The Indus Valley Civilization was among the pioneers of Bronze Age technology, its advances in metallurgy and measurement influencing cultures across Asia, Europe, and North Africa. Yet archaeologists have been puzzled by one feature: many of the civilization’s greatest cities lacked a nearby permanent water flow to meet their needs. The dating of an old riverbed demonstrates much of the area really did flourish without a great river.
Even today, the vast majority of the world's cities lie on sources of fresh water. Urban areas that outgrow the rivers that once fed them rely on great feats of engineering to bring water from neighboring valleys. How, then, could some of the largest settlements in the world 4,600-3,900 years ago have sat between the Ganges and Indus river systems, not close enough to either to survive?
In the 19th century, geographers discovered that the glacial-fed permanent Sutlej River once ran past several of these great metropolises. Case solved? Not quite. A paper in Nature Communications reports that dating of the sediments in Sutlej's former and current routes show the major tributary of the Indus had taken its current course by 8,000 years ago, long before these sites became substantial settlements.
When populations eventually congregated along the old riverbed (paleochannel), any water that flowed was seasonal – a product of the monsoon. Exactly how people managed to collect enough water to support a civilization through the dry season remains a mystery, particularly since there is no evidence of major dams. Groundwater left over from the era when the Sutlej flowed through the valley may have been a factor.
However, senior author Professor Sanjeev Gupta of Imperial College London argues the Sutlej's absence would have had its advantages. The intact rivers of the region are subject to devastating floods, frequent enough to have threatened cities. Even in a rainy year, however, the temporary flow through the paleochannel was probably insufficient to threaten those on its banks.
Where once it was thought the diversion of the Sutlej caused the abandonment of the population centers along its former banks, it now seems it was this redirection that made its former course so suitable for cities.
In the soft soils of deltas, rivers can constantly change their course, but usually fairly subtly. On the other hand, landslides or the dramatic collapse of an upstream riverbank can occasionally trigger major changes in a river's route. In 2008, the breaking of a beach of a levee of the Kosi River, northern India, caused the entire waterway to re-route 60 kilometers (40 miles) eastwards. Often, as in the Kosi River's case, such changes cause rivers to return to a route they had followed once before, sometimes switching back and forth several times.