Theme A: River Enviroments Cheat Sheet by etait4502

Building a dam or reservoir in the upper course of the river can control the discharge in the river. They will reduce flooding and the resulting reservoir can be used for leisure and hydroe­lec­tri­city. The disadv­antages of building dams can be:

- good farmland is flooded

- habitats are destroyed

Building high embank­ments along the sides of the river increases the river's capacity to contain any floodw­ater. Levees do reduce flooding but they are expensive to strengthen and heighten (can be eroded easily). There will be catast­rophic flooding if they are breached.

These walls are built around settle­ments and important factories or roads. They are quite expensive and do not look very natural but are usually effective.

Known as channe­lising. By straig­htening and deepening the river channel, the cross-­sec­tional area of the river is increased allowing it to contain more water. The straighter channel makes the water move faster through that part of the river so it does not build up and is less likely to flood. This totally changes the ecosystem in the river and spoils the natural look of the area. Often further downstream where the river is not channe­lised, the water builds up and floods occur there.

The water is pumped out of the river and stored in temporary lakes. Then it is pumped back in after the water in the river has returned to normal flow. This strategy is effective but a large amount of unused land is needed so it can be flooded.

As more and more tribut­aries flow into the main river. By the time it reaches the mouth it will have gathered water from hundreds of smaller streams, increasing its volume of water.

The Somerset Levels are a low lying region in the South West of England. As it has a naturally high water table and poor drainage it is prone to flooding. The Winter of 2013-14 saw it experience prolonged flooding which was described as "the worst in over a centur­y". Villages were stuck in polluted water for 6 weeks. Although the river was naturally supposed to flood twice every 100 years it flooded twice in the last 2 years.

- Combin­ation of imperm­eable bedrock and low interc­eption levels means land is naturally at risk of flooding.

- Series of severe winter storms in Southern England in 2013-14. One of the stormiest December's on record and one of the windiest since January 1993.

- In addition, extra water was being sent to the levels from Taunton and Bridge­water as part of a scheme where water is pumped away from areas to protect new homes build on former floodp­lains. Pumping stations couldn't remove water fast enough. The enviro­nment agency brought in more pumps but it was 'too late'. Owen Paterson was slow to respond which made things worse.

Both human and physical reasons are to blame for the prolonged flooding experi­enced in the Somerset Levels from 2013-14.

The Missis­sippi river is located in the South-East of the USA, with one of the largest drainage basins in America that drains water from a third of the USA and Canada.

Around 25,000 people were evacuated when it flooded in 2011, damage costs estimated at around 3 billion.

- Raised levees, levels raised to 15m and streng­thened to enclose river for a stretch of 3000km.

- Dams: the flow of the main tribut­aries, such as Ohio River, were controlled by over 100 dams.

- Affore­station in upper course, trees have been planted in areas such as Tennessee Valley to intercept some rainfall and stabilise soil.

- Washlands; in 2011 the Maganza Spillway was opened to flood around 2000km squared of farmland in Louisiana, delibe­rately preventing that water from reaching the city of New Orleans.

- Missis­sippi River is very important to USA as 18million rely on it for water supply.

- For current and future genera­tions, the lack of silt reaching the land means that the fertility of the soil is no longer being naturally completed during the deposition of alluvium in the floods. Eventually more and more artificial fertil­isers will have to be added to the soil.

- The soft engine­ering strategy of affore­station is ineffe­ctive as not applied over wide enough area and takes too long for trees to become large enough for noticeable runoff differ­ence.