Sarasvati and the palaeodelta of the Great Rann

Considerable debate has taken place about Sarasvati’s entry in the northern part of the Great Rann. Scholars have pointed to references in Rigveda, Manusmriti and Mahabharata about Sarasvati disappearing in the sands at Vinäsana and not in the sea; but at the same time, there is also reference in some of these ancient texts about a narrow sea, possibly a creek, coming right upto Bikaner, but which disappeared during the Vedic times10,22. Rigvedic and archaeological references describe how Sarasvati supported inland and marine trade and travel and that, around 3000 BC, there was continuous flow of this river upto even the Little Rann13.

The topography at the Great Rann is typically deltaic, developing usually at the mouth of rivers, confirming entry of a few rivers in the sea at this place. Neotectonism, reactivating faults and lineaments which are seen criss-crossing this region, as well as frequent seismicity, apart from Holocene sea-level changes all appear to have influenced development of a peculiar drainage topography in this area. The tilting and sinking of land resulting from the tectonic events have carved characteristic uplands (locally called Bets) representing areas of river mouth deposits, and lowlands which are sites of distributary channels17,28. Satellite imagery, as well as detailed mapping, have revealed network of distributaries and extensive graded deposits, products of Holocene marine regression17. It appears that Indus (Sindhu), Shatadru (Sutlej), Sarasvati, Drishadvati (palaeo-Yamuna) and Lavanavati (possibly an ancestor of present day Luni river) had independent courses and opened into the Rann separately. According to Maliket al.17, at least three rivers – proto-Shatadru (Hakra), Sarasvati and Drishadvati must have drained into the Rann around 2000 BC, of which only Sindhu (Indus) has survived. The original delta complex with relict channels, including that of Nara, a continuation of Ghaggar, is today better preserved on the western side but covered by wind-borne deposits on the eastern part of the Great Rann17,43,44.

Yash Pal et al.32 argue that though in the satellite imagery Sarasvati/Ghaggar appear to debouch into the sea or a lake near Marot or Beriwala (Pakistan) (Figure 3), this place is far interior, and unlikely to be a palaeo-seacoast, even allowing for rise of sea level during the Holocene marine transgression. In fact studies about coast line changes along the west coast have shown a much lower sea level some 12,000 y back which rose to the present level only later and had remained there for the last 7000 y. These findings, therefore, discount the possibilities of a seacoast at this place45,46 though they do not rule out the river’s entry into the sea that must have existed further south of this site in those times. It may be mentioned that Quaternary neotectonism has submerged vast areas of palaeodelta complex, possibly along with palaeochannels. In this context, it is relevant to take note of the observation that Sarasvati’s ancient course in this region is in continuity with another dry river bed–Hakra or Sotra which can be traced through Bikaner to Bhahawalpur and Sind in Pakistan, and finally upto the Rann of Kutch. Such a course appears likely if we backtrack the delta distributaries inland, when it is noticed they connect up with the existing palaeochannels there. Some of these are actually extensions of relict channels seen beneath the sands of Thar Desert, as found out by geophysical and hydrogeological surveys 16,17,35,38.

While tectonism had certainly a major role in shaping the fate of Sarasvati and other rivers, this could not have been the only agent bringing about various changes that led to its downfall. Even though the role of climate on the disappearance of Sarasvati system was underestimated by some of the earlier workers, undoubtedly it must have exercised considerable sway during the Holocene, a period during which major climatic swing has been noted globally26,27,36,47. It is well known that variation in earth’s orbit and tilt of earth’s axis affect the earth’s climate (Milankovitch and albedo forces). A drastic weather change related to these phenomena had peaked around 7000 BC26. Recent studies have shown that the onset of an arid climate occurred in two pulses – at 4700–3700 and at 2000–1700 BC26, both of which had fairly wide impact not only in India in the desertification of western Rajasthan but in other countries also, like Africa in the development of Saharan and Nubian deserts. The desertification is thought to have occurred 5400 y ago (3400 BC) and its onset greatly affected the monsoon rains and consequently the river systems too. The change from wetter to arid condition destroyed steadily the vegetation, which in turn affected soil moisture, its evaporation, atmospheric circulation and precipitation, all important links in the monsoon evolution chain and, ultimately the climate over the region. However, a recent study48 of water-table fluctuations and radiocarbon estimates from the Lunkansar Lake deposit do not support the views about aridity around 3500 BC, the period when Sarasvati and Indus Valley culture were thought to have collapsed. The chronology emerging from these studies show that the once perennial lakes had ceased to be so and they had dried and desiccated more than 1500 y before the dated collapse of the civilization.

Computer based climate simulation studies26, to reproduce the changes to solar heating of the atmosphere due to variations in earth’s tilt and orbit have shown that climate-induced weakening of monsoons over India and north Africa led to desertification in a span of just 300 years. Needless to point out, when one traces the topographic evolution of a place, the influence of a combination of many natural phenomena can be recognized in its build up. It becomes, therefore, very difficult to point out any one reason for some of the major changes to the topography or river systems. The climatic swing that led to sweeping changes in northwestern India was triggered by variations in earth’s orbit and tilt and these departures are known to recur periodically. The latter should, therefore, rise the possibilities for a favourable orientation of these parameters of earth at some future time to initiate climatic conditions for a re-greening of the Rajasthan desert, rejuvenation of the dry river beds and, hopefully, for a rebirth of Sarasvati, like Phoenix out of the ashes.

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