Dams
Dams + reservoirs. Dams are barriers and reservoirs are artificial lakes. Reservoirs store water and generate Hydroelectric power. Dams are expensive, creating a reservoir can flood existing settlements. Farmland downstream can become less fertile.
Channel Straightening
Meanders are removed by buildings straight, artificial channels. Water leaves the area quickly, so flood risk is lower. Flooding can happen downstream though, faster moving water can cause more erosion downstream.
Embankments
Raised walls are built along river banks. The river can hold more water, so floods are less frequent. They’re expensive, and a risk of flooding if water level rises.
Flood relief channels
Channels built to divert water around built-up areas or to divert excess water if the river level gets too high. Gates on the channels mean that the release of water can be controlled, reducing flood risk. There will be increased discharge where the relief channel rejoins the river, which could cause flooding in that area
Flood warnings and preparation
Give people time to move possessions. But don’t actually prevent floods.
Flood plain zoning
Planting trees in river valley increases interception of rainwater. Discharge and flood risk decrease. Vegetation reduces soil erosion in the valley and provides habitats for wildlife. Less land available for farming.
River restoration
Making river more natural, removing man made levees for example. Discharge is reduced, Less chance of flooding downstream. Little maintenance is needed and there are better habitats for wildlife. Local flood risk can increase especially if nothing is done to prevent major flooding
Upper Course of a River has:
- Steep Gradient
- V-Shaped Valley
- Steep Sides
- Narrow, shallow channel
Middle course of a river has:
- Medium Gradient
- Gently sloping valley dies
- Wider, deeper channel
Lower course of a river has:
- Gentle gradient
- Very wide, almost flat valley
- Very wide, deep channel.
Vertical Erosion:
- Deepens the river valley and channel, making it V-Shaped.
- Happens in the upper course
- High turbulence causes the rough, angular particles to be scraped along the river bed, causing intense downwards erosion.
Lateral Erosion:
- Widens the river valley and channel during the formation of meanders.
- Dominant in middle and lower courses of a river.
Hydraulic Action:
- The force of the river weather colliding with rocks breaks rock particles away from the river channel.
Abrasion:
- Eroded rocks picked up by the river scrape and rub against the channel, wearing it away.
Attrition:
- Eroded rocks picked up by the river crash into each other and break into smaller fragments.
- Their edges get rounded off as they rub together.
- The further material travels, the more it is eroded - attrition causes particle size to decrease from a river’s source to its mouth.
Solution (erosion):
- River Water dissolves some types of rock like chalk and limestone.
Traction:
- Large particles like boulders are pushed along the river bed by the force of the water.
Saltation:
- Pebble-sized particles are bounced along the river bed by the force of the water.
Suspension:
- Small particles like silt and clay are carried along by the water.
Solution (transportation):
- Soluble materials like limestone dissolve in the water and are carried along.
When does deposition occur?
- When a river loses velocity and energy.
Why do rivers slow down and deposit material?
- Volume of water falls
- Amount of eroded material increases
- River is shallower e.g on inside of a bend
- River reaches its mouth
How does particle size affect how far material is transported?
- Smaller particles are transported further and deposited closer to the river’s mouth.
How do waterfalls form? (6 marks)
- They form where a river flows over an area of hard rock followed by an area of softer rock.
- The softer rock is eroded by hydraulic action and abrasion more than the hard rock, creating a ‘step’ in the river.
- As water flows over the step it erodes more and more of the softer rock.
- A steep drop is eventually eroded, which is called a waterfall.
- The hard rock is eventually undercut by erosion. It becomes unsupported and collapses.
- 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.
- Over time, more undercutting causes more collapses. The waterfall retreats, leaving behind a steep-sided gorge.
