Palaeogeography
The Sarasvati River is mentioned in all books of the Rigveda except the fourth. It is the only river with hymns entirely dedicated to it: RV 6.61, RV 7.95 and RV 7.96.It is mentioned as a divine and large river,which flows "from the mountains to the samudra," which some take as the Indian Ocean. Talageri states that "the references to the Sarasvati far outnumber the references to the Indus" and "The Sarasvati is so important in the whole of the Rigveda that it is worshipped as one of the Three Great Goddesses". However, the reason for the predominance of the Sarasvati in the Rigveda is the late Harappan (1900-1300 BCE) population shift eastwards to Haryana; the latter part of the period corresponds to the common scholarly opinion of the date of this text.
Another reference to the Sarasvati is in the geographical enumeration of the rivers in the late Rigvedic Nadistuti sukta (10.75.5, this verse enumerates all important rivers from the Ganges in the east up to the Indus in the west in a strict geographical order), as "Ganges, Yamuna, Sarasvati, Shutudri", the Sarasvati is placed between the Yamuna and the Sutlej, consistent with the Ghaggar identification. It is clear, therefore, that even if the river had unmistakably lost much of her former prominence, the Sarasvati remained characterized as a river goddess almost throughout the Rigveda.
According to some palaeo-environmental scientists and Archaeologists,between 2500 BCE and 1900 BCE some tectonic disturbances caused tilt in topography of Northwest India resulting in the migration of river. The Sutlej moved westward and became a tributary of the Indus River while the Yamuna moved eastward and became a tributary of the Ganges. The Yamuna shifted its course eastwards, supposedly in the early 2nd millennium BCE, allegedly reaching its current bed by 1st millennium BCE, while the Drishadvati bed retained only a small seasonal flow. The Sutlej several times over shifted its channel northwards and was eventually captured by the Indus system. The water loss due to these movements caused the Sarasvati river to dry up in the Thar Desert.
However, Henri-Paul Francfort, utilizing images from the French satellite SPOT already two decades ago, found that the large river Sarasvati is pre-Harappan altogether and started drying up already in the middle of the 4th millennium BC; during Harappan times only a complex irrigation-canal network was being used. The date should therefore be pushed back to c 3800 BC. R. Mughal (1997), summing up the evidence, concludes that the Bronze Age Ghaggar-Hakra sometimes carried more, sometimes less water (for example derived from the Sutlej). The latter point agrees with a recent isotope study. Painted Grey Ware sites (ca. 1000 BCE) have been found in the river bed and not on the banks of the Ghaggar-Hakra river, which suggests that river was certainly dried up by this period.
The Rig Vedic hymn X 75, however, gives a list of names of rivers where Sarasvati is merely mentioned while Sindhu receives praise. It is commonly agreed that the tenth Book of the Rig Veda is later than the others. Some revisionists think, ahistorically, that this may indicate that the Rig Veda could be dated to a period after the first drying up of Sarasvati (c. 3500 BCE) when the river lost its preeminence.
Scholars, however, commonly date the Rig Veda to after the Indus Valley culture, arguing for example, that the lack of clear evidence of domesticated equids at Indus Valley culture sites contrasts with the Rig Veda's frequent references to domesticated horses. Scholars also interpret frequent use in the Rig Veda of the word "ratha", which in later Sanskrit can mean any kind of carriage, to be references specifically to horse-drawn, spoked-wheeled war chariots, whereas the only carts (called 'anas' in Vedic) found at Indus Valley culture sites are solid-wheeled bullock carts. There are indeed a number of mentions in the Rig Veda of spoked (ara) wheels, horse-drawn chariots and the use of chariots in sport, competition and battle, including also the deity Indra's vehicle. Indra is described as throwing his vajra weapon from a heavenly "ratha" pulled by two "hari" horses, a noun form of "bay".
The many archeological sites along the bed of Sarasvati (variously given as 414 or even 600) dwarf the number of sites so far recorded along the Indus River, which number less (about three dozen). However, most of the Harappan sites along the Sarasvati are found in desert country, and have remained undisturbed since the end of the Indus Civilization. This contrasts with the heavy alluvium of the Indus and other large Panjab rivers that have obscured Harappan sites, including part of Mohenjo Daro. About 80 percent of the Sarasvati sites are datable to the fourth or third millennium BCE, suggesting that the river was flowing during (part of) this period, which is also indicated by the fact that some Indus sites are found inside the bed of the Ghaggar-Hakra.
Read more about this topic: Ghaggar-Hakra River