River Processes | Geography for GCSE/IGCSE - Year 11 PDF Download

River Processes

• Erosion is the process of wearing down surfaces over time.
• River channels are shaped by four main erosion processes:
  • Hydraulic action: This is the force of the water against the riverbanks and bed, dislodging and eroding material through sheer pressure.
  • Abrasion: This is the process where rocks and stones carried by the river wear down the riverbed and banks as they are dragged along.
  • Attrition: In this process, rocks and stones carried by the river collide and break into smaller pieces, becoming smoother over time.
  • Corrosion (solution): This is the process where certain minerals in rocks are dissolved by the water, leading to erosion.

Erosion can be categorized as mainly vertical or lateral:

• Vertical erosion: This is predominant in the upper course of rivers where the river erodes downwards, deepening the river and valley over time.
• Lateral erosion: This type is more common in the middle and lower courses of rivers, causing the river and valley to widen as it erodes sideways.

Transportation in Rivers

Types of Transportation: There are four primary processes of transportation in rivers:

• Traction: This process involves the rolling of large particles along the riverbed.
• Saltation: Here, smaller particles are bounced along the riverbed in a leapfrogging motion.
• Suspension: Fine particles are carried within the flow of the river without settling to the riverbed.
• Solution: Dissolved materials are transported within the body of the river.

Deposition in River Systems

• When a river lacks the energy to transport materials, it deposits them. This process is known as deposition.
• The primary reasons for reduced energy in rivers leading to deposition include:
  • Decreased discharge caused by factors like low precipitation or water abstraction upstream.
  • Reduced gradient along the river's course.
  • Slower flow, particularly on the inner side of river bends or where the river is shallower.
  • When the river reaches a sea, ocean, or lake.
• Heavier materials are typically deposited first, constituting the bedload.
• Lighter materials such as gravel, sand, and silt, termed as alluvium, are carried further downstream.
• Dissolved materials are transported out to the sea.

Changes in River Characteristics

• As a river undergoes erosion, transportation, and deposition, its characteristics evolve as it progresses downstream.
• These alterations are succinctly described in the Bradshaw model.

River characteristics

• Every river possesses longitudinal and transverse profiles.
• While each river's profiles are distinct, they share some common traits.
• These profiles depict alterations in river attributes as they progress from their origin to the outlet.

Long Profile

• The long profile of a river illustrates the variations in gradient from its origin to its terminus.
• Typically, long profiles exhibit a concave shape and share several common features:
  • The source commonly originates in elevated regions.
  • The upper reaches of the river often encompass steep terrain with irregular surfaces.
  • As the river progresses into its middle course, the gradient tends to diminish.
  • In the lower section, the gradient further reduces, eventually reaching near-flat levels.

Cross profiles

• River cross profiles are representations of cross-sections from one riverbank to the other.
• Cross profiles across the upper, middle, and lower courses depict alterations in the river channel.
• Upper course attributes consist of:
  • Shallow depth
  • Steep valley sides
  • Narrow width
  • Low velocity
  • High bedload
  • Irregular channel bed
  • Elevated friction levels
  • Vertical erosion

River Course Characteristics

• 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:

• Deeper than middle course channel
• Flat floodplains
• Wider than middle course channel
• Greater velocity than the middle course channel (except as the river nears the mouth)
• Material carried mainly sediment and alluvium
• Smooth channel bed
• Lowest friction
• Deposition is dominant