Landforms

How are waterfalls formed?

• Drop in the river bed from one level to another due to changes in the hardness of the rock, where hard rock overlies soft rock

• Hydraulic action and abrasion are the main erosional processes:

• The soft rock erodes quicker, undercutting the hard rock - creating a plunge pool

• leads to the development of an overhang of hard rock which eventually collapses

• overhang falls into the plunge pool increasing abrasion and making the plunge pool deeper

• process then repeats and the waterfall retreats upstream leaving a steep-sided gorge

What is a V-shaped valley, and how does it form?

    - Narrow, steep-sided valley in the upper course.

Formed by:

    - Vertical erosion is dominant in the upper course of the river

• This cuts down into the river bed and deepens the river channel 

• Weathering and mass movement lead to material from the valley sides collapsing into the river, forming a steep v-shaped valley

What are interlocking spurs?

    In lowland areas, lateral erosion is dominant

• Meanders increase in size

• The fastest water flow (thalweg) is on the outside of the river bends, leading to erosion:

  • The erosion undercuts the riverbank, forming a river cliff

  • The riverbank collapses and the edge of the meander moves further out

• The slowest flow is on the inside of the river bends, leading to deposition:

  • The deposits form a slip-off slope

• Deposition on one side and erosion on the other leads to the meander migrating across the valley

Formation of oxbow lakes

• With distance downstream, the size of the meanders increases

• The erosion on outside bends can eventually lead to the formation of a meander neck

• At a time of the flood, the river may cut through the neck of the meander, forming a straighter course for the water

• The flow of water at entry and exit from the meander will be slower, leading to deposition

• The meander becomes cut off from the main river channel, forming an oxbow lake

Describe floodplains and levees.

    - Floodplains: Flat land formed by meander migration.

    - Levees: Natural embankments formed by deposition.

What factors influence delta formation?

    - Sediment load, drop in river velocity, flocculation.

    - Bioconstruction (vegetation slowing water).