Bulletin No. 12 of National Geogrpahical Society of India Banaras

Fergusson3 considered that the combined Kosi and the Mahananda then probably flowed farther east to join the old course of the Brahmaputra to the west of the present position of Dacca. [...] Likewise the confluences of the Gogra the Son and the Punpun with the Ganges have all shifted to the west. [...] the Gandak the Gogra the Ganges the Son and the Punpun. [...] But one thing is certain that throughout the Medieval period and up to the middle of the 14th century there were two rivers instead of one flowing in parallel courses to the sea and these two rivers divided between themselves the vast volume of water from the five rivers of the Punjab as well as from the old Ghaggar and Chitang to the east. [...] Conclusion It is noteworthy that such a marked westward migrtion of the Brahmaputra the Bhagirathi the Kosi the Gandak the Gogra the Son the Sutlej the Ravi and the Indus is not a fortuitous coincidence.