Irrigation from Indus waters has provided the basis for successful agriculture since time immemorial. Modern irrigation engineering work commenced around 1850, and large canal systems were constructed by the British administration. In many cases old canals and inundation channels in Sindh and Punjab were revived and modernized; thus the greatest canal irrigation system in the world was created. At partition in 1947, the international boundary between India and West Pakistan cut the irrigation system of the Bari Doab and the Sutlej Valley Project, originally designed as one scheme, into two parts. The headwork fell to India while the canals ran through Pakistan. This led to a disruption in the water supply in some parts of Pakistan. The dispute that thus arose and continued for some years was resolved through the mediation of the World Bank by a treaty between Pakistan and India (1960) known as the Indus Waters Treaty. According to this agreement, the flow of the three western rivers of the Indus basin--the Indus, Jhelum, and Chenab (except a small quantity used in Kashmir)--is assigned to Pakistan, whereas the entire flow of the three eastern rivers--the Ravi, Beas, and Sutlej--is reserved exclusively for India.

In India a number of dams, barrages, and link canals have been built to distribute water from the eastern Indus tributaries to the Punjab and neighbouring states. The Harike Barrage, at the confluence of the Beas and Sutlej, channels water into the Indira Gandhi (Rajasthan) Canal, which runs for some 400 miles to the southwest to irrigate some 1.5 million acres (607,000 hectares) of desert in western Rajasthan. The main canal was completed in 1987.

Pakistan has diverted water from its western rivers to replace reduced flows in the Sutlej Valley Project region. A number of new link canals and barrages have been completed by the Pakistan Water and Power Development Authority. The biggest of these link canals is the Chashma-Jhelum link joining the Indus River with the Jhelum River, with a discharge capacity of some 21,700 cubic feet (615 cubic metres) per second. Water from this canal feeds the Haveli Canal and Trimmu-Sidhnai-Mailsi-Bahawal link canal systems, which provide irrigation to the Multan and Bahawalpur divisions in the lower Punjab.

The Indus Waters Treaty also made provision for the construction of two major dams in Pakistan. The Mangla Dam on the Jhelum River, near the town of Jhelum, has a crest length of about 10,300 feet and a maximum height of more than 450 feet and is one of the largest rolled earth-fill dams in the world. Mangla Reservoir, created by the dam, is 40 miles long and has an area of 100 square miles. The project also generates some 800 megawatts of hydroelectricity. The reservoir is being developed as a fishing centre and a tourist attraction as well as a health resort.

A second gigantic project is the Tarbela Dam on the Indus, 50 miles northwest of Rawalpindi. The dam, of the earth- and rock-filled type, is 9,000 feet long and 470 feet high, and its reservoir is 50 miles long. The dam's generating capacity is some three times that of the Mangla Dam, and its total potential is considerably greater.

On the Indus itself there are several important headworks, or barrages, after the river reaches the plain. In the mountainous region the principal waterways west of the Indus are the Swat Canals, which flow from the Swat River, a tributary of the Kabul River. These canals help in the irrigation of the two chief crops of the area, sugarcane and wheat. The Warsak multipurpose project on the Kabul River, about 12 miles northwest of Peshawar, provides irrigation for food crops and fruit orchards in the Peshawar valley and is designed to produce 240,000 kilowatts of electricity. In the plains region the Kalabagh, or Jinnah, Barrage controls the system of canals in the Thal Project, the development authority for which was set up in 1949. The project irrigates a former desert area. It is an integrated project aimed at extending agriculture, developing rural industry, and promoting the settlement of population in villages and towns. Farther downstream is the Chashma Barrage. Still farther the Taunsa Barrage, designed for the irrigation of land in the Dera Ghazi Khan and Muzaffargarh districts, also produces about 100,000 kilowatts of electricity. Within the Sindh there are three major barrages on the Indus--Guddu, Sukkur, and Kotri, or Ghulam Muhammad. The Guddu Barrage is just inside the Sindh border and is some 4,450 feet long; it irrigates cultivated land in the region of Sukkur, Jacobabad, and parts of Larkana and Kalat districts. The project has greatly increased the cultivation of rice, but cotton also has become a major crop on the left bank of the river and has replaced rice as a cash crop. The Sukkur Barrage was built in 1932 and is about one mile long. The canals originating from it serve a cultivable area of about five million acres of land producing both food and cash crops. The Kotri Barrage, also known as the Ghulam Muhammad Barrage, was opened in 1955. It is near Hyderabad and is nearly 3,000 feet long. The right-bank canal provides additional water to the city of Karachi. Sugarcane cultivation has been extended, and crop increases have been achieved in the cultivation of rice and wheat.

Experience in the Indian subcontinent and elsewhere has shown that canal irrigation, unless carefully controlled, can cause much damage to the cultivated land. The water in the unlined canals seeps through the soil and raises the water table so that the soil becomes waterlogged and useless for cultivation. As irrigation by canals has expanded in the Indus and its tributary lands, in some areas underground water has appeared on the surface to form shallow lakes. Elsewhere the water has evaporated in the intense summer heat, leaving behind layers of salt that make crop production impossible. Steps have been taken to provide adequate drainage systems to avoid waterlogging and salt buildup.