Lecture 18

Question 1

Downstream variation in sediment

Answer 1

• in general, faster flow moves larger particles
• but rapid mean velocity is more effective if depth is shallow
• net effect: large particles are generally deposited near source, smaller particles further downstream
• suspended load is deposited where flow slows or stops
• dissolved load typically carried to sea, deposited as non-clastic sediment

Question 2

River channel patterns

Answer 2

Most common:

• braided
• meandering

Question 3

Braided channels

Answer 3

• multiple channels often separated by bars (often silt or clay)
• channels and bars change position and size as the stream erodes and deposits sediment
• typically form where:
  
  • discharge is variable
  • banks are easily eroded
• common in glacier fed rivers

Question 4

Meandering channels

Answer 4

• large sinuous loops-each bend is a meander
• most common in fine gained alluvium, low gradient channels
• deposition on inner bank forms point bar
• erosion on outer bank causes meanders to migrate downstream over time
• meander cut off May form an oxbow lake

Question 5

Meander cut offs result in

Answer 5

Oxbow lakes

Steeper gradients

Question 7

Flood plains continued

Answer 7

• most meandering rivers flood at times of high discharge
• water spreads over flood plain
• fine grained sediment is deposited from flood water
• natural levees form at point where water first spills from channel

Question 8

Terraces

Answer 8

• many rivers alternate between downcutting and lateral erosion
• lateral erosion occurs by meander migration
• downcutting may occur if:
  
  • sea level falls
  • tectonism removes an obstacle
  • discharge increases
• down cutting may leave old flood plains as terraces (e.g. North sask river)

Question 9

Deposition all landforms

Answer 9

1. Flood plains
2. Alluvial fan
   - fan shaped body of alluvium at base of upland area
3. Delta
   - triangular shapes deposit formed when a stream enters the standing water of a sea or lake (mostly clay and silt sediment)

Question 10

Flooding

Answer 10

• occurs when streams discharge exceeds capacity of channel

- major floods infrequent but can be devastating and catastrophic

Question 11

Flooding continued

Answer 11

• peak discharge comes after rains that produce it

- after rainfall, surface runoff moves into stream channels, increasing discharge rapidly

Question 12

Steps in flooding

Answer 12

A. Rainfall begins (0 hours): the peak discharge is delayed as the runoff collects and runs down the stream channel

B. 1 hour: stream can still contain the increased volume

C. 2 hours: stream reaches its peak flow and cannot be contained by its banks anymore

Question 17

Flood plains

Answer 17

• natural levees form at point where water first spills from a channel
• thickest and coarsest sediments deposited at channel edges
• thin and fine sediments deposited over outer parts of floodplain
• natural levees become built up by many floods