Friday, June 15, 2012

Fluvial History And The Fortunes Of The Harappan Civilization

In a recent issue of PNAS Liviu Giosan use a combination of high resolution topographic data, geomorphologic analysis and sediment dating to establish a chronology of the evolution of fluvial landforms of the Indus and its tributaries.

The Harappan civilization over a 600 year period from around 4500 B.P (before present) to about 3900 B.P flourished in this region and then went into decline with urban centers abandoned and populations moving eastwards towards the Himalayan foothills and the Gangetic plains. Goisan's work - and there is also independent evidence for this -  shows that this 600 year period was a kind of a Goldilocks period. The region became arid, but not too arid.

Indus and its tributaries get water from two climatic regimes, the summer monsoons from the Arabian sea and winter rains from the northwesterly winter disturbances bringing moisture from the Mediterranian, Caspian and Black sea region. Most of the sediment load of these rivers is generated during the heavy erosion that takes place in the Himalayas during the summer monsoons. When rivers carry and deposit sediment along their course they are said to be in an aggradational mode i.e. the stream bed and the surrounding floodplains get raised as more and more sediment is deposited.

Over much of the earliest part of the Holocene, the Indus and its tributaries were aggradational. Then the monsoons weakened and the sediment load reduced. Winter rains falling as snow though kept river discharge active. Rivers without sediment or less sediment incise or cut into their own deposits. So, by mid Holocene all these rivers had developed a characterized profile of incised valleys and river terraces marking the original river bed and broad surrounding plains that because of  reduced rainfall were less prone to severe flooding.

Giosan suggest that the Harappan people took advantage and adapted to these circumstances. There are Harappan and even pre-Harappan sites within these incised valleys of the Indus and its tributaries like the Beas and the Sutlej. This suggests that incision had occurred in the early Holocene and agriculture evolved to take advantage of manageable floods and a perennial water supply within the valleys and adjoining plains. 

After around 3900 B.P. the aridification intensified. The agricultural heartland of this civilization was along the  Ghaggar/Hakra river, located between the Sutlej and the Yamuna. This river was monsoonal fed and would have been perennial until then. As it slowly dried up, urban population centers could not be sustained and the Harappan civilization went into decline. There was a movement of people to the east towards the wetter Himalayan foothills.

This eastward civilizational shift is captured well in the figure to the left (Source Giosan Red dots are Harappan sites. White dots are  Painted Grey Ware sites. Painted Grey Ware is a cultural phase that overlaps with the late Harappan.

This study also lends additional support that the Ghaggar river which has been identified as the mythical Saraswati river mentioned in the Rig Veda was not a glacial river in the Holocene. It has been proposed that the Sutlej and the Yamuna flowed into the Ghaggar and changed course around 4000 B.P to 3900 BP, triggering a water crisis. This study points out that the landforms of incised valleys typical of these glacial rivers are not present across the Ghaggar Hakra region. Sediment dates of river terraces along the Sutlej also indicate that the Sutlej had incised and was flowing along its present course by late Pleistocene.

Work by Clift, using zircon dating to fingerprint sediment and Himalayan catchment areas of these rivers, which I reviewed in an earlier post  indicates that although the Sutlej and Yamuna likely did flow into the Ghaggar in the Pleistocene they changed course towards the Indus and the Ganges by late Pleistocene, thousands of years before civilization was established in this region. And there is some independent isotope studies on ancient water in buried channels along the Ghaggar that also suggests that there were no glacial rivers flowing into the Ghaggar during the Holocene.

Sometime back I got involved in a debate with indologist Michel Danino on my blog. I had pointed out that glacier rivers like Sutlej and the Yamuna have developed a characteristic morphology of a relatively narrow belt of incised valley and terraces. As has been suggested, if such large perennial rivers were flowing across the plains of Punjab in the upper reaches near the Siwaliks and joining the Ghaggar they would have prevented the formation of the alluvial fans systems that have developed in the upper reaches of the interfluve areas around Ghaggar.

Michel though was convinced that the Sutlej had flowed into the Ghaggar until in fact medieval times. I bring this debate up because Michel, who is not a geologist, ultimately came to this conclusion based on his reading of the geological literature. And here, I think, that Indian geologists working on this problem have a lot to answer to.

That the Sutlej or Yamuna may have flowed into the Ghaggar in the past is a perfectly reasonable geological hypothesis. But until these recent studies by Clift and Giosan et. al. there was no convincing evidence for it. Earlier, geological evidence of a glacial Ghaggar/Saraswati has been based primarily on the satellite mapping of dried up channels near the Ghaggar river which indicated that in the past the Ghaggar was a bigger river. But, these channels were never shown to be connected to the present day glacially connected Sutlej and Yamuna basins. They could just as well be interpreted as belonging to a monsoon fed river originating in the lower Himalayan ranges. Nor was there any firm sediment provenance that could identify a specific Himalayan source for these channels or a sediment chronology to establish the long term fluvial history of this region.

Paleobotanical and sedimentological criteria had always indicated that increasing aridification and reduction in monsoon strength better explained the drying of the Ghaggar around 3900 B.P.  Despite all this, the Sutlej or the Yamuna changing course at around 3900 B.P  became the favored explanation for the drying of the Ghaggar. This scenario of a once large Ghaggar neatly fitted the description in the Rig Veda of a mighty Saraswati, a holy river that just like the Ganges was thought to have its source in the high glacial Himalayas. I suspect that the glacial river theory had more emotional appeal and gained acceptance among some geologists.

The strong assertions by geologists that the diversion of glacial rivers from the Ghaggar coincided with the decline of the Harappan civilization was used by archaeologists like Prof. B.B. Lal to place the composers of the Rig Veda on the plains of the Punjab before the Ghaggar dried up, apparently bolstering the theory that the Harappan people and the Vedic people were one and the same. A geological narrative constructed without rigorous evidence has been promoted to support a theory of cultural evolution in northwest India.

Unfortunately, this glacial past of the Saraswati timed to the demise of the Harappan civilization is now enshrined in textbooks written by senior geologists like K.S. Valdiya. They should now be revised or at the very least these geologists  need to admit that their theory has been seriously challenged. If geologists working on this problem still want to stick to the theory of a glacial Saraswati, they will need to come up with a more convincing data driven rebuttal to the work of Clift and Giosan et. al.

Giosan, L., Clift, P., Macklin, M., Fuller, D., Constantinescu, S., Durcan, J., Stevens, T., Duller, G., Tabrez, A., Gangal, K., Adhikari, R., Alizai, A., Filip, F., VanLaningham, S., & Syvitski, J. (2012). PNAS Plus: Fluvial landscapes of the Harappan civilization Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1112743109

Additional Posts:
K.S.Valdiya on the glacial Saraswati in Current Science

1 comment:

  1. If the map didn't have the Arabian Sea and the Lat-Long marks as reference, I might have concluded that the Himalayas were distributaries ^_^