Mention any two advantages and disadvantages of producing hydroelectricity by building dams on rivers.

Advantages
(i) The use of water energy does not cause any pollution.
(ii) It is a cheap and renewable source of electricity, which will never get exhausted.
Disadvantages
(i) The construction of dams on rivers results in to a variety of ecological changes in the downstream area of the river.
(ii) The soil in the downstream area becomes poor in quality because of the lack of annual floods to deposit nutrient rich slit on the banks of the river.