The drying up of the great river Saraswati must have resulted in great conflict, equally great changes, and left its mark on the social practices of the population. The Mahabharata, suitably emphasized, can be viewed as the story of this disaster. Rather than a conflict over succession to small kingdom in Northern India, the real story is social policy conflicts over how to respond to the crisis. The winning side’s preferred policies became entrenched as part of “Hinduism”. That accounts for the importance of the Mahabharata for these policies have stayed with us for 4000 years!
A number of social practices unique to Hinduism and spread across the sub-continent are first referenced in the Mahabharata and/or the Harivamsa. Caste, the sacredness of the cow, forest practices (until very recently), river-crossings (“teertha”) considered sacred, limits to imperial power, the preference for male children (not unique to Hinduism but a much longer history in South Asia than in any other region of the world).
What is the evidence?
We know now that the Saraswati dried up around 2000 BCE. The Mahabharata has a single reference to this event – Krishna’s brother Balarama arrives at Kurukshetra just in time to see Bhima break Duryodhana’s thighs. After Krishna calms him down and saves Bhima, Balarama explains that he missed the war because he walked the entire bed of the Saraswati from Dwarka to Kurukshetra and it was dry.
Talk about the elephant in the room. The Saraswati was a snow-fed river with two major tributaries – the Yamuna and the Sutlej. Around 2000 BCE, tectonic events in the Himalayas and in the northern end of the Aravalli Hills near modern-day Delhi caused the Yamuna to change direction from going west to going east (and join the Ganga at Prayag). Meanwhile the Sutlej makes a sharp right hand turn a little to the west and joins the Indus. This converted the Saraswati from the mammoth river described in the Rig-Veda (over five and ten miles wide in places) to a rain-fed stream limping through the Thar Desert.
What was the elephant in the room? Balarama’s statement is greeted with sorrow but not surprise. The drying up of the Saraswati was an expected event. The approximately 2000 settlements that have been identified by archaeologists on the putative banks of the Saraswati must have been depopulated in the preceding few generations, i.e. during the period covered by the stories of the Mahabharata. People do not vanish – they migrate. Imagine that! -- while the Pandavas and Kauravas were fighting over who would inherit the (small!) kingdom a vast migration was in progress. A million (at least) people were moving away from the banks of the Saraswati and going north, south, west, and east. South would put them in lands dominated by Dwarka; East would make them move through Hastinapur and Kampilya (the capital of Panchala).
About this vast movement the Mahabharata is silent. The Harivamsa even has an opposite migration, from Mathura (on the banks of the east-flowing Yamuna) to Dwarka!
The Mahabharata is not a sacred text. Within its text, the Mahabharata mentions two occasions when the story was told as entertainment and the text was expanded in response to questions from the audience. It is designed to be drama, not oral history, and was not written down until sometime between 300 BCE and 400 CE (one scholar has imputed this to Pushyamitra Sunga around 100 BCE). Unlike the RigVeda which has been maintained unchanged for many thousand years, the Mahabharata is under no such constraint.
I believe that the stories in the Mahabharata were changed to reflect the audience and the reciter’s understanding of events – if you do not know that the Yamuna used to join the Saraswati out to the west it does not make sense to talk about a conflict between immigrants/refugees and residents. Over the 1900 years to Pushyamitra Sunga, the story changed to focus on what was easy to understand – a conflict over succession.