There was a fall in the average rainfall in the cities leading to the formation of desert-like condition. This led to the decline in agriculture on which most of the trade was dependent. Owing to this, people of the Indus Valley started shifting to some other location leading to the decline in the entire civilization. As per some scholars, the reason of the decline is the change in the course of River Ghaggar-Harka that had led to an increase in aridity of the place. Around 2000 BC, there was found an increase in arid conditions. The location where the Indus valley civilisation once flourished is a desert today.
Frequent flood, invasion by foreign tribes ,decline in trade were some of the reasons for the decline of the Harappan Civilization.Some regions become arid due to weakening of monsoon , drying up of rivers basin, earthquakes ,changes in sea-level ,etc.People migrated to other places leading to the decline of cities.
Many theories have been formulated and provided, but all the theories met with one or another form of criticism. Archeological evidences do prove that there was no sudden collapse of the Indus Valley Civilization but it declined over a period of time and got mingled with other civilizations of that period.
The Harappan Civilization was a prosperous and flourishing urban civilization .It laid the foundation of the Indian Civilization.s