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

(N/A) Advantages:
$(a)$ Production of cheap electricity without causing pollution.
$(b)$ Dams are used for irrigation and flood control.
Disadvantages:
$(a)$ Large land areas get submerged,leading to the loss of biodiversity and displacement of local populations.
$(b)$ It adversely affects the ecosystem of adjoining areas as well as areas downstream by altering natural water flow and sediment transport.