Human interaction with surface processes (Three Gorges Dam, China)

When people build dams and levees to control river flooding, it has several impacts on the operation of natural river processes, e.g. the Three Gorges Dam in China.

Building dams affects river transport processes. Building dams across a river to control flooding interferes with the ability of the river to carry its load of sediment from source to sea

Huge reservoirs are created behind the dams and the river’s load is trapped in the reservoir

At the Three Gorges Dam in China, a dam 185 metres high and 1.6km wide was built across the River Yangtze

A reservoir over 560km long was created behind this dam

This dam is used to generate power for major cities such as Shanghai and to prevent devastating floods which in the past have killed hundreds of thousands of people

However, damning the river has prevented the transport of fertile alluvium downstream

The floods have also stopped. This has saved lives but farmland downstream from the dam is becoming infertile as it is starved of the life-giving alluvium which would have been deposited by the floods.

Farmers now have to use expensive artificial fertilisers on their land

Building levees prevents natural flood processes from occurring, e.g. along the River Rhine in Germany

Levees are raised river banks deposited in the lower stage of a river’s course. These are natural river landforms but humans also build levees & raise existing levees to stop a river flooding onto its flood plain

Flood plains are flat, fertile areas

Over the centuries many have become densely populated areas. Large cities have been built on them. E.g. Duisberg - the largest inland port in the world was build on the flood plain of the River Rhine.

Preventing rivers overflowing onto their flood plains by interfering with the natural flood process has stopped damage to economically important development such as factories and transport infrastructure

Because the levees are so high, the river cannot flood naturally; so it deposits its load on the riverbed. Over time this raises the riverbed above the level of the flood plain and so the levees are the only thing containing the river

This creates a need for higher, stronger levees which are expensive to build and maintain. Floodwater which would normally escape onto the flood plain is now trapped in the river and is funnelled downstream until it can overflow where it then creates extreme damage

Levees also cause the river to flow faster as it is constrained in narrower channels. The Rhine now flows 30% faster than before the levees were built