V-shaped valley, steep sides, narrow, shallow channel
Gentle sloping valley sides, wider, deeper channel
Very wide, almost flat valley, very wide, deep channel
This deepens the river valley ( and channel ), making it v-shaped.
It's dominant in the upper course of the river.
This widens the river valley ( and channel ).
It's dominant in the middle and lower courses.
1) Hydraulic action
The force of the water breaks rock particles away from the river channel.
Eroded rocks picked up by the river scrape and rub against the channel, wearing it away. Most erosion happens by abrasion.
Eroded rocks picked up by the river smash into each other and break into smaller fragments.
Their edges also get rounded off as they rub together.
River water dissolves some types of rock, e.g. Chalk and limestone.
1) Traction - large particles like boulders are pushed along the river bed by the force of the water.
2) Saltation - pebble-sized particles are bounced along the river bed by the force of the water.
3) Suspension - small particles like silt and clay are carried along by the water.
4) Solution - soluble materials dissolve in the water and are carried along.
1) when a river drops the eroded material it's transporting.
2) it happens when a river slows down ( loses velocity ).
3) there are a few reasons why rivers slow down and deposit material:
- the volume of water in the river falls
- the amount of eroded material in the water increases.
- the water is shallower, e.g. On the inside of a bend
- the river reaches it's mouth
1) The current ( the flow of the water ) is faster on the outside of the bend because the river channel is deeper ( there's less friction to slow the water down ).
2) So more erosion takes place on the outside of the bend, forming river cliffs.
3) The current is slower on the inside of the bend because the river channel is shallower ( there's more friction to slow the water down ).
4) So eroded material is deposited on the inside of the bend, forming slip-off slopes.
1) Erosion causes the outside bends to get closer...
2) until there's only a bit of land left between the bends ( called the neck ).
3) The river breaks through this land, usually during a flood...
4) and the river flows along the shortest course.
5) Deposition eventually cuts off the meander...
6) forming an ox-bow lake.
Waterfalls and gorges
1) Waterfalls form where a river flows over an area of hard rock followed by an area of softer rock.
2) The softer rock is eroded more than the hard rock, creating a 'step' in the river.
3) As water goes over the step it erodes more and more of the softer rock.
4) A steep drop is eventually created, which is called a waterfall.
5) The hard rock is eventually undercut by erosion. It becomes unsupported and collapses.
6) The collapsed rocks are swirled around at the foot of the waterfall where they erode the softer rock by abrasion. This creates a deep plunge pool.
7) Over time, more undercutting causes more collapses. The waterfall will retreat ( move back up the channel ), leaving behind a steep-sided gorge.
1) This is the wide valley floor on either side of a river which occasionally gets flooded.
2) When a river floods onto the flood plain, the water slows down and deposits the eroded material that it's transporting. This builds up the flood plain ( makes it higher ).
3) Meanders migrate ( move ) across the flood plain, making it wider.
4) The deposition that happens on the slip-off slopes of meanders also builds up the flood plain.
These are natural embankments ( raised bits ) along the edges of a river channel.
During a flood, eroded material is deposited over the whole flood plain.
The heaviest material is deposited closest to the river channel, because it gets dropped first when the river slows down.
Over time, the deposited material builds up, creating levees along the edges of the channel, e.g. Along the yellow river in China.
1) Peak Discharge:
The highest discharge in the period of time you're looking at.
2) Lag time:
The delay between peak rainfall and peak discharge.
3) Rising limb:
The increase in river discharge as rainwater flows into the river.
4) Falling limb:
The decrease in river discharge as the river returns to its normal level.
Factors affecting river discharge
Amount and type of rainfall
- lots of rain and short, heavy periods of rainfall means there's more runoff. Lag time is decreased, so discharge increases.
- hot, dry conditions and cold, freezing conditions both result in hard ground - this increases runoff. Lag time is decreased, so discharge increases.
Previous weather conditions
- after lots of rain, soil can become saturated. More rainwater won't be able to infiltrate into the soil so runoff will increase. Lag time is decreased, so discharge increases.
- urban areas have drainage systems and they're covered with impermeable materials like concrete - these increase runoff. Lag time is decreased, so discharge increases.
- water infiltrates through pore spaces in permeable rock and flows along cracks in pervious rio is - this means there isn't much runoff. Lag time is increased, so discharge decreases. Water can't infiltrate into permeable rock - this means there's a lot of runoff. Lag time is decreased, so discharge increases.
- lots of runoff occurs on steep slopes. Lag time is decreased so discharge increases.
Physical factors of flooding
- After a long period or rain, the soil becomes saturated. Any further rainfall can't infiltrate, which increases runoff into rivers. This increases discharge quickly, which can cause a flood.
- when a lot of snow or ice melts it means that a lot of water goes into a river in a short space of time. This increases discharge quickly which can cause a flood.
- heavy rainfall means there's a lot of runoff. This increases discharge quickly, which can cause a flood.
- if a river is in a steep-sided valley, water will reach the river channel much faster because water flows more quickly on steeper slopes. This increases discharge quickly, which can cause a flood.
Human factors of flooding
- trees intercept rainwater on their leaves, which then evaporates. Trees also take up water from the ground and store it. This means cutting down trees increases the volume of water that reaches the river channel, which increases discharge and makes flloodinh more likely.
- buildings are often made from impermeable materials, e.g. concrete, and they're surrounded by roads made from tarmac (also impermeable). Impermeable surfaces increase runoff and drains quickly take runoff to rivers. This increases discharge quickly which can cause a flood.
Flooding is increasing
Some rivers in the UK have been flooding more frequently over the last 20 years. For example the River Ouse in Yorkshire reached a high water level 29 times between 1966 and 1986. But between 1987 and 2007 it reached the same level 80 times.
UK water demand
1) The north and west of the UK have high rainfall, whcih means there's a good supply of water.
2) The south east and midlands have high population densities, which means there's a high demand for water.
3) The south east and midlands are areas of water defecit ( there's greater demand than supply ).
4) The north and west are areas of water surplus ( there's greater supply than demand ).
Over the past 25 years, the amount of water used by people in the UK has gone up by about 50%
Managing the UKs water supply
1) One way to deal with the supply and demand problem is to transfer water from areas of surplus to areas of deficit. For example Birmingham ( an area of defecit ) is supplied with water from the middle of Wales ( an area of surplus ).
2) Water transfer can cause a variety of issues:
- The dams and aqueducts ( bridges used to transport water ) that are needed are expensive
- It could affect the wildlife that lives in the rivers, e.g. fish migration patterns could be disrupted by dam building
- There might be potential political issues e.g. people may not want their water given to another country