11.6 Meanders and oxbow lakes

Question 1

Why is there more water in the middle course

Answer 1

There’s more water because tributaries have joined and because more water has arrived through throughflow and surface run-off

Question 2

Describe the characteristics of a meander

Answer 2

• on the indies de there is a curbed beach like feature called a slip off slope
• there is sand, gravel and pebbles
• the river is slow flowing
• on the outside of the bend there is a river cliff
• sometimes this is steep and a few metres high
• the water is fast flowing

Question 3

How does a meander form

Answer 3

• As a river goes around a bend, most of the water is pushed towards the outside. This causes increased speed and therefore increased erosion (through hydraulic action and abrasion)
• The lateral erosion on the outside bend causes undercutting of the bank to form a
  river cliff
• Water on the inner bend is slower, causing the water to slow down and deposit the eroded material, creating a gentle slope of sand and shingle.
• The build-up of deposited sediment is known as a slip-off slope

Question 4

Outline the formation of a meander and oxbow lake (4-6 marks)

Answer 4

• A meander is a winding curve or bend in a river. Water flows faster on the outer curve and so it erodes the river bank. Material is deposited on the inner curve as the water in the river has less energy.
• Lateral erosion results in the undercutting of the river bank and the formation of a steep sided river cliff.
• On the inside of the bend, where the river flow is slower and water is shallower, material is deposited on a slip off slope, as there is more friction.
• Over time, because of erosion and deposition, meanders gradually change shape and move across the floodplain and migrate downstream.
• The neck of the meander narrows due to later erosion over time. This is known as a swans neck meander. During flood or storm conditions when the river’s energy is high, the river current cuts through the meander neck and the river takes a new shorter course, bypassing the meander loop
• Deposition occurs at the new straight section, cutting off the meander loop.
• An oxbow lake is formed which is separate from the main river.
• This will dry up over time and be filled in with silt (alluvium). It will create fertile marshland, leaving a ‘scar’ on the floodplain

Question 5

What is alluvium

Answer 5

The fine silt that is deposited from the river when it floods onto the plain. It contains minerals and nutrients to fertilise the land