<P> A question about the river system in Bangladesh is when and why the Brahmaputra changed its main course, at the site of the Jamuna and the "Old Brahmaputra" fork that can be seen by comparing modern maps to historic maps before the 1800s . The Brahmaputra likely flowed directly south along its present main channel for much of the time since the last glacial maximum, switching back and forth between the two courses several times throughout the Holocene . </P> <P> One idea about the most recent avulsion is that the change in the course of the main waters of the Brahmaputra took place suddenly in 1787, the year of the heavy flooding of the river Tista . It is, however, well known that the Tista has always been a wandering river, sometimes joining the Ganga, sometimes being shifted westwards by the superior strength of that river and forced to join the Brahmaputra . </P> <P> In the middle of the 18th century, at least three fair - sized streams flowed between the Rajshahi and Dhaka Divisions, viz., the Daokoba, a branch of the Tista, the Monash or Konai, and the Salangi . The Lahajang and the Elengjany were also important rivers . In Renault's time, the Brahmaputra as a first step towards securing a more direct course to the sea by leaving the Mahdupur Jungle to the east began to send a considerable volume of water down the Jinai or Jabuna from Jamalpur into the Monash and Salangi . These rivers gradually coalesced and kept shifting to the west till they met the Daokoba, which was showing an equally rapid tendency to cut towards the east . The junction of these rivers gave the Brahmaputra a course worthy of her immense power, and the rivers to right and left silted up . In Renault's Altas they very much resemble the rivers of Jessore, which dried up after the hundred - mouthed Ganga had cut her new channel to join the Meghna at the south of the Munshiganj subdivision . </P> <P> In 1809, Buchanan Hamilton wrote that the new channel between Bhawanipur and Dewanranj "was scarcely inferior to the mighty river, and threatens to sweep away the intermediate country". By 1830, the old channel had been reduced to its present insignificance . It was navigable by country boats throughout the year and by launches only during rains, but at the point as low as Jamalpur it was formidable throughout the cold weather . Similar was the position for two or three months just below Mymensingh also . </P>

Which indian river has most number of tributeries