The decline of the Harappan civilization:

(i) Some scholars have suggested that natural calamities such as floods and earthquakes might have caused the decline of the civilization. It is believed that earthquakes might have raised the level of the flood plains of the lower course of Indus river. It blocked the passage of the river water to the sea and resulted in the floods which might have swallowed the city of Mohenjodaro. However, this only explains the decline of Mohenjodaro and not the Harappan civilization.

(ii) Some scholars believe that increased aridity and drying up of the river Ghaggar - Harka on account of the changes in river courses might have contacted to the decline. According to this theory, there was an increase in arid conditions by around 2000 BC. This might have affected agricultural production, and led to the decline.

(iii) Some other scholars believe that Harappan civilization declined due to Aryan invasion. According to this theory, the Harappan civilization was destroyed the Aryans who came to India from the north - west around 1500 BC. However, on the basis of closer and critical analysis of data, this view is completely negated today.

Thus, there is no single factor that can explain the decline of the Harappan civilization in totality. At the maximum these can explain the decay of certain sites or areas only. Hence each theory has been criticised. Nevertheless, the archaeological evidence indicates that the Harappan civilization did not collapse all of a sudden but declined gradually and ultimately merged with other local culture.