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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.