Rivers change the landscape over time.
Water flows down steep slopes near the rivers source, causing vertical erosion.
The slope gradually becomes flat land as the river approaches the mouth.
- Erosion is the wearing away of rock and soil along the river bed and banks.
- Hydraulic Action is when the river forces air into cracks and crevices, gradually wearing the river bank away.
- Abrasion is where rocks get carried along by the river, wearing down its beds and banks.
- Attrition is when rocks, carried by the river, smash together and break into smaller, smoother particles.
- Solution is when particles of rock dissolve into the water.
Rivers Transport material by solution,suspension, traction and saltation.
- Solution is when dissolved materials get transported.
- Saltation is when small pebbles and stones bouce along the river bed.
- Suspension is when small fine material is carried along.
- Traction is when large boulders and rocks roll along the river bed.
- Rivers have an upper, middle and lower course.
- In the upper course, the river erodes the landscape and bends to avoid hard rock, creating interlocking spurs.
- Rapids and waterfalls can form when it runs over alternating layers of hard and soft rock. This is because soft rock erodes more easily than hard rock.
- In the middle course the river flows on a gentler slope with more energy and volume. Sideways erosion widens the channel to the right and then the left forming horseshoe-like loops called meanders.
- Over time the meander becomes tighter. Eventually the river can flow over two ends leaving an oxbow lake.
- In the lower course, a high volume of water flows over flat land. It now has a wide floodplain which is the area around a river that is covered when it floods. Floodplains are fertile and good for agriculture because of the richalluvium in floodwaters.
River Flooding and Management Issues
- A flood occurs when a river is swollen by heavy rain. It bursts its banks and the water spills onto the flood plain.
- Factors in the landscape can make flooding more likely. A steep-sided channel can cause fast surface run-off where the soil cannot absorb the water and so water runs over it. Rivers can flood if there are no trees and plants to catch and drink the water.
- Floods can be catastrophic in the short term but they can have good long term effects too such as boosting soil fertility.
- Floods tend to affect LEDCs more than MEDCs, partly because they depend on agriculture and need the fertile floodplains.
- Hard engineering options usually cost more and have a greater impact on the river and landscape.
- Soft engineering options are cheaper and have less impact on the environment.
- Managed flooding is allowing one area to flood to prevent flooding towns or villages.
- Governments and developers often prefer hard engineering options, while environmental groups prefer softer options, like planting trees. Flood management options need to be sustainable.