Tuesday, January 9, 2018

Note On The Sutlej Paleochannels

The topographic relief rendition posted below shows beautifully the incised valleys of the glacially sourced Yamuna and Sutlej rivers. This rendition has been derived from the NASA Shuttle Radar Topography Mission (SRTMv3) DEM (Digital Elevation Model) with a 1 arc-second or 30m spatial resolution. The region depicted in the figure is immediately west of the Himalaya frontal ranges covering parts of Punjab and Haryana.


Source: Ajit Singh et. al. 2017 - Counter-intuitive influence of Himalayan river morphodynamics on Indus Civilisation urban settlements.

This incision began during the early Holocene, beginning about 10,000 to 8700 years ago and continuing over the next few thousand years, as proposed in an earlier study by Liviu Giosan and colleagues. A decline in monsoon strength over northwest India resulted in low sediment load carried by the rivers. Under such conditions, starved of sediment, the river starts cutting down or incising into its older deposits. Over time they carve out large valleys as the Yamuna and Sutlej have.

During the mid Holocene, from about 6000 years to 3800 years ago, the region between the Yamuna and the Indus was extensively settled and farmed by the Harappan people. The river Ghaggar flows through this region. One popular theory supported by many geologists was that during Harappan times the river Sutlej flowed into the river Ghaggar, switching to its present course only about 4000 years ago. However, Giosan and colleagues had argued that had that been the case, a large incised valley should have been carved by the Sutlej from the point it exits the Himalaya to the point it joins the Ghaggar. The absence of a wide NE-SW oriented incised valley in the interfluve between the Yamuna and the Indus indicates that the Sutlej did not flow into the Ghaggar during most of the Holocene.

 A recent study led by geologist Sanjeev Gupta (Ajit Singh et. al. 2017)  has validated this scenario using geochemical criteria. They have shown that the Sutlej river once did flow into the Ghaggar but changed course and joined the Indus in the late Pleistocene -early Holocene between 15000- 12,000 years and 8000 years ago. Additional data from Giosan and colleagues shows (SI Text) fluvial deposits of Late Pleistocene-Early Holocene age (latest being 10,000 years old) along the present day Sutlej floodplain. These staggered dates imply that a major channel of the Sutlej avulsed or changed course as early as 15000 to 12000 years ago. A smaller strand of the river continued to flow into the Ghaggar until about 8000 years ago or so.

All this means that the Harappan settlements and agriculture in this region was not sustained by a large perennial glacial fed river. Rather, the Harappans adapted their water usage strategy and farming practices to exploit a smaller and maybe an ephemeral river and more distributed water sources.

The geochemical  work by Gupta and colleagues has been rightly praised and highlighted in many media reports. What did go unnoticed and unappreciated was the relief rendition of the incised channels. They provide a very powerful visual representation of the Holocene fluvial history of this region.

The modified relief rendition below also shows the course of the abandoned Sutlej incised valley. Note that this valley is much narrower than the Sutlej and Yamuna incised valleys. Also, trace these narrower incised valleys upstream and you can see that they originate in the Siwaliks. There are no deep extensive incised valleys along the route I have marked in blue. The Sutlej would have carved a prominent incised valley roughly along the blue route had it been flowing into the Ghaggar during most of the early and mid Holocene. Its absence suggests to me that the valley annotated as the abandoned Sutlej incised valley was really carved out in the earlier part of the Holocene by the smaller Ghaggar river originating in the Siwaliks.


Modified from :  Ajit Singh et. al. 2017 - Counter-intuitive influence of Himalayan river morphodynamics on Indus Civilisation urban settlements

Aside: After Liviu Giosan's paper came out, the archaeologist Shereen Ratnagar asked me whether incised valleys are diagnostic of glacial rivers. She was puzzled because the monsoonal rivers Marakand, Ghaggar and a number of smaller streams which originate in the Siwaliks have also carved incised valleys. The answer is no, they are not. What Giosan's work was pointing out was that wide incised valleys of a particular telltale orientation were absent, thus providing a clue as to when the Sutlej changed its course.

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