L2 - Long and cross profiles

• a long profile shows the gradient of a river as it journeys from source to mouth
• a river will try to achieve a smooth curve in order to reach its 'base level', which is basically sea level, this is called the graded profile
• the long profile can be split into 3 sections - the upper course, the middle course and the lower course
• discharge is the volume of water passing through a given point on a river at a given time, it is measured in cubic metres per second (cumecs)
• discharge = velocity x cross sectional area
• discharge increases with distance downstream because tributaries join, bringing more water and add to the volume of water in the main river
• the average velocity of a river increases with distance downstream
• velocity is determined by several factors: gradient is one but the most important is the amount of water in contact with the bed and banks of the river channel - the more there is , the more friction and the slower it will go, therefore in the upper course there is less water so more of it will be in contact with the banks and bed and the slower it will be
• as you move downstream, more water is added and the channel gets wider and deeper, there will be less water in contact with the bed and banks and therefore the river will speed up with distance downstream
• a cross profile is a section from the side across a river channel and/ or valley
• a channel cross profile only includes the river itself
• a valley cross profile includes the channel, the valley floor and the slopes up the side of the valley
• watershed

  boundary between two drainage basins
• source

  start of the river
• tributary

  a small stream joining the main river
• drainage basin

  area drained by river
• confluence

  point at which tributary joins the river
• the upper course:

  • mostly vertical erosion
  • steep sided
  • interlocking spurs (not meanders)
  • rapids and waterfalls
  • v-shaped valleys
  • slower moving water due to friction (small discharge)
  • large material (not eroded, large angular rocks)
• meander

  a bend in the river
• ox bow lake

  a cut -off meander
• the middle course:

  • flatter
  • wider due to tributaries joining
  • smaller and smoother sediment
  • lateral erosion taking over
  • transportation + erosion
  • fairly high velocity
  • quite high discharge
• floodplain

  flat, fertile land next to the river
• mouth

  end of the river
• delta

  material deposited
• the lower course:

  • very flat
  • very wide
  • fine material (sand, smooth river bed)
  • a large load
  • more deposition
  • faster flowing (high velocity)
  • high discharge
• in the upper course, the river travels through mountains
• in the middle course, the river travels through hills
• in the lower course the river travels through a very large area of low-lying land
• upper course erosion:

  mostly vertical erosion by hydraulic action
• middle course erosion:

  less vertical erosion, more lateral erosion, lots of attrition and abrasion, some solution
• lower course erosion:

  very little erosion, only lateral erosion
• upper course transport:

  mostly traction, large boulders moved
• middle course transport:

  mostly suspension, increased traction, load becomes smaller and less angular
• lower course transport:

  mostly suspension and solution, very small particles of load
• upper course deposition:

  large boulders deposited
• middle course deposition:

  more deposition, especially on the inside bend of meanders
• lower course deposition:

  deposition is now the main fluvial process, fine material is now deposited
• in the upper course, the river erodes its bed by hydraulic and abrasive action, as the river travels downstream, it is joined by a number of tributaries which increase the volume of water and give the river more kinetic energy, a higher velocity and more erosive power, so the channel gets much deeper
• the channel becomes wider downstream because as the gradient becomes less steep, there is less vertical erosion, by the time the river is in the middle course, lateral erosion is dominant which erodes the riverbanks which makes the channel wider
• in the upper course, there is a steep v-shaped cross-profile, the vertical erosion of the river creates a slope that weathered material from the valley sides can fall down, on reaching the river, this material is removed. the rock is harder which means the valley sides are not broadened out much by weathering and so slopes remain steep