Among the sacred rivers of India, the absent presence of the Sarasvati has long been a source of speculation. At the sangam in Prayag (Allahabad), she is supposed to form the third of a triveni, her underground flow inter-braiding with the visible waters of the Ganga and Yamuna. But when did this river, so vividly described in the Vedas, disappear from view and why

Michel Danino&rsquos book on the Sarasvati draws us away from the Indo-Gangetic plain and directs our attention at what is today Haryana and Rajasthan. Until about 4,000 years ago, it was this now-arid region that was watered by the Sarasvati, a mighty river that flowed from the Himalayas to the Arabian Sea, a distance of 1,500 kilometres. The Vedas were composed by people living on its grassy banks, for whom the river was also the goddess of speech and learning.

I have never been particularly keen on ancient Indian history and archaeology. But I was at once fascinated by the thought of present-day Haryana, not exactly known for either learning or poetry, being the fount of the sacred Hindu hymns. Even if it was all of 4,000 years ago. I was even more intrigued by the thought that this desert-like region was once richly fertile and densely populated. And then the Sarasvati disappeared, her wide bed abandoned. Three thousand years later, the same dried-up watercourse provided a convenient route into the subcontinent for Mahmud of Ghazni and, later, Ibn Battuta and Timur. The British eyed the passage with the same military motive as Mahmud and Timur, but oriented in the opposite direction troops could move swiftly from Delhi to Sind, &ldquocamels may march by it fifty abreast on either side of a column of troops&rdquo.

From being a major river to a military route through the desert &mdash what explains the Sarasvati&rsquos demise Danino deftly moves between ancient religious texts, the research of early colonial geographers and recent findings that rely on more sophisticated techniques, to triangulate the shifting contours of this landscape. He shows how the Sarasvati was once fed by the waters of the Sutlej and Yamuna, and how geological changes moved these tributaries away &mdash a &lsquodouble desertion&rsquo that slowly starved the river to death. The traces of that once-mighty river still remain in what is now the seasonal stream of the Ghaggar-Hakra.

Danino&rsquos narrative is part-detective deduction &mdash stumbling along with British explorers and German Indologists, puzzling over facts that don&rsquot add up &mdash and part scholarly argumentation, taking on Irfan Habib and other historians with tart polemical posers. His style brings arcane academic debates to life he takes sides, proposing just how fragments of the past should be stitched together to create a whole cloth. In a field frustratingly full of factual uncertainties, Danino masterfully deploys his strategy of careful conjecture to, bit by bit, create a compelling story.

There is first the scriptural evidence the most ancient Indian text, the Rig Veda, describes the great Sarasvati and its environs with precision. Subsequent Vedic literature &mdash the Mahabharata, the Brahmanas and Puranas &mdash corroborates this description, and goes on to mention the river&rsquos break-up and recession. Then comes the evidence from 19th-century British geographers who correlate the Sanskrit texts with their empirical findings. In the next century comes the archaeological evidence excavations show a large number of settlements along the river's course, dating back to the Indus Valley civilisation. Of these, Kalibangan is the best known today. Changes in the distribution pattern of these sites suggest that the Sarasvati started shrinking 5,000 years ago and, by 1900 BCE, much of its central basin was dry. But if the Sarasvati died during the Indus Valley period, how could she be described and praised in the Vedas which, according to Max M&uumlller, were composed by &lsquoAryans&rsquo who settled in the region after 1400 BCE &mdash four centuries after it had dried up Why would the Rig Veda call a long-dead stream &ldquomighty&rdquo, &ldquoimpetuous&rdquo and &ldquobest of rivers&rdquo To find out, read The Lost River.