L3 - Fluvial processes

• a river carries water from the land to the sea, the journey it takes is the long profile
• fluvial processes are the processes the river goes through along its journey including: erosion, transportation and deposition
• erosion

  the wearing away of the land
• the ability of the river to erode depends on its velocity
• erosion will occur in the whole of the long profile (upper, middle and lower courses), but there are different types
• vertical erosion

  eroding downwards on the river bed (this will generally happen in the upper course). it happens because of gravity and the river trying to reach base level as quickly as possible
• lateral erosion

  eroding sideways on the river banks and happens in the middle and lower courses of a river
• there are 4 types of erosion:

  • hydraulic action
  • abrasion
  • solution
  • attrition
• load

  the sediment in the river. fluvial transport is the process by which rivers transport this load, and the load of a river will differ in size and shape
• in the upper course, where there has been little erosion, the load will be larger - sometimes even boulders that are angular
• in the middle and lower courses, the load will become smaller and rounder in size due to erosion
• load comes from weathered material from the sides of the valley and also from the rivers bed and banks
• the amount of sediment carried will depend on:

  • the volume of the water - the greater the volume, the more load it can carry
  • the velocity - a fast flowing river has more energy to transport and can move larger particles
  • the local rock types - some rocks are more easily eroded than others
• the total amount of load in the river increases with distance downstream
• solution

  minerals are dissolved in the water. this is a chemical change affecting rocks such as limestone and chalk. load transported this way is called solute load
• suspension

  fine, light material (such as alluvium) is held up and carried within the rivers flow. this is called suspended load.
• saltation

  small pebbles and stones are bounced along the river bed. the load is alternately lifted and then dropped in line with a local rise and fall in the velocity of the water
• traction

  large boulders and rocks are rolled along the river bed, load carried in this way is called bedload
• deposition

  the process by which a river drops its load
• sediment

  the material deposited by a river
• the bigger the particle, the more energy (in the form of velocity) is needed to keep it moving. when velocity falls, the largest particles will be dropped first and the finest particles will be deposited last
• along the river profile, whenever velocity falls, there will be deposition, such as at the base of a waterfall, or the inside bend of a meander, or where the river enters a sea or lake
• more sediment is deposited in times of drought when discharge is low (and therefore velocity is low due to more friction with the bed and banks)
• velocity increases as discharge rises, and this enables a river to pick up larger particles
• as velocity and discharge decrease, particles are generally deposited according to their size, with the largest size being deposited first
• it takes less energy to keep particles moving than it does to pick them up
• abrasion

  • responsible for both vertical and lateral erosion
  • small boulders and stones scratch and scrape their way down a river during transport and wear away the rivers bed and banks
  • stones that have fallen into the river after being broken off will be sharp and angular and will increase the process of abrasion
  • this process is most effective in short, turbulent periods, when the river is at bank full or in flood
• hydraulic action

  • caused by the sheer power of fast-flowing moving water
  • water hits the river bed and banks and forces both air and water into cracks, this compresses air in the cracks
  • the repeated changes in air pressure weakens the channel, leading to erosion of the bed and banks
  • hydraulic action is responsible for vertical erosion in the upper course
  • in the lower course it contributes to lateral erosion of the banks, especially on the outside bend of meanders
  • it can be locally strong within rapids or below waterfalls
• solution

  • most active on rocks that contain carbonates, such as limestone and chalk
  • the minerals in the rock are dissolved by weak acids in the river water and carried away in solution
• attrition

  • the reduction of particle size within a river due to them colliding and knocking against other particles
  • they hit into each other as well as the bank and bed
  • as they move downstream, they become smoother, smaller and more rounded
  • consequently, larger, more angular particles / rocks tend to be found upstream