The Indus Valley civilization flourised between the time period of 3300 – 1700 BCE. Around 1900 BCE, people started leaving and the cities started to decline. There are many reasons attributed for this decline, and the theories include tectonic activity along the Indo-Asian plate boundary, or flooding. Another reason could be the disappearance of the Ghaggar-Hakra river system which was part of Sarasvathi. Then there is the infamous Aryan invasion theory.
Now according to new research, it was not Aryans, but monsoons, which were responsible for the demise of the Indus Valley Civilization. Geologist Anil Gupta of IIT Kharagpur studied the effect of monsoons over the past 10,000 years and have come to the conclusion that a strong monsoon helped the civlization grow, while a weakening monsoon might have led to its decline.
The Arabian Sea sediments and other geological studies show that the monsoon began to weaken about 5,000 years ago. The dry spell, lasting several hundred years, might have led people to abandon the Indus cities and move eastward into the Gangetic plain, which has been an area of higher rainfall than the northwestern part of the subcontinent.
“It’s not high temperatures, but lack of water that drove the people eastward and southward,” Gupta said.
About 1,700 years ago, the monsoon began to improve again, leading to increased farm produce for several centuries and contributing to the relative prosperity in India during the medieval ages, from AD 700 to 1200. [Indus cities dried up with monsoon]