Before 1975, Hilsa fish from the Bay of Bengal could reach the confluence in Allahabad and even the Yamuna in Agra. During monsoon, these fish would swim upstream for incredible distances, a fact documented in various reports from the Central Inland Fisheries Research Institute. However, this migration halted after the construction of the Farakka Barrage on the Ganges River in 1975.

The Farakka Barrage is a massive dam on the Ganges River in West Bengal's Murshidabad district, merely 16-18 kilometers from the Bangladesh border. Approximately 2.25 kilometers long with 109 gates, its primary purpose was to clear silt from the Hooghly River, enabling large ships to access the Kolkata and Haldia ports. British engineer Sir Arthur Cotton had proposed diverting Ganges water into the Hooghly as early as 1853, but the plan wasn't executed then.

The Ganges, flowing from Bihar-Jharkhand, enters the Malda district of West Bengal and remains the Ganges until Farakka. The Farakka Barrage regulates the Ganges' flow, where it splits into two branches. The right branch, known as the Bhagirathi (later Hooghly) in India, flows past significant cities like Kolkata and Haldia. A 38-42 km feeder canal from the Farakka Barrage diverts Ganges water into this branch. The left branch enters Bangladesh, where it is called the Padma.

The problem arose when diverting water towards the Hooghly reduced the Ganges' flow into Bangladesh, especially during the dry season. This adversely affected agriculture, fisheries, and drinking water in Bangladesh, leading to a prolonged dispute between India and Bangladesh. Bangladesh alleged damage to its agriculture, fisheries, and environment due to water scarcity in the dry season. Temporary agreements were reached in 1977 and the 1980s.

A breakthrough came with the 'Ganges Waters Sharing Treaty' signed in 1996 to resolve this dispute. On December 12, 1996, then-Prime Minister H.D. Deve Gowda of India and Sheikh Hasina of Bangladesh signed the agreement in New Delhi. Valid for 30 years, the treaty is set to expire in December 2026. It primarily governs the sharing of available water at the Farakka Barrage during the dry season (January 1 to May 31).

According to the agreement, if the water availability at Farakka is 70,000 cusecs or less, both countries receive half the water. If it's between 70,000–75,000 cusecs, Bangladesh gets 35,000 cusecs, and India receives the rest. If it's 75,000 cusecs or more, India gets 40,000 cusecs, and Bangladesh receives the remainder. Both countries are guaranteed 35,000 cusecs alternately during March-May. A joint committee monitors the water flow. Recently, following a change in government in Bangladesh, the country has demanded more water, bringing the agreement back into focus.