Question

write two main advantages and two ill effects of constructing a big dam

Answer 1

Two advantage are
1) Control over flood
2) Production of electricity
Two ill effects are
1) Increases salinity of the soil
2) Destroy aquatic life

Answer 2

ADVANTAGES: 
1. Once a dam is constructed, electricity can be produced at a constant rate.

2. If electricity is not needed, the sluice gates can be shut, stopping electricity generation. The water can be saved for use another time when electricity demand is high. 

3. Dams are designed to last many decades and so can contribute to the generation of electricity for many years / decades. 

4. The lake that forms behind the dam can be used for water sports and leisure / pleasure activities. Often large dams become tourist attractions in their own right.

5. The lake's water can be used for irrigation purposes. 

6. The build up of water in the lake means that energy can be stored until needed, when the water is released to produce electricity.

7. When in use, electricity produced by dam systems do not produce green house gases. They do not pollute the atmosphere.

DISADVANATGES:
1. Dams are extremely expensive to build and must be built to a very high standard. 

2. The high cost of dam construction means that they must operate for many decades to become profitable.

3. The flooding of large areas of land means that the natural environment is destroyed.

4. People living in villages and towns that are in the valley to be flooded, must move out. This means that they lose their farms and businesses. In some countries, people are forcibly removed so that hydro-power schemes can go ahead.

5. The building of large dams can cause serious geological damage. For example, the building of the Hoover Dam in the USA triggered a number of earth quakes and has depressed the earths surface at its location.

6. Although modern planning and design of dams is good, in the past old dams have been known to be breached (the dam gives under the weight of water in the lake). This has led to deaths and flooding. 

7. Dams built blocking the progress of a river in one country usually means that the water supply from the same river in the following country is out of their control. This can lead to serious problems between neighbouring countries.