River Landforms Flashcards
How do waterfalls form
Waterfalls form when a layer of hard rock lies on a layer of softer rock
As the river flows over the harder rock onto the softer rock it erodes the softer rock faster creating a step in the river bed
As the step develops the drop from the harder to softer rock increase
At the bottom of the drop due to abrasion and hydraulic action, this erodes a plunge pool
Also through abrasion and hydraulic action, the river undercuts the hard rock to form an overhang.
As the undercutting continues. the over hang of hard rock increases until it cannot support it’s weight and collapses into the plunge pool
Constant repetition of this process causes the waterfall to retreat backwards (upstream) and over time a gorge forms
What is a meander
A meander is a bend in the river
How are river cliffs formed in meanders
As the river flows around the meander the water is directed to the outside of the bend. more erosion occurs here therefore the channel is deepened. the water the flows faster and more efficiently in the deeper part of the channel. this causes even more erosion - the sheer force of the fast flowing water (hydraulic action) plus the throwing of the river load against the bank (abrasion) undercuts the river bank. through time as erosion continues a river cliff is formed eventually the river cliff will collapse and the process begins again
How are slip off slopes formed
With water directed towards the outside of the bend, the inside of the river bed has much slower flow. Here the river does not have enough energy to transport the material it is carrying and so begins to deposit it’s load. As this continues, deposited material builds up on the inside of the bend to form a slip off slope.
How are floodplains formed
Rivers flood naturally. When a river overflows it’s banks the water spreads out over the valley floor on either side of the channel. The velocity of the water and therefore the energy, is much less. And so the considerable load the lower course river is carrying can no longer be transported. The material is deposited and over time, the layers of deposited material (silt/alluvium) build up on the wide flat valley floor. This landform is known as a floodplain
How are levees formed.
When the river floods its banks and deposits material, it deposits the heaviest and coarsest materials first. on its banks, then the next heaviest and gradually the finer particles in suspension as the water spreads across the floodplain. the natural sorting of the load means that banks are built up highest by deposition creating natural embankments known as levees.
