The Long Profile, Cross Profile and Channel Characteristics

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

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

Channel Roughness

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.

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

Transportation

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

Deposition

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 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