In 1900 BC lots of scholars believed that the Indus Valley Civilization collapsed because of climate change.Experts believe the drying of the Sarawati River was the main cause for climate change.There was a flood that struck the area.
The Indus Valley climate grew colder and drier in 1800 BC, and a get event may have diverted or disrupted river systems, which were the lifelines of the Indus Valley Civilization. The Harappans migrated toward the Ganges basin in the east, where they could have established villages and isolated farms.
In 1800 BC there was an invasion. The Indo-European tribe called the Aryans invaded and conquered the Indus Valley Civilization.Although, more recent evidence tends to contradict this claim.