What factors does how much work a river does depend on?
- Velocity of water - faster = can carry bigger pieces of material
- Volume of water - greater volume of water = greater load
- Bedrock type - hard rock = hard to erode
Give and example of hard and soft rock.
- Hard:
- Granite
- Soft:
- Shale
What can cause deposition to occur?
- A decrease in gradient
- A decrease in discharge as water drains away after heavy rain
- River meets sea or lake
- River flows more slowly on the inside of bends
What is discharge?
The volume of water flowing down the river at any one time.
How do you measure discharge of a river?
Measured in cubic metres per second (a.k.a. cumecs).
What can cause a variable annual discharge?
- Climates with wet and dry seasons
- Melting glacial ice in spring
What are the three main shapes of river channel?
- Braided (river splits into channels and flows around banks of material it has deposited)
- Sinuous
- Straight
What are the characteristics of the river in its upper course?
Velocity
Low
Turbulence
High
Depth
Shallow
Breadth
Narrow
Process
Vertical erosion
Gradient
High
How do potholes form?
- Stone trapped in a hollow in the river bed.
- Eddies in the water swirl stone around, causing corrasion.
- Hollow becomes deeper and wider.
- Hollows join together to make a pothole.
How do rapids form?
- Band of soft rock eroded morethan band of hard rock.
- Creates a changing gradient.
- Changing gradient creates turbulence.
How does a waterfall form?
- Waterfalls can develop from rapids.
- Horizontal layer of hard rock lies on top of a layer of soft rock in a river valley.
- Soft rock eroded faster. Plunge pool develops.
- Splashing from water (hydraulic action) and stones (corrasion) undercuts cliff.
- Cliff cannot support itself and eventually collapses.
- Waterfall retreats upstream over time, leaving behind a gorge (steep sided valley) and escarpment (cliff).
What can waterfalls be used for?
- Tourism
- Hydroelectricity
- Rapids good for water sport
What problems do waterfalls and rapids bring?
Navigational problems.
What are interlocking spurs, and how do they form?
- Water avoids hard rock and takes path that uses least energy.
- Water therefore forced to wind around spurs.
What are the characteristics of the river in its middle course?
Velocity
Higher
Turbulence
Lower
Depth
Deeper
Breadth
Wider
Process
Transportation and lateral erosion
Gradient
Lower
How do meanders and oxbow lakes form?
- Water flow is naturally helical (corkscrew).
- Water meanders as it erodes soft rock and flows around hard rock.
- Helical flow sends energy laterally. Fastest flow is forced to outer bend (creating a river cliff where erosion takes place) and slowest flow is in the inner bend (creating a slip-off slope where deposition takes place).
- Continued erosion creates a narrow neck between two meanders.
- Faster flow around the neck means that the neck is cut through.
- Old meander becomes an oxbow lake when deposition (at the sides in a straight river) seals off the ends.
- Eventually dries up to become a marsh and then meander scar.
What is a flood plain?
Area of flat land next to a river that is liable to flooding, enclosed by a bluff. Often marshland that is poorly drained.
How does a flood plain form?
- Deposition occurs at slip-off slopes at the inner bend of a meander. As meanders change over time, these deposits spread across the valley.
- Gravel is deposited on the river bed.
- During floods, the river bursts its banks and deposits fine alluvium on the plain itself, far away from the river because suspended load is lightest.
How do levées form?
- River floods.
- Lightest load deposited far away from the river.
- Heaviest load deposited on the banks of the river.
- This build up levées on either side of the channel.
Why is the river bed higher than the flood plain?
- During normal flows, the river deposits its material from its load onto the river bed within the channel.
- This happens more often than floods.
What is a delta?
- An area of flat, low-lying, marshy land where the river meets the sea.
- Often contains lakes and lagoons.
- River channel has divided into distributaries (braided channel).
How does a delta form?
- River carrying its load meets the slow, still water of the sea or lake.
- River loses velocity.
- River deposits load.
- Load builds up over time to form a delta.
- Continued deposition blocks the main river channel, causing the development of distributaries.
What is a long profile?
A line drawn from the source to the mouth of a river to show how gradient changes through its course.
Why live on a floodplain or delta?
- Flat land is easy to build settlements and roads on.
- Fertile alluvium soils good for farming.
- River valleys often natural route ways.
- Rivers good for transportation.
- Rivers used for drinking.
- Rivers used for tourism and recreation.
