Using archaeological data and geoscience technology, a team of scientists has shown that a major, gradual decline in monsoon rains led to the collapse of the ancient Harappan culture, which relied on river floods to sustain its system of agriculture.
     Once extending more than 1 million square kilometres across the plains of the Indus River from the Arabian Sea to the Himalayas and the Ganges - over what is now Pakistan, northwest India and eastern Afghanistan - the Indus civilisation was the largest but least known of the first great urban cultures that also included Egypt and Mesopotamia. Now mostly arid desolation, over 4,000 years ago the area supported a sophisticated urban culture with various trade routes and maritime connections with Mesopotamia, standards for building construction and sanitation systems, the arts, and a writing system that still eludes epigraphers.
     Between 2003 and 2008 the team developed and analysed digital landform maps of the Indus Valley area, then probed the ground to determine origins and ages of sediments, eventually developing a 10,000-year chronology of landscape change.
     "We reconstructed the dynamic landscape of the plain where the Indus civilisation developed 5200 years ago, built its cities, and slowly disintegrated between 3900 and 3000 years ago," said geologist Liviu Giosan of the Woods Hole Oceanographic Institution (WHOI), and lead author of the report.
     At first, the declining monsoon rains actually played a role in the rise of the Harappan civilisation. Adds Giosan: "As monsoon drying subdued devastating floods, the land nearby the rivers - still fed with water and rich silt - was just right for agriculture. This lasted for almost 2,000 years." By about 3900 years ago the river system had dried to the point where the Harappans were compelled to move and disperse eastward toward the Ganges basin, where the monsoon rains were still plentiful and more reliable.
     Dorian Fuller, an archaeologist with the University College London and co-author of the report, explains: "cities collapsed, but smaller agricultural communities were sustainable and flourished. Many of the urban arts - such as writing - faded away, but agriculture continued and actually diversified."
     The researchers also believe they have discovered the mythical river Sarasvati, portrayed in ancient Sanskrit scriptures as "surpassing in majesty and might all other waters", and long considered lost. They have evidence that the current Ghaggar-Hakra river was the ancient Sarasvati, based on sedimentary, topographical, and archaeological data of settlement near the river during the Harappan era. Their findings suggest the ancient river was fed by perennial monsoons - not Himalayan glaciers, as was previously supposed - and that the increasingly arid climate reduced it to the short seasonal flows of today.

Edited from Popular Archaeology (28 May 2012)