The Long Profile
A long profile shows how the gradient of the river channel changes from the river's source to its mouth. It does this by showing the height of the river bed above base level (the lowest point the river can erode to) for the whole length of the river.
The total amount of erosion and deposition along the full course of a river are balanced, but the rates of erosion and deposition change along its course. This results in landforms such as waterfalls, making the profile uneven.
Upper course: steep gradient, high above sea level, lots of potential energy
Middle course: gradient decreases, potential energy converted to kinetic energy
Lower course: little potential energy, lots of kinetic energy
Channel Characteristics/Velocity and Discharge
Efficiency is measured by hydraulic radius. The larger its HR the more efficient a river is.
- HR is the channel's cross-section area divided by the length of its wetted perimeter.
- Contact between the water and the WP creates friction - increases energy loss, slows river
- A large HR means a smaller proportion of water is in contact with the WP - friction is lower, reducing energy loss - increasing velocity and discharge
- Smooth, narrow, deep channels have a larger HR - more efficient
As channel roughness increases, as does turbulence, causing greater erosion.
Channel roughness is greatest in the upper stages of the river, and lowest in the lower stages.
River Processes - Erosion
Upper course: vertical erosion, abrasion, hydraulic action, occurs during high-energy conditions e.g. high discharge & velocity, rough channel causing turbulence and the large angular bedload causing vertical erosion
Middle course: lateral, abrasion, attrition of larger particles - particle size decreases from source to mouth
Lower course: less erosion due to low turbulence and reduced sediment particle size, abrasion reduced, lateral erosion
River Processes - Transportation and Deposition
Upper Course: traction - boulders, saltation, occurs during high-energy conditions
Middle Course: most material carried in suspension, some moved by saltation
Lower Course: mostly suspension - silt and clay, solution
Upper Course: little deposition
Middle Course: sand and gravel, accross the floodplain
Lower Course: smaller particles deposited on floodplain and river mouth
The Cross Profile
The cross profile of a river shows you what a cross-section of the river channel or valley looks like.
Upper course valley: steep V shape, narrow valley floor, steep-slope sides
Middle course valley: wider, gently sloping sides, deposition creating flood plain on valley floor
Lower course valley: wide, very slightly sloping sides, wide flood plain