It is believed that the Bhagirathi was the main flow of Ganga, hundreds of years ago. The present channel of the Bhagirathi, with its sacred traditions and ruined cities, marks the ancient course of the river Ganga. Captain Sherwill said it was the main river from Rajmahal (রাজমহল) to Sagar Island (সাগর দ্বীপ) in olden days, practically along the course of the present Hooghly River, which in due course became insignificant. The main river appears to have frequently changed its course below Gour in the last six centuries and successively discharged into the sea at different mouths, such as Matla, Kalinai, Kabadakh and Haringhata in the Sunderbans.
The original river Ganga used to flow across the entire north and east India from Uttarakhand (a new province carved out of Uttar Pradesh on 9th November 2000) to West Bengal (then only Bengal) before the 16th century. Geologists say, before it diverted to the Padma (পদ্মা)eastward, there might have been two major channels, flowing more or less independently and building the deltaic tract in the part of Bengal, west of Madhupur jungle, viz.., the Ganga flowed through central Bengal and the Teesta (তিস্তা)through south Bengal. Earlier, the Teesta was reinforced by the Mahananda (মহানন্দা) and the Kosi (কোসি)and still earlier, perhaps also by the Brahmaputra (ব্রহ্মপুত্র) before it coursed eastward to the Meghna (মেঘনা), i.e., before it merged with the Tsan Po (সান পো) of Tibet as a much smaller stream than now. These north Bengal Rivers flowed and fell together into the sea, probably through the meghna estuary. This hypothesis fits in with the historical and mythological evidences, supporting the contention that the Bhagirathi was the main flow of the Ganga in olden days.
Bhagirathi was the main trading link between north India and the south Asian countries, through the Bay of Bengal. Sir William Willcock (the renowned irrigation engineer) described the Bhagirathi, the Jalangi, and the Mathabhanga as the “overflow irrigation systems” in ancient Bengal, built up by great engineers like Bhagirath. Other experts believed that the Bhagirathi was a natural river and was once the main channel of the Ganga, diverting its discharge towards the sea. From time to time like other rivers too Bhagirathi has changed its course affecting the rise and fall of many cities on its banks, as it happened in the case of Murshidabad.
Account of Change of Course of Bhagirathi:
Gradually silting of Bhagirathi caused it to change its course. Captain Sherwill, in his Report on the Rivers of Bengal, quotes an extract from a letter written, In January 1666 AD by French traveller Jean-Baptiste Tavernier that he saw the mouth of Bhagirathi by boat, closed by sand bank (Tavernier’s voyages in India). In 1683 AD William Hedges travelled on a Palki (পালকি) on his way to Cossimbazar from Mahula (মহুলা), because of shallow water on the river. John Zephaniah Holwell on his way to Murshidabad by boat, was detained by shallows at Shantipur below the confluence of the Bhagirathi and Jalangi, in 1756 AD. In 1781 AD Major James Rennell, the famous English geographer, historian and a pioneer of oceanography, surveyed Bengal, during his work at India and sketched up the most detail map of river Bhagirathi. From his map of “Cossimbazar island”,1781 AD we get a vivid description of the river Bhagirathi in this region.