Severe and persistent river droughts, each lasting longer than 85 years, affected the Indus basin between 4,400 and 3,400 years ago, according to new research. The findings may help explain why this ancient civilization slowly declined, and highlights how environmental factors could shape ancient societies.
An artist’s reconstruction of the gateway and drain at 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 modern-day Pakistan and northwest India.
At its peak, between 4,500 and 3,900 years ago, its culture was characterized by advanced cities and sophisticated water management systems.
However, the reasons for the civilization’s prolonged decline after this peak are not yet fully understood.
“The Indus River was central to the development of the ancient Indus Valley Civilization, providing a stable water source for agriculture, trade, and communication,” said Dr. Vimal Mishra from the Indian Institute of Technology Gandhinagar and colleagues.
“The civilization flourished around the Indus River and its tributaries around 5,000 years ago and evolved over time.”
“During the Mature Harappan stage (4,500-3,900 years ago), the Indus Valley Civilization featured well-planned cities, advanced water management systems, and a sophisticated writing system.”
“After 3,900 years ago, however, the Harappan Civilization began to decline and eventually collapsed.”
“The causes of this decline remain debated. Proposed factors include climate change, seawater retreat, droughts, floods, and shifting river dynamics, interacting with social and political changes.”
“Understanding ancient hydroclimatic events and their societal impacts provides critical insight into the vulnerability of complex societies to environmental stress.”
For the study, the authors simulated climate conditions across the Indus Valley region between 5,000 and 3,000 years ago.
They combined these results with data from several indirect measures of past climate conditions, including the geochemistry of stalactites and stalagmites in two Indian caves, and the water level records of five lakes in northwest India.
From this, they identified a temperature increase over the period of approximately 0.5 degrees Celsius, and a decrease in average annual rainfall across the region of between 10% and 20%.
They were also able to identify four long-term droughts between 4,450 and 3,400 years ago, each lasting more than 85 years and affecting between 65% and 91% of the Indus Valley region.
“These droughts influenced settlement location choice in the Indus Valley Civilization,” the researchers said.
“Between 5,000 and 4,500 years ago, the settlements were concentrated in areas with more rainfall.”
“However, from 4,500 years ago onwards, they shifted closer to the Indus River, probably as the droughts started to affect water availability.”
“The final 113-year-long drought that they identify, between 3,531 and 3,418 years ago, coincides with archaeological evidence of major deurbanization in the Indus Valley Civilization.”
“The Indus Valley Civilization likely did not collapse suddenly as a result of any one climate event, but instead declined slowly, with the prolonged droughts a major contributory factor,” the researchers concluded.
Their paper was published November 27 in the journal Communications Earth & Environment.
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H. Solanki et al. 2025. River drought forcing of the Harappan metamorphosis. Commun Earth Environ 6, 926; doi: 10.1038/s43247-025-02901-1
