Indus Valley Civilization
The Indus Valley Civilisation (also known as the Harappan Civilisation) was a civilised society that flourished between 2600BC and 1900BC in what is now Pakistan and northern India, in the valley of the River Indus. It was characterised by highly developed urban life, particularly in the two main cities to have been excavated, Mohenjo-daro and Harappa, and the port city of Lothal. The basis on which the cities could exist was agricultural surplus, and this was ensured by a widespread and advanced system of irrigation and land drainage.
The reasons for the eventual decline of the Indus Valley society are unclear. Competing theories contend (1) that it was destroyed by the incursion of Aryan peoples from central Asia; and (2) that the shifting of rivers had a catastrophic effect on the agricultural base.