No river meant no floods for ancient Indus settlements

Kalibagnan Archaeological Site Indus CivilizationThis remnant of a street was excavated at the site of the Indus Civilization urban center of Kalibangan, which was situated along the Ghaggar-Hakra palaeochannel. Credit: S. Gupta, Imperial College London

Large Middle Eastern rivers like the Nile, Tigris, Euphrates and Tiber were critical for the development of early urban societies in Egypt, Mesopotamia and Ancient Rome. And researchers have long thought that the rise, and eventual decline, of cities in the ancient Indus Civilization, which spread across about 1 million square kilometers of what’s now northwestern India and Pakistan from roughly 4,600 to 3,900 years ago, also depended on major rivers, namely those emerging from the Himalayas. But a new study looking at river sediments from the time of the civilization and earlier suggests that wasn’t the case for every ancient Indus city; some may have benefited from being farther away from large rivers and their periodic floods.

Topographic Map of Indus CivilizationThe locations of most known Indus Civilization sites, including Kalibangan, are not situated along major present-day Himalayan rivers. Credit: P.J. Mason/S. Gupta, Imperial College London; data for map courtesy of NASA and U.S. Geological Survey

Geologist Sanjeev Gupta of Imperial College London and his colleagues were particularly interested in studying sediments beneath a large paleoriver channel, called the Ghaggar-Hakra, along which many urbanized Indus city-states were located. A prevailing idea among many archaeologists has been that major changes to the river’s location and flow through the Ghaggar-Hakra caused the channel to dry up, leading to the abandonment of the settlements built near the river. But Gupta’s team concluded that the river dried up before the occupation of these urban settlements. While the paleochannel was adjacent to the Indus city-states, the river “didn’t exist during the time of the civilization,” Gupta says.

The researchers wanted to study sites close to a big Indus city, so they drilled five sediment cores along a transect perpendicular to the Ghaggar-Hakra paleochannel, two of which were just kilometers from the archaeological remains of Kalibangan, a major Indus Civilization community.

Gupta and his colleagues then analyzed the core sediments, dating minerals in each layer using radiometric and luminescence techniques. They found sediments deposited between 60,000 and 8,000 years ago that indicated a large river likely flowed through the paleochannel during that time. Strong rivers in this region carry sand and larger sediments eroded from the foothills of the Himalayas. But sediments in the cores that were deposited after 8,000 years ago were composed of only smaller-grained silts and clays. This indicated that the paleoriver was no longer strong enough to carry sand or larger grains after that time, suggesting a substantially weakened flow through the channel or a drying of the paleoriver.

Landsat Imagery Indus CivilizationThe path of the Ghaggar-Hakra palaeochannel can be seen in this composite Landsat image. The modern Sutlej River may have flowed through the paleochannel prior to the development of Indus Civilization cities. Credit: P.J. Mason/S. Gupta, Imperial College London; Landsat imagery courtesy of NASA Goddard Space Flight Center and U.S. Geological Survey

Gupta says he thinks less water flowed through the paleochannel after 8,000 years ago because the river had jumped to a new channel. The team analyzed sediments from nearby modern rivers and compared them to sediments older than 8,000 years from the Kalibangan archaeological site.  The results suggested the Sutlej River — which today lies roughly 100 kilometers to the northwest — is the large river that once flowed through the channel. The valley abandoned by the river, meanwhile, would have been “an ideal site for urban development because of its relative stability compared to Himalayan river channel belts that regularly experience devastating floods,” the researchers wrote.

The new work is “an excellent contribution” and sheds light on a longstanding question about the amount of water flowing from glaciers through the Ghaggar-Hakra-Saraswati river system, says J. Mark Kenoyer, an anthropologist at the University of Wisconsin-Madison. This study “provides strong evidence that there was less water flowing from glaciers in this system [during the time of the Indus Civilization] than previously thought,” he says.

If the river that filled the Ghaggar-Hakra paleochannel was already gone by the time Indus cities lining it arose, then these settlements’ demise couldn’t have been linked to the river drying up. So, Gupta says, “the jury is still out as to what caused the decline of the [Indus] civilization.”

However, Kenoyer says, the work “does not address the issue [of] water flow farther down the river system,” where other large Indus settlements, like Mohenjo-Daro, were located. So, he says, how the big Himalayan rivers affected or didn’t affect the decline of these other large urban settlements of the Indus Valley remains a mystery.

Alex Lopatka

Lopatka is an extern at EARTH interested in the geosciences and their intersection with policy.

Tuesday, January 23, 2018 - 06:00

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