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- By 1800 BCE, the Indus Valley Civilization saw the beginning of their decline:
- Writing started to disappear,
- Standardized weights and measures used for trade and taxation purposes fell out of use, and
- Some cities were gradually abandoned.
- Possible reasons: The reasons for this decline are not entirely clear, but it is believed that:
- The drying up of the Saraswati River, a process which had begun around 1900 BCE, was the main cause.
- Recent Study by IIT Kharagpur, ASI, PRL (2020): The decline of the Harappan city Dholavira was attributed to the drying up of rivers such as the Saraswati River and the Meghalayan drought.
- Natural disaster: At several Indus cities, such as Mohenjodaro, Chanhudaro, and Lothal, there are layers of silt debris intervening between phases of occupation, indicating the possibility of damage caused by inundations from swollen rivers.
- Multiple layers of silt at Mohenjodaro provide evidence of the city being affected by repeated episodes of Indus floods, ultimately contributing to the decline of the Harappan civilization.
- It seems that in Mohenjodaro, various periods of occupation were separated by clear evidence of deep flooding.
- The shifting course of the Indus: Lambrick suggests that alterations in the path of the Indus River could be the root cause of Mohenjodaro's destruction. The Indus, characterized by its unstable river system, frequently changes its course.
- It is believed that the river Indus shifted and the inhabitants of the city and the nearby villages engaged in food production abandoned the area due to water scarcity. This occurrence occurred repeatedly throughout the history of Mohenjodaro.
- The silt observed in the city is a result of wind action, depositing significant amounts of sand and silt.
- Theory of Aryan invasions:The idea that the civilization was destroyed by Aryan invaders was first put forward by Ramaprasad Chanda and was elaborated on by Mortimer Wheeler.
- Wheeler believed that the Harappan civilization was destroyed by the Aryan invaders.
- Wheeler pointed to certain human skeletal remains found in the late phases of occupation at Mohenjodaro as proof of the Aryan massacre.
- This was a large migration and used to be seen as an invasion, which was thought to be the reason for the collapse of the Indus Valley Civilization, but this hypothesis is not unanimously accepted today.
- Gradual desiccation and climate change: According to D.P. Agarwal and Sood, the decline of the Harappan civilization can be attributed to the escalating aridity in the region and the subsequent drying up of the Ghaggar-Hakra River.
- Monsoon Link Theory of 2012: The theory posits that climate change is accountable for the downfall of the Harappan civilization.
