Rivers part 3

• Waterfalls

  Waterfalls typically form in the upper stages of a river. They occur where a band of hard rock overlies a softer rock. Falling water and rock particles erode the soft rock below the waterfall, creating a plunge pool.

  The softer rock is undercut by erosional processes such as hydraulic action and abrasion creating a plunge pool where water and debris swirl around eroding the rock further deepening it and creating an overhang. The hard rock overhang above the plunge pool collapses as its weight is no longer supported. Erosion continues and the water retreats upstream leaving behind a gorge
• Gorge


  A steep sided valley cut by a waterfall
• Plunge pool


  A pool at the base of a waterfall formed by erosion
• What is a meander?

  A meander is a bend in the river that is created by lateral erosion and deposition
• How does a meander form?

  As water is flowing down a river, lateral erosion will be occurring

  At times when the water is flowing at a higher velocity and rocks are softer, more erosion will occur-creating the outside bend and river cliff

  Where the water has a low velocity and rocks are harder more deposition will occur-creating an inside bend and slip-off slopes
• What is deposition


  rock which has been eroded doesn't get picked up by river since the river has less energy
• The inner bend of a meander will be:

  Shallower

  have deposition

  a slip-off slope and the water will be moving at a low velocity
• The outer bend of a meander will be:

  deeper

  have erosion

  a river cliff and the water will be moving at a high velocity
• What is an oxbow lake


  an oxbow lake forms when the neck of a meander is broken through so you cant have an oxbow lake without a meander
• The formation of an oxbow lake

  1) Corrasion and hydraulic action erode outer bends to form a steep bank to the channel called an undercut river cliff

  2) An underwater current carries eroded material to the inside of the bend where flow of water is slower

  3) Material is deposited to form a slip off slope (gentle bank)

  4) As lateral erosion continues, bend of meander becomes more pronounced. Neck of meander narrows

  5) Eventually usually in times of flood, narrow neck may be broken and crossed so the river flows straight again

  6) Meander sealed off by deposition to form ox bow lake
• define the term floodplains


  an area of low-lying ground next to a river, formed mainly of river sediments that is likely to flood
• what are floodplains made of

  Made of alluvium. This is sediment deposited y a river when it floods
• Why are floodplains often used for farming

  the soils are very fertile
• explain the two processes responsible for the formation of a floodplain

  migration of a meander: when the meander reaches the edge of a floodplain they erode the valley side which is why floodplains are so large

  formation of levees: when a river floods, it deposits silt. Over the years this builds up forming a thick deposit of fertile alluvium
• Define the term levee

  A build up of material on the river bank, deepening the river over-time caused by flooding
• Explain the formation of a levee

  When a river bursts its banks (floods) friction with the land reduces velocity which causes deposition. Heavy sediment that has been transported from upstream is deposited closest to the river and the smaller sediments are further. With each flood the banks are built up higher. This high ridge of land is called a levee
• Define the term estuary

  A body of water found where freshwater from rivers and streams mixes with saltwater from the ocean
• What two factors effect estuaries

  wave action

  river processes
• When is deposition occurring in estuaries


  When the sea retreats the amount of water in the estuary is reduced
• What forms a s a result of deposition and why are these important


  The river deposits fine silt to form mudflats which are an important habitat for wildlife
• What can mudflats eventually develop into

  saltmarshes
• example of an estuary

  Severn Estuary