Many scholars believe that the collapse of the Indus Valley Civilization was caused by climate change. Some experts believe the drying of the Saraswati River, which began around 1900 BCE, was the main cause for climate change, while others conclude that a great flood struck the area.
According to another theory by British archaeologist Mortimer Wheeler, a nomadic, Indo-European tribe, called the Aryans, suddenly overwhelmed and conquered the Indus River Valley.