The most important question in connection with the river system of Tangail vis-a-vis Mymensingh district is when and why the Brahmaputra River changed its main old channel. During the last 150 years or so, this diversion of the old Brahmaputra to its present Jamuna channel has considerably prompted the geographers and geologists to enquire deep into it. Two theories are advanced: As explanation of the diversion, one theory describes the gradual uplift of the Madhupur Tract and a final trigger action of the Teesta diversion in 1787 as the chief factor; and the other theory states that the Brahmaputra diversion resulted directly from a major increase in its volume of water due to beheading of the Tsangpo river of Tibet by Dihang, a tributary of the then small Brahmaputral. It has now been proved that the great Tibetan river Tsangpo joined the Brahmaputra about 1780 and this accession was more important than the Teesta floods in deciding the Brahmaputra to try a shorter way to the sea.