What reasons are suggested for the disappearance of the Indus Valley civilization?

The reasons for the suggested disappearance of the Indus Valley civilization were droughts and a decline in trade with Egypt and Mesopotamia, ancient civilizations which were not too extended far away from the Indus Valley. The droughts would have caused famine. Other reports equally suggest that immigration by new peoples, deforestation, floods or changes in the course of river could have affected the Indus Valley Civilization.