Severe and persistent river droughts, each lasting more than 85 years, affected the Indus Basin between 4,400 and 3,400 years ago, according to a new study. The findings may help explain why this ancient civilization slowly declined and illustrate how environmental factors may have shaped ancient societies.
Artistic reconstruction of the gates and drains of the ancient city of Harappa. Image credit: Chris Sloan.
The Indus Valley Civilization was one of the earliest urban cultures, existing between 5,000 and 3,500 years ago around the Indus River and its tributaries in what is now Pakistan and northwestern India.
At its peak, between 4,500 and 3,900 years ago, its culture was characterized by developed cities and complex water management systems.
However, the reasons for the long decline of civilization after this peak are not yet fully understood.
“The Indus River played a central role in the development of the ancient Indus Valley civilization, providing a stable source of water for agriculture, trade and communications,” said Dr Vimal Mishra of the Indian Institute of Technology Gandhinagar and colleagues.
“Civilization flourished around the Indus River and its tributaries about 5,000 years ago and evolved over time.”
“During the mature Harappan phase (4500–3900 years ago), the Indus Valley Civilization featured well-planned cities, advanced water management systems, and a sophisticated writing system.”
“However, after 3,900 years ago, the Harappan civilization began to decline and eventually collapsed.”
“The reasons for this decline remain controversial. Proposed factors include climate change, seawater retreat, droughts, floods and changing river dynamics interacting with social and political changes.”
“Understanding ancient hydroclimatic events and their social consequences provides critical insight into the vulnerability of complex societies to environmental stress.”
For the study, the authors simulated climate conditions in the Indus Valley region between 5,000 and 3,000 years ago.
They combined these results with data from several indirect measurements of past climate conditions, including the geochemistry of stalactites and stalagmites in two Indian caves, and data on water levels in five lakes in northwestern India.
Based on this, they identified an increase in temperature over the period of approximately 0.5 degrees Celsius and a decrease in average annual precipitation across the region by 10–20%.
They were also able to identify four long droughts between 4450 and 3400 years ago, each lasting more than 85 years and affecting between 65% and 91% of the Indus Valley region.
“These droughts influenced the choice of settlement sites in the Indus Valley Civilization,” the researchers say.
“Between 5,000 and 4,500 years ago, settlements were concentrated in areas with high rainfall.”
“However, starting 4,500 years ago, they moved closer to the Indus River, probably because droughts began to affect water availability.”
“The last 113-year drought they identified between 3531 and 3418 years ago coincides with archaeological evidence of large-scale disurbanization in the Indus Valley Civilization.”
“The Indus Valley Civilization most likely did not collapse suddenly as a result of any climate event, but instead declined slowly, with prolonged droughts being a major contributing factor,” the researchers concluded.
Their paper was published on November 27 in the magazine Connection Earth and environment.
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H. Solanki etc.. 2025. River drought triggering Harappan metamorphosis. Commune Earth Environment 6, 926; two: 10.1038/s43247-025-02901-1
