During the rainy season these moribund rivers act as excellent drainage channels draining off a large volume of water and have a considerable current. Most of these rivers are narrow and flow in well-defined channels. Tangail District is flanked on the west by the mighty river Jamuna, which is nowhere less than 4 miles wide during the rainy season. The Dhaleshwari, first an old channel of the Ganges and then of the Brahmaputra, cuts across the south-western corner of the district on its powerful sweep to join the Meghna near Narayanganj. The old name of Dhaleshwari was "Gajghata". It used to flow afterwards by the Salimabad Channel and then at last by Porabari Channel. A part of the eastern boundary of the district runs close to the Banar River. The river Bangshi flows almost down the middle of the district, branching out from the old Brahmaputta to the north from near Jamalpur. Bangshi falls into Dhaleswari near Savar, in Dhaka district. The Bangshi forms a natural barrier to the Madhupur Jungle on the Tangail side, all the way from Madhupur to Mirzapur. It is fordable at only two or three places near Basail on its my to river Meghna. Dhaleswari itself however takes out from the Jamuna from inside Tangail district. Among other important rivers of the district, Lohajang is worth mentioning. It flows past the district headquarters of Tangail and is almost dead at present (in moribund condition). Other rivers are Khiru, Nanglai, Atia, and Jhinai. The old Brahmaputra's most important offshoot is the Jhinai; striking off near Jamalpur it rejoins the Jamuna north of Sarishabari, while another branch flows past Gopalpur. Now these sub-systems of rivers, viz, Bangshi and Banar, and the Lohajang, Khiru, Nangtai, Atia and Jhinai are all dying out because of the shift of the old Brahmaputra river from its former channel to the present Jamuna channel. 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.