- 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.
- The drying up of the Saraswati River, a process which had begun around 1900 BCE, was the main cause.
- Another reason may be a great flood in the area. Either event would have had catastrophic effects on agricultural activity, making the economy no longer sustainable and breaking the civic order of the cities.
- Later, a large group of nomadic cattle-herders, the Aryans, migrated into the region from central Asia. The Aryans crossed the Hindu Kush Mountains and met the Indus Valley Civilization.
- 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.