The Sarasvati River described in the Rigveda as flowing from the ‘mountains to the sea’ is very ancient, several centuries before 3000 BC or well over 5000 years old counting from today. What is interesting is that the Yamuna and Sutlej flowed into the Sarasvati. The made the mighty river mightier. Sometimes before 3000 BC, probably due to tectonic shifts that followed earthquakes, the Yamuna changed course and began to flow into the Ganga. This reduced the flow into the Sarasvati, leading to a weakening of its upper course that soon disappeared into the desert at a place called Vinashana. Archaeologists have identified the place as Kalibangan in Rajasthan where Harappan and pre-Harappan settlements have been found along with signs of at least one earthquake. This corresponds to the description found in the post Rigveda literature like the Jaiminiya Brahmana and also the Mahabharata.
While the upper Sarasvati weakened and disappeared into the desert, the lower Sarasvati continued to flow through what is now the Thar Desert, fed by the Sutlej and other smaller rivers. This is what sustained the Harappan civilisation in Rajasthan, Sindh and Cholistan down to the Rann of Kutch. But this lower Sarasvati too dried up in stages in the 2200-1900 BC period, ending with it the Harappan civilisation. The reason for this was ecological degradation and not any invasion – Aryan or otherwise. First as it happened with the Yamuna, the Sarasvati lost the Sutlej when it changes course and joined the Indus again due to tectonic shifts. This was compounded by massive worldwide drought in and around 2200-1900 BC period that struck across a wide belt from southern Europe, North Africa to China. It ended the civilisations of the Dynastic Egypt and Sumer-Akkad in Mesopotamia along with Harappan. The Harappan civilisation ended in around 1900 BC and not 1500 BC as stated by some history books. Which means the civilisation ended a good 400 to 500 years earlier than thought.
This ecological degradation was a major blow to the region. This resulted in migration of the Harappan civilisation into Gujarat, east into the Gangetic plain and into west Asia. There are records of Kassite rulers of Indian origin establishing an empire in Babylon that lasted for 500 years