Editing Geography of Bangladesh (section)

==River systems==
[[Image:Ganges River Delta, Bangladesh, India.jpg|thumb|right|300px|Ganges River Delta, Bangladesh and India]]
[[File:Urban-Rural Population and Land Area Estimates, v2, 2010 Bangladesh (13873798283).jpg|thumb|300px|Population density and height above sea level in Bangladesh (2010). Bangladesh is especially vulnerable to [[sea level rise]].]]
The rivers of Bangladesh mark both the physiography of the nation and the life of the people. About 700 in number, these rivers generally flow south. The larger rivers serve as the main source of water for cultivation and as the principal arteries of commercial transportation. Rivers also provide fish, an important source of protein. Flooding of the rivers during the monsoon season causes enormous hardship and hinders development, but fresh deposits of rich silt replenish the fertile but overworked soil. The rivers also drain excess monsoon rainfall into the Bay of Bengal. Thus, the great river system is at the same time the country's principal resource and its greatest hazard.

The profusion of rivers can be divided into five major networks. The Jamuna-Brahmaputra is {{convert|292|km}} long and extends from northern Bangladesh to its confluence with the Padma. Originating as the [[Yarlung Tsangpo River (Tibet)|Yarlung Tsangpo River]] in China's Xizang Autonomous Region ([[Tibet]]) and flowing through India's state of [[Arunachal Pradesh]], where it becomes known as the Brahmaputra ("Son of Brahma"), it receives waters from five major tributaries that total some {{convert|740|km}} in length. At the point, where the Brahmaputra meets the [[Teesta River|Tista River]] in Bangladesh, it becomes known as the [[Jamuna River (Bangladesh)|Jamuna]]. The Jamuna is notorious for its shifting subchannels and for the formation of fertile silt islands ([[List of islands of Bangladesh|chars]]). No permanent settlements can exist along its banks.

The second system is the Padma-Ganges, which is divided into two sections: a {{convert|258|km}} segment, the Ganges, which extends from the western border with India to its confluence with the Jamuna some {{convert|72|km}} west of Dhaka, and a {{convert|126|km}} segment, the Padma, which runs from the Ganges-Jamuna confluence to where it joins the [[Meghna River|Meghna]] River at Chandpur. The Padma-Ganges is the central part of a deltaic river system with hundreds of rivers and streams—some {{convert|2100|km}} in length—flowing generally east or west into the Padma.

The third network is the [[Surma-Meghna River System]], which courses from the north-eastern border with India to Chandpur, where it joins the Padma. The Surma-Meghna, at {{convert|669|km}} by itself the longest river in Bangladesh, is formed by the union of six lesser rivers. Below the city of Kalipur it is known as the Meghna. When the Padma and Meghna join, they form the fourth river system—the Padma-Meghna—which flows {{convert|145|km}} to the Bay of Bengal.

This mighty network of four river systems flowing through the Bangladesh Plain drains an area of some {{convert|1.5|e6sqkm|sqmi}}. The numerous channels of the Padma-Meghna, its distributaries, and smaller parallel rivers that flow into the Bay of Bengal are referred to as the Mouths of the Ganges. Like the Jamuna, the Padma-Meghna and other estuaries on the Bay of Bengal are also known for their many chars.

[[File:Goalundo -1 (10).jpg|thumb|[[Goalundo Ghat]] in central Bangladesh]]
A fifth river system, unconnected to the other four, is the [[Karnaphuli]]. Flowing through the region of Chittagong and the Chittagong Hills, it cuts across the hills and runs rapidly downhill to the west and southwest and then to the sea. The [[Feni District|Feni]], [[Karnaphuli]], Sangu, and [[Matamuhari]]—an aggregate of some {{convert|420|km}}—are the main rivers in the region. The port of Chittagong is situated on the banks of the Karnaphuli. The Karnaphuli Reservoir and Karnaphuli Dam are located in this area. The dam impounds the Karnaphuli River's waters in the reservoir for the generation of hydroelectric power.

The Ganga–Brahmaputra rivers contribute nearly 1000&nbsp;million tons/yr of [[sediment]]. The sediment contributed from these two rivers forms the Bengal Delta and [[Submarine fan]], a vast structure that extends from Bangladesh to the south of the Equator which is up to 16.5&nbsp;km thick, and contains at least 1130&nbsp;trillion tonnes of sediment accumulating over the last 17 million years at an average rate of 665&nbsp;million tons/yr.<ref>{{cite journal|url=http://www.iisc.ernet.in/currsci/apr252003/1041.pdf|title=A sediment budget for the Ganga–Brahmaputra catchment|first=Robert|last=Wasson|year=2003|journal=Current Science|volume=84|issue=8|pages=1041–1047|access-date=4 October 2014|archive-date=24 September 2015|archive-url=https://web.archive.org/web/20150924033520/http://www.iisc.ernet.in/currsci/apr252003/1041.pdf|url-status=live}}</ref> The Bay of Bengal used to be deeper than the [[Mariana Trench]], the present deepest ocean point.

During the annual monsoon period, the rivers of Bangladesh flow at about {{convert|140000|m3/s}}, but during the dry period they diminish to {{convert|7000|m3/s}}. Because water is vital to agriculture, more than 60% of the net arable land, some {{convert|91000|sqkm}}, is cultivated in the rainy season despite the possibility of severe flooding, and nearly 40% of the land is cultivated during the dry winter months. Water resources development has responded to this "dual water regime" by providing flood protection, drainage to prevent over flooding and waterlogging, and irrigation facilities for the expansion of winter cultivation. Major water control projects have been developed by the national government to provide irrigation, flood control, drainage facilities, aids to river navigation and road construction, and hydroelectric power. In addition, thousands of tube wells and electric pumps are used for local irrigation. Despite severe resource constraints, the Government of Bangladesh has made it a policy to try to bring additional areas under irrigation without salinity intrusion.

Water resources management, including gravity flow irrigation, flood control, and drainage, were largely the responsibility of the [[Bangladesh Water Development Board]]. Other public sector institutions, such as the Bangladesh Krishi Bank, the Bangladesh Rural Development Board, the Bangladesh Bank, and the [[Bangladesh Agricultural Development Corporation]] were also responsible for the promotion and development of minor irrigation works in the private sector through government credit mechanisms.