Decline set in around 1900 BC and after 1700 BC, little remained in Harappan civilization. However, it did not come to an abrupt end within particular decade or century. Until recently, it was believed that Indus Cities were destroyed by the Aryans who entered India from Iran and Afghanistan through northwestern passes such as the Bolan and the Khyber. The evidences of this theory were two pronged. Firstly, there is a reference in the Rig-Veda, that Indra destroyed hostile people of Hariyuppa (Harappa) called Dasyus who lived in forts called Pur (Thus, one name of Indra is Purandhar, destroyer of Pur). Secondly, there was a discovery of some skeletons of men, women and children from the lower city of Mohenjo Daro, killed during the so-called last massacre. Most ardent propounder of this theory was Sir Mortimer Wheeler.
However, most scholars and archaeologists dismiss the Wheeler’s theory to be too simplistic. The argument against this is that a Pur of Rigveda was not a fort of Indus Valley but just a structure of filmy ramparts and stockades etc. Further, Rig-Veda never mentions anything about some of the recognizable features of the sites such as streets, houses, wells, drains, granaries etc.
Another theory considers a combination of natural and socio-economic factors behind decline of the Indus cities. The natural factors could be geological and climatic. It is believed that the Indus Valley region experienced several tectonic disturbances which caused earthquakes. These upheavals not only disturbed their life but also changed courses of rivers or dried them up. The modern satellite imagery confirms dramatic shifts in the river courses, which might have caused great flood cutting the food producing areas from urban centres. This is evident from the quantities of silt layers in the upper levels of Mohenjo Daro indicating heavy floods.
Another natural reason might be changes in patterns of rainfall. During the mature Harappan age in 2500 BC, there was a great rise in the amount of rainfall, but by the beginning of the second millennium BC it had dropped dramatically thus affecting food production adversely. With the rivers shifting their courses, the rainfall declining and sufficient food failing to arrive from the countryside, there was a slow but inevitable collapse of the Indus system. Its final outcome was catastrophic for the Mature Harappan phase.
This decline theory of environment degradation was given by John Marshall. The dogma says that cutting of forests for agriculture and timber for fuel may have resulted in the barren land and silting of rivers. It has been postulated that in Saraswati region, the civilization declined mainly because of the shifting of the river channels.