Upper-course river features include steep-sided V-shaped valleys, interlocking spurs, rapids, waterfalls and gorges.

Middle-course river features include wider, shallower valleys, meanders, and oxbow lakes.

Lower-course river features include wide flat-bottomed valleys, floodplains and deltas.

As the river moves through the upper course it cuts downwards. The gradient here is steep and the river channel is narrow. Vertical erosion in this highland part of the river helps to create steep-sided V-shaped valleys, interlocking spurs, rapids, waterfalls and gorges.

Interlocking spurs on a tributary of the Yangtse

• As the river erodes the landscape in the upper course, it winds and bends to avoid areas of hard rock. This createsinterlocking spurs, which look a bit like the interlocking parts of a zip.
• When a river runs over alternating layers of hard and soft rock, rapids andwaterfalls may form.

Meanders

A meander on the River Cuckmere

In the middle course the river has more energy and a high volume of water. The gradient here is gentle and lateral (sideways) erosion has widened the river channel. The river channel has also deepened. A larger river channel means there is less friction, so the water flows faster:

• As the river erodes laterally, to the right side then the left side, it forms large bends, and then horseshoe-like loops called meanders.
• The formation of meanders is due to both deposition and erosion and meanders gradually migrate downstream.
• The force of the water erodes and undercuts the river bank on the outsideof the bend where water flow has most energy due to decreased friction.
• On the inside of the bend, where the river flow is slower, material isdeposited, as there is more friction.
• Over time the horseshoe become tighter, until the ends become very close together. As the river breaks through, eg during a flood when the river has a higher discharge and more energy, and the ends join, the loop is cut-off from the main channel. The cut-off loop is called an oxbow lake.

In the lower course, the river has a high volume and a large discharge. The river channel is now deep and wide and the landscape around it is flat. However, as a river reaches the end of its journey, energy levels are low and deposition takes place.

Floodplains

The river now has a wide floodplain. A floodplain is the area around a river that is covered in times of flood. A floodplain is a very fertile area due to the richalluvium deposited by floodwaters. This makes floodplains a good place for agriculture. A build up of alluvium on the banks of a river can create levees, which raise the river bank.

The likelihood of a river bursting its banks and flooding is determined by factors in the surrounding landscape, such as steepness of the river valley, the amount of vegetation and the prevailing rock-type. The short-term impact of floods can be catastrophic, but they can have positive long-term effects as well.

A flood occurs when a river bursts its banks and the water spills onto thefloodplain. Flooding tends to be caused by heavy rain: the faster the rainwater reaches the river channel, the more likely it is to flood. The nature of the landscape around a river will influence how quickly rainwater reaches the channel.

The following factors may encourage flooding:

Helicopter carrying British aid across flood water, Mozambique

• A steep-sided channel - a river channel surrounded by steep slopes causes fastsurface run-off.
• A lack of vegetation or woodland - trees and plants intercept precipitation (ie they catch or drink water). If there is little vegetation in the drainage basin then surface run-off will be high.

• A drainage basin, consisting of mainlyimpermeable rock - this will mean that water cannot percolate through the rock layer, and so will run faster over the surface.
• A drainage basin in an urban area - these consist largely of impermeable concrete, which encourages overland flow. Drains and sewers take water quickly and directly to the river channel. Houses with sloping roofs further increase the amount of run-off.

Floods can cause damage to homes and possessions as well as disruption to communications. However, flooding can also have positive impacts on an area.

Flooding deposits fine silt (alluvium) onto the floodplain, making it very fertile and excellent for agriculture. People living on or near floodplains may rely upon regular flooding to help support their farming and therefore provide food.

LEDCs tend to be affected more than MEDCs by the effects of flooding. This is partly because LEDCs have more farms, and farming communities are attracted to fertile flood plains. LEDCs often do not have the resources to prevent flooding or deal with the aftermath of flooding.

The Kariba dam on the Zambezi river

Steps can be taken to manage flooding. Often these steps involve trying to lengthen the amount of time it takes for water to reach the river channel, thereby increasing the lag time. Flood management techniques can be divided into hard- and soft-engineering options.

Hard options tend to be more expensive and have a greater impact on the river and the surrounding landscape.

Soft options are more ecologically sensitive. The tables summarise the main flood management techniques.

River engineering

• The river channel may be widened or deepened allowing it to carry more water. A river channel may be straightened so that water can travel faster along the course. The channel course of the river can also be altered, diverting floodwaters away from settlements.
• Altering the river channel may lead to a greater risk of flooding downstream, as the water is carried there faster.

Afforestation

• Trees are planted near to the river. This means greater interception of rainwater and lower river discharge. This is a relatively low cost option, which enhances the environmental quality of the drainage basin.

Managed flooding (also called ecological flooding)

• The river is allowed to flood naturally in places, to prevent flooding in other areas - for example, near settlements.

Planning

• Local authorities and the national government introduce policies to control urban development close to or on the floodplain. This reduces the chance of flooding and the risk of damage to property.
• There can be resistance to development restrictions in areas where there is a shortage of housing. Enforcing planning regulations and controls may be harder in LEDCs.

Dam construction

• Dams are often built along the course of a river in order to control the amount of discharge. Water is held back by the dam and released in a controlled way. This controls flooding.
• Water is usually stored in a reservoir behind the dam. This water can then be used to generate hydroelectric power or for recreation purposes.
• Building a dam can be very expensive.
• Sediment is often trapped behind the wall of the dam, leading to erosion further downstream.
• Settlements and agricultural land may be lost when the river valley is flooded to form a reservoir.

River engineering

• The river channel may be widened or deepened allowing it to carry more water. A river channel may be straightened so that water can travel faster along the course. The channel course of the river can also be altered, diverting floodwaters away from settlements.
• Altering the river channel may lead to a greater risk of flooding downstream, as the water is carried there faster.

Different interest groups have different views about flood management techniques: Governments and developers often favour large hard engineering options, such as dam building. Building a dam and a reservoir can generate income. Profits can be made from generating electricity or leisure revenue. Environmental groups and local residents often prefer softer options, such as planting trees. Soft options cause little damage to the environment and do not involve the resettlement of communities. Effective flood management strategies should be economically, environmentally and socially sustainable. Sustainable strategies allow management without compromising the needs of future generations.