The Living World
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Shaping River Landscapes

Erosion

Erosion is the process through which rivers wear away the riverbed and banks, which gradually reshapes the landscape. Different types of erosion contribute to this transformative force:

  • Hydraulic Action: The forceful impact of the water collides with the riverbanks. As air becomes trapped in the cracks of the riverbank and bed, the rocks break apart.
  • Abrasion: The rocks the river carries rub against the riverbed and banks, causing them to wear away.
  • Attrition: As the river carries rocks along, they crash into each other and gradually break down into smaller, smoother pieces.
  • Solution: Over time, water can dissolve specific types of rocks, like limestone, which leads to their gradual erosion.

Transportation

Rivers pick up and transport various sediments downstream, using different transportation methods based on the sediment’s size and weight:

  • Suspension: Lighter sediment is suspended within the water and transported by the river’s flow. This is often seen near the mouth of the river.
  • Saltation: Saltation occurs when pebbles bounce along the river bed, usually near the source.
  • Traction: The traction process involves rolling big and heavy pebbles on the river bed. This is usually seen near the river’s origin, where the load is heavier.
  • Solution: Dissolved chemicals are moved by the water, and the extent varies depending on the presence of soluble rocks.

Deposition

Deposition occurs when a river loses its energy and releases the sediment it has been carrying. Multiple factors contribute to this process:

Shallow Water: When a river becomes shallower, it can no longer carry sediment effectively. This leads to the buildup of sediment in the riverbed.

River’s Mouth: At the end of its journey, where the river meets larger bodies of water, the reduced energy often leads to deposition.

Decreased Volume: If a river’s water volume decreases, it can’t carry sediment as effectively.

Changing River Shape

Rivers are nearly constantly changing aspects of our geography. Their shape is always evolving, and they leave evidence of their development carved into the land.

A River’s long profile

The long profile of a river refers to the changes in elevation along its course, from its source to its mouth.

1. Rivers start in highland areas, at their source and flow swiftly downhill. At the start of the river’s profile, we are in the upper course. In the upper course, the river carries large amounts of rock, called the load. The rocks and material in this load are yet to be broken down by erosion, so the rock creates a great deal of turbulence, and a rougher path for the river. 

2. As rivers move downstream, towards their lower course, their slope becomes gentler, and they meander through flatter landscapes. Erosion has broken down the material in the river, and the load is now a fine sediment. 

3. Eventually, rivers reach their mouth, where they meet larger bodies of water, such as oceans or lakes.

Changing cross-sections

The cross-section of a river represents its shape and size when viewed from the side.

1. At the river’s source, the cross-section is narrow and shallow, with swift water flowing over rocks and boulders. As the river flows downhill, vertical erosion increases. Vertical Erosion is the erosion that happens as the water is pulled down by gravity, eroding the rock vertically.

2. As more water from tributaries and rainfall contributes to the river’s flow, the cross-section widens downstream. Vertical erosion still happens, but at a lower rate. Instead, lateral erosion becomes the main force of erosion. Lateral erosion is the erosive effects of a river moving sideways. 

3. The cross-section in the middle and lower reaches (closer to the mouth) becomes wider and deeper, allowing for smoother flow and sediment transport. Lateral erosion is now the only erosive effect, though there is a lot less erosion overall.

Erosion and deposition change the river’s cross-section over time, reshaping its banks and bed. During floods, the cross-section may expand as the river overflows its banks.

Landforms Shaped by River Processes

Through the combined forces of erosion, transportation and deposition, rivers create breathtaking landforms that shape our landscapes.

Here are some examples:

  • Waterfalls: Formed when a river encounters a sudden drop in its course, which results in a vertical descent of water.
  • Gorges: Deep and narrow valleys carved by the erosive power of rivers.
  • Meanders: Bends in a river’s course, formed as the river erodes its outer banks and deposits sediment on its inner banks.
  • Oxbow Lakes: Formed when a meander gets cut off from the main river channel, creating a curved lake.
  • Levees: Raised banks formed by the deposition of sediment during floods.
  • Floodplains: Flat areas of land next to rivers and waterways that experience periodic flooding, which often have large sediment deposits.
  • Deltas: Landforms created when a river deposits sediment at its mouth, forming a fan-shaped or triangular landform.

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