Friday, July 13, 2018

Sutlej Paleochannels- More Details

A while back I had written a post in response to a paper (Ajit Singh et. al. 2017) on the paleo-Sutlej river. The study used geochemical analysis to identify ancient channels of the river. Today, the Sutlej flows out of the Himalaya and joins the Indus River. But this study showed that until about 8000 years ago, the Sutlej (or at least a strand of the river) flowed along a different course. Its paleo-channels coincide with  the course of the river Ghaggar in Haryana and Rajasthan in northwest India. This topic is of relevance in studying how rivers may have impacted agricultural practices and settlement patterns of the Harappan Civilization. The geography of the river Ghaggar also matches that of the river Saraswati, described in the Rig Ved. There is therefore considerable interest in working out the detailed history of these rivers.

I had pointed out that an earlier paper by Liviu Giosan and colleagues has used topographic criteria to come to a similar conclusion as Ajit Singh and colleagues. Giosan's study stressed that today the Sutlej and Yamuna flow along deeply incised valleys that were cut in the early Holocene (~10,000 to 8700 years ago). The absence of such valleys in the region between the present day Sutlej and Yamuna indicates that the Ghaggar channel was not being fed by glacially sourced rivers since 10,000 -8700 years ago.

The relief rendition below is from Ajit Singh's paper. It shows clearly the incised valleys of the Sutlej and Yamuna. I had overlain the blue line and suggested that if the Sutlej had flowed into the Ghaggar in early Holocene, there should have been an incised valley along the blue line.

As it happens, Sanjeev Gupta, who was the lead scientist of the study by Ajit Singh and colleagues, is a reader of my blog. He emailed me and has provided more insights regarding these paleo-channels.

I am posting his comments below with his permission.

My comment from the earlier post- 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.

Sanjeev Gupta's reply - Just to respond... we only see the incised channel in the SRTM where the valley is not completely infilled. Indeed where we see the valley in SRTM it is not the base of the incised valley. but a partially infilled valley. So along the blue line you have drawn there is likely to have been a valley but it is entirely infilled.

My comment (with regards to the 'abandoned Sutlej incised valley' in the above relief rendition) - (it)....was really carved out in the earlier part of the Holocene by the smaller Ghaggar river originating in the Siwaliks.

Sanjeev Gupta's reply- This is not possible because all the geochemistry signature is of the Sutlej - the base of the valley actually occurs in the stratigraphy.

We have some newer data that better constrain the timing of incision but I stress the topographic surface is not the base of the incised valley - that lies in the subsurface.


So, Sanjeev Gupta's view is that present day topography is not necessarily a more reliable guide to the course of these ancient rivers. Geochemical fingerprinting is the way to go.

1 comment:

  1. The truth is as always in the middle Suvrat. The modern landscape is the continuously degrading reflection of past landscapes. Some features and some areas preserve more of the past at the surface than others. Combined geomorphological analysis, subsurface geophysics/geology and geochemistry fingerprinting is a must.