2.2 - Rivers

Question 1

What is a drainage basin? Describe 7 features of one:

Answer 1

-drainage basin (area that is drained by a river)

-watershed
-source
-tributary (smaller, adjoining river)
-trunk stream (main channel)
-confluence (joining of 2 rivers)
-flood plain (land next to river which floods, made of alluvium)
-mouth

Question 2

What is a watershed?

Answer 2

a ridge that surrounds a drainage basin

Question 3

Describe drainage density and its formula:

Answer 3

DD = total length of all rivers ÷ area of DB

-high DD, high chance of flash floods (water can get into river faster)

Question 4

Drainage basin’s flowchart:

Answer 4

Question 5

What is discharge?

Answer 5

volume of water carried by a river

Question 6

Describe the processes of transportation in a river:

Answer 6

-traction (rolling)
-saltation (bouncing)
-suspension (floating)
-solution (dissolved)

Question 7

Describe the processes of erosion in a river:

Answer 7

-abrasion (wearing away, sandpapering effect)
-attrition (loose rocks impacting each other)
-hydraulic action (force of water impacting bed/banks)
-solution (rocks dissolve in water over time)

Question 8

Describe the courses involved in a river’s journey from source to mouth:

Answer 8

-upper (V valley, waterfalls, rapids, interlocking spurs)
-middle (meanders, floodplains)
-lower (oxbow lakes, floodplains, levees, deltas)

Long profiles are used to portray this information

Question 9

How does a river change as it flows downstream?

Answer 9

(through the upper/middle/lower course)

-wider, deeper, less steep, more sloping sides
-higher discharge, higher sediment load

Question 10

Why does a river’s depth and width usually increase downstream?

Answer 10

-increased discharge (from tributaries joining trunk stream)
-beds and banks are eroded more by increased HA and abrasion

Question 11

How are waterfalls formed?

Answer 11

-less resistant rock (clay) underneath more resistant rock (limestone)
-undercut by erosional process (HA and abrasion)
-overhang develops
-overhang gets too big, collapses into plunge pool (area under waterfall)
-process repeats, waterfall retreats (forms gorge)

Question 12

What are river potholes and how are they formed?

Answer 12

round holes found in a river bed

-sediment scours bed and forms depressions in river bed
-turbulent flow causes pebbles (grinders) inside it to spin and erode the depression deeper by abrasion
-hole largens, larger material can get trapped and support further erosion

Question 13

Explain how a meander/oxbow lake is formed:

Answer 13

-obstruction in river so it flows faster around it
-outside of bend eroded faster, forms river cliff
-inside eroded slower, more deposition, forms slip-off slope

-meander erodes laterally (outside of bend undercuts river cliff), causes collapse and retreat, meander gets bigger
-neck narrows and breaks through, cutting off an oxbow lake
-former meander sealed off by deposition

Question 14

Describe a meander cross section:

Answer 14

-asymmetrical channel
-slip off slope on inside, river cliff on outside
-inside is shallow (more deposition), outside is deepest (more erosion)
-thalweg (fastest river flow)

Question 15

What is a delta?

Answer 15

a depositional landform that is made when the deposition rates (of alluvium) exceeds erosion rates at the mouth of a river where it flows into an ocean/estuary/lake

Question 16

Describe the structural features of a delta:

Not the layers

Answer 16

-a braided channel
-distributaries (river splits up)
-deltaic plain (made of alluvium, clay/silt/gravel/sand)
-eyot (sediment island)

Question 17

What kinds of deltas are there? Give an example for each:

Answer 17

-arcuate (Nile)
-bird-foot (Mississippi)
-cuspate (Ebro, Spain)

Question 18

Explain how a delta/braided channel is formed:

Name the layers

Answer 18

-deposition rates exceed erosion rates

bottomset - fine clay/silt flung out by turbidity current
foreset - coarse gravel/sand, built by avalanching
topset - coarse build by fluvial deposition

-eyots build up and form distributaries and braided channels

Mention deposition rates in any delta formation question

Question 19

Requirements for a delta to form:

Answer 19

-deposition rates must exceed erosion rates

-high sediment load
-river + sea water densities must be different
-no strong sea currents

Question 20

How are levees + floodplains formed?

Answer 20

-when river floods, velocity drops (increased friction)

-coarse material deposited closer to banks, creates a taller levee
-fine material deposited further out across floodplain, creates sloped levees

-after many floods, layers of the rich + fertile alluvium builds up the floodplain + levees

Question 21

Pros and cons of larger levees:

Answer 21

-increases channel capacity, holds more water in future so it needs more to flood, so people can build on the flood plain
-larger flood risk as there is now more water

Question 22

What are some benefits of living near a river?

Answer 22

-fishing
-transportation of people/materials
-flat, fertile land (good for farming)
-water supply

Question 23

Why might living near a river be a problem?

Answer 23

-flood risk
-difficulty of crossing
-erosion of land

Question 24

Suggest 4 hazards caused by flooding:

Answer 24

-damage to buildings/roads/possessions (eg cars)
-loss of water/electricity/gas supply
-risk of drowning
-spread of disease (cholera)

Question 25

Draw a storm hydrograph:

Answer 25

bars are rainfall, line is discharge

Question 26

What is the danger of having a shorter lag time and why?

Answer 26

-higher chance of having a flash flood
-rainfall gets into river too fast and floods

Question 27

Give some reasons as to why a flash flood might happen:

Answer 27

-short-lived but intense rainfall
-short lag time (water gets into river faster)
-shortage of permeable surfaces (eg in an urban area with lots of concrete)
-higher drainage density

Question 28

Why do pebbles generally get smaller downstream?

Answer 28

-added discharge at confluences from tributaries
-increased force of HA on the rocks, making loose rocks impact each other more frequently (more attrition)

Question 29

How does a culvert work?

Answer 29

-culvert restricts width of river upstream so less water gets to the town
-backed-up water is kept in field storage away from the town

Question 30

Name 2 human features of a river:

Answer 30

-bridges/roads over it
-marinas

Question 31

Suggest why most of the major flooding occured at Cape Girardeau:

Answer 31

-further downstream and several tributaries have joined the Mississippi, higher discharge
-likely to have a large flat flood plain around the river