We do not know the causes of the decline of the Indus Civilization. Great civilizations of ancient ages have disappeared for mysterious reasons. When definite causes can not been known, scholars have to imagine several possible reasons leading to such declines. About Indus civilizations, the following arguments have been advanced as possible causes of its decline.
According to Sir Mortimer Wheeler, Harappa and Mohenjo-Daro were destroyed by the Aryan invader. In the Vedas the early Aryans described their God Indra as the destroyer of cities of the Asuras. He was called Purandara. It may be that when the Aryans entered into India, they saw some great cities, only the Indus valley belonging to other people. They fought with them. In those battles the Aryan invaders seem to have been victorious. Possibly, they destroyed the cities of the defeated people and drove them from their original homes.
According to Wheller, the fall of Mohenjo-Daro and Harappa perhaps occurred about 1500 years before the birth of Christ. To other scholars the above cause appears unsound. They believe that climatic changes led to the decline of the cities. It may be that the rivers Indus and Ravi changed their courses for which the cities were badly affected. It also appears possible that the frequent floods of those rivers made it difficult people to live. Being deserted, the cities perhaps got buried under earth in course of many years.