against the bed and banks, removing material. most erosion of river beds and banks happens by abrasion
what is attrition
eroded rocks smash into each other and break into smaller fragments
their edges also get rounded off as they rub together
attrition doesn’t erode the bed and banks - it just makes the particles of rock in the river smaller and more rounded
what is cavitation
air bubbles in turbulent stretches of water implode causing shockwaves that break pieces of rock off the banks and bed
what is corrosion/solution
the dissolving of rocks by chemical processes
carbon dioxide dissolves in water to form a weak acid which reacts with rocks like limestone and chalk, breaking them down
4 types of transportation
solution
suspension
saltation
traction
what is solution
substances that dissolve are carried along in the water - e.g limestone is dissolved in water thats slightly acidic
what is suspension
very fine material, like silt/clay particles, is whipped up by turbulence (erratic swirling of water) and carried along in the water. most eroded material is transported this way
what is saltation
larger particles, like pebbles/gravel, are too heavy to be carried in suspension. instead the force of the water causes them to bounce along the river bed
what is traction
very large particles, e.g boulders, are pushed along the river bed by the force of the water
material transported by traction/saltation is called
the rivers bedload
how can the speed and energy of a river be reduced (5)
- reduced rainfall causes lower discharge, which means the river slows down and has less energy
- increased evaporation or abstraction also causes lower discharge
- friction e.g in shallow areas close to the banks, reduces speed of river, reducing energy
- when the river is forced to slow down, e.g before a narrow section of the channel, it loses energy
- a lot of energy is lost when the river meets the sea (the sea absorbs the energy)
what is the capacity of a river
the total load (measured in volume, weight or mass) that a river can transport at a given point
the load of a river can be divided into diff categories according to particle size - sizes range from fine silt/clay (less than 0.1mm diameter) to big boulders
what does the competence describe
the maximum particle size that a river is capable of transporting at a given point
what does the Hjulstrom curve show
relationship between river velocity and competence
also shows how processes of erosion, deposition and transportation vary with river velocity
what is the critical erosion velocity curve
shows the minimum velocity needed for the river to put up (erode) and transport particles of diff sizes
it takes a higher velocity to erode material than it does to just transport material
what does mean settling velocity curve show
the velocities at which particles of diff sizes are deposited. i.e it shows the competence of the river at diff velocities
