After 1900 BC. all the major Indus cities were abandoned. The true reason for the decline of the Indus Valley civilization is not known, but here are some ideas of what happened to them.
It was once widely thought that Aryan nomadic invaders had attacked their lands, slaughtering them. This idea is no longer accepted because recently scientists have found out that the Indus people were peaceful and traded with the nearby nations.
Nowadays, scientists think that the change in climate, and the change of monsoon seasons had caused the decline of the Indus people. Some scientists think that people migrating from other regions had disrupted the lives of the civilians who lived in the ancient cities. Most of the migrants were herders, so they probably destroyed the waterways and canals by taking their animals there. This theory is backed up by the rapid change of pottery between 2700-2000 BC.