GUWAHATI, June 6 - The Brahmaputra is a snow-fed perennial river. It gets floods between May and September. Over 80 per cent of its flow and over 95 per cent of its sediment come during this period.
According to the Master Plan of the Brahmaputra Basin Part-I Main Stem prepared by the Brahmaputra Board and published in 1986, the maximum annual suspended load of the river assessed at Pandu was found to be 36,609 ha m (hectare metre) in 1958 and the minimum suspended load at this gauge-discharge point was found to be 1,487 ha m in 1973, that is the variation in the annual sediment concentration at Pandu was found to be from a maximum of 0.0679 per cent in 1958 to a minimum of 0.00363 per cent in 1974.
The composition of the suspended load at Pandu was 5.27 per cent coarse, 26.05 per cent medium and 68.68 per cent fine grade. Significantly, the Master Plan said the sediment load at Pandu was found gradually decreasing from 1965 to 1976 and increasing thereafter.
The tributaries which make major contributions to the silt load of the Brahmaputra are Lohit, Dibang, Subansiri, Ranganadi, Dikrong, Jia Bharali, Dhansiri (North), Puthimari, Pagladiya, Manas, Aie, Champamati and Sankosh on the north bank.
On the south bank, the tributaries that use to make major contributions to the silt load of the Brahmaputra are Noa-Dihing, Burhi Dihing, Disang, Dikhow, Jhanji, Kopili, Bhogdoi and Dhansiri (South).
However, it is found that the north bank tributaries carry considerably more silt than the south bank tributaries. The Dihang, Dibang and Lohit carry the highest sediment load and over 70 per cent of the annual average silt load at Pandu is contributed by these three rivers, said the Master Plan.
Of the north bank tributaries, the Subansiri, Jia Bharali, Manas and Sankosh used to carry high sediment load during the preparation of the Master Plan, which states that the remaining north bank tributaries used to carry comparatively low sediment load.
The south bank tributaries relatively carry low sediment load. But among them, the Burhi Dihing carried the highest sediment load. The Kopili, Dhansiri (South) and Disang also used to carry significant silt load.
The suspended silt load carried by the north bank tributaries comprised 14.47 per cent coarse, 29.25 per cent medium and 56.28 per cent fine grade, against the 8.24 per cent coarse, 17 per cent medium and 74.76 per cent fine grade carried by the south bank tributaries.