River formations

Question 1

Waterfalls

Answer 1

• A waterfall is a steep section of of the channel where the water falls freely.
• A waterfall forms when a river, after flowing over hard rock meets a band of soft rock.
• As the river meets the edge of a plateau, the velocity increases because the water in front of it lose contact with the bed.
• The water flows over the hard rock and the underlying soft rock is rubbed away easily.
• Overtime, the rock is under cut to form an overhang, which eventually collapses due to no support.
• The collapsed rock at the foot of the falls is swirled around by turbulence to form a plunge pool.
• The main erosional processes at work are: Hydraulic action, abrasion and to lesser extent attrition
  
  • The process is repeated and the waterfall retreats upstream to leave a steep sided gorge.

Question 2

Floodplains

Answer 2

• When a river overflows its banks, it will spread out across the surrounding flat land.
• There is a sudden increase in the wetted perimeter, and the hydraulic radius. This leads to an increase in friction and a corresponding decrease in velocity, resulting in the deposition of materials held in suspension.
• As a river floods it deposits a thin layer of silt which gradually builds up the height of the flood plain (with successive flooding). The land is very fertile.
• The edge of the floodplain is marked by a prominent slope a bluffline.

Question 3

Delta

Answer 3

• When a river flows into an area of slow moving water like a lake or sea, the falling velocity causes the deposition of the river’s load; furthermore when the river reaches the sea, the meeting of fresh and salt water produces an electric charge, which causes clay particles to coagulate.
• This process is called flocculation.
• Deposits are laid on the bed in the 3 fold sequence:
  
  • Bottom set beds, finest material
  • Foreset beds, silts and clay forms slopes
  • Top set beds, coarser deposit
• The river often divides into seperate channels, or distributaries.
  
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Question 4

Types of deltas

Answer 4

• Arcuate: (the Nile Delta)
• Bird’s foot

Question 5

Why are deltas uncommon in the UK?

Answer 5

The large tidal range, and high energy environment prevent deposition.

Question 6

What are the problems/ benefits of Deltas?

Answer 6

• Nubricht rich land
• But can flood, because it is low lying

Question 7

Outline the conditions necessary for Delta formation

Answer 7

Flocculation. Small tidal range, and weak currents. Shallow seated.

Deposition rate must exceed that of the erosion rate.

Question 8

Oxbow Lakes

Answer 8

• As a river meanders downstream it is possible for one side of the meander to catch up with the river channel downstream.
• Over time the sinuosity of the meander becomes so pronounced that during a flood the river cuts through the narrow neck of land to shorten it’s course.
• This temporary straightening means the main current will flow in the middle of the channel.
• Deposition occurs next to the banks.
• Eventually the old curve is abandoned to leave the crescent shaped oxbow lake.
• Overtime the lake gradually silts up to form a marsh area.

Question 9

Pools and Riffles

Answer 9

Pool: an area of deeper water

Riffle: an area of shallower water

• The water is forced to flow around the riffles.
• The riffles direct the water flow towards the zone of max velocity and deepest water on one side of the channel resulting in erosion.
• Erosion occurs on the outside bank and deposition on the inside.
• This movement is made by a corkscrew motion called heliocoidal flow.

Question 10

Meanders

Answer 10

• Water flows fastest on the outside bend, where discharge and velocity are greatest and friction at a minimum.
• This results in erosion which undercuts the bed and banks, creating a steep sided river cliff.
• Water flows slowest on the inside bend, where discharge and velocity are at a minimum and friction at its greatest. This results in deposition forming a point bar. Particles usually graded in size.