River Characteristics

Characteristics of river profiles

Characteristics of river profiles

River characteristics

• All rivers have long and cross-profiles

• Each river's long and cross profiles are unique, but they do have some characteristics in common

• These profiles show changes in river characteristics from the source to the mouth

Long profile

• The long profile of a river shows the changes in the river gradient from the source to the mouth

• Most long profiles have a concave shape with similar characteristics:

  • The source is usually in an upland area

  • The upper course of the river includes steep areas with uneven surfaces

  • In the middle course, the gradient decreases

  • In the lower section, the gradient decreases further until it becomes almost flat

Cross profiles

• The cross profiles of a river are cross-sections from one bank to another

• Cross profiles of the upper, middle and lower courses show the changes in the river channel 

• Upper course characteristics tend to include:

  • shallow

  • steep valley sides 

  • narrow

  • low velocity

  • large bedload

  • rough channel bed

  • high levels of friction

  • vertical erosion

• Middle course characteristics tend to be

  • deeper than upper course channel

  • gentle valley sides

  • wider than upper course channel

  • greater velocity than upper course channel

  • material in river decreases in size

  • smoother channel bed

  • lower levels of friction than upper course channel

  • lateral erosion

• Lower course characteristics tend to be

  • deeper than middle course channel

  • flat floodplains 

  • wider than middle course channel

  • greater velocity than the middle course channel (apart from as the river enters the mouth) 

  • material carried mainly sediment and alluvium

  • smooth channel bed

  • lowest friction

  • deposition is dominant

The Bradshaw Model

The Bradshaw Model

• As a result of erosion, transportation and deposition, the character of a river changes as it moves downstream as shown in the cross-profiles

• These changes are summarised in the Bradshaw model

• Bradshaw's model shows that the following characteristics increase with distance downstream (away from the source):

  • discharge

  • width

  • depth

  • velocity

  • load quantity

• Other characteristics decrease:

  • load size

  • bed roughness

  • gradient (slope angle)

Wetted perimeter and hydraulic radius

• The wetted perimeter is the length of the cross-sectional area where the water is in contact with the banks and bed

• The efficiency of the river is measured using hydraulic radius (cross-sectional area divided by wetted perimeter)

• The higher the ratio the greater efficiency and less friction

  • This increases the amount of erosion, which means the channel becomes wider and deeper

River processes

River processes

Erosion

• Erosion is the wearing down of surfaces 

• There are four erosion processes that change the shape of the river channel:

  • Hydraulic action

    • The force of the water, which removes material from the bed and banks of the river

  • Abrasion

    • When the materials carried by the river scrape away the banks and bed

  • Attrition

    • When the material being carried by the river hits each other, the pieces become rounder and smaller

  • Corrosion (solution)

    • When rocks are dissolved in slightly acidic water

• Erosion can be mainly vertical or lateral: 

  • Vertical erosion is dominant in the upper course of rivers. It increases the depth of the river and valley as the river erodes downwards

  • Lateral erosion is dominant in the middle and lower courses of rivers. It increases the width of the river and valley as it erodes sideways

Transportation

• There are four processes of transportation:

  • Traction

    • Occurs when larger rocks and materials are rolled along the riverbed

  • Saltation

    • Smaller material, which can be lifted by the water, bounces along the riverbed

  • Suspension

    • Lighter material carried within the river flow

  • Solution

    • When materials are dissolved in the river water

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Deposition 

• When a river does not have enough energy to carry materials, it drops them

  • This is deposition

• The causes of reduced energy include

  • Reduced discharge due to a lack of precipitation or abstraction upstream

  • Decreased gradient

  • Slower flow on the inside of a river bend or where the river is shallower 

  • When the river enters a sea, ocean or lake

• The heaviest material is deposited first; this is known as the bedload

• The lighter materials, gravel, sand and silt, are known as alluvium and they are carried further downstream

• The dissolved materials are carried out to sea