I would like to bring to the readers’ attention this paper on Indus Valley Civilization and a blog in New York Times. It talks about the collapse of the Harappan civilization and how it was affected by the changes in climactic conditions thousands of years back. The article talks about how the changes in monsoon pretty much destroyed the agriculture as the agricultural process were based not on irrigation channels like canals/dams, but on monsoon and monsoon only. Over a period of time,
Hydroclimatic stress increased the vulnerability of agricultural production supporting Harappan urbanism, leading to settlement downsizing, diversification of crops, and a drastic increase in settlements in the moister monsoon regions of the upper Punjab, Haryana, and Uttar Pradesh.
As time passed, the monsoons continued to weaken until the rivers no longer flooded, and the crops failed. People began abandoning the cities and moved eastward toward the Ganges basin, where rains were more dependable (though not dependable enough to sustain urban metropolises). The civilization dispersed, fracturing into small villages and towns.
This demonstrates two very important points. First, that there is a continuity of civilizations and Aryan invasion theory is bogus. The same people who lived in Indus valley were the ones who moved to the plains of Ganga and other rivers. Secondly, we are still living the same nightmare where we are depending on periodic monsoons. We had a prime minister who talked about linking rivers and so called intellectuals laughed at him.