COMMENTARY
CURRENT 1668 SCIENCE, VOL. 95, NO. 12, 25 DECEMBER 2008
The 18 August 2008 Kosi river breach: an evaluation
D. V. Reddy, D. Kumar, Dipankar Saha and M. K. Mandal
In the Himalayan terrain, the Kosi river
is formed by the confluence of seven
smaller streams, viz. Indravati, Sun Kosi,
Tama Kosi, Likhu Kosi, Dudh Kosi, Arun
Kosi and Tamar Kosi at different places
in China, Tibet and Nepal, before its entry
into the Indian state of Bihar (Figure 1).
In India, the Kosi river merges with the
mighty Ganges near Kursela village,
Katihar District, Bihar. Kamla, Baghmati
(Kareh) and Budhi Gandak are the major
tributaries of Kosi in India, besides many
minor tributaries like Bhutahi Balan. The
river travels a distance of 729 km from
its source to the confluence with the
Ganga, and drains a total area of about
69,300 km2.
The Holocene Kosi river mega fan in
Bihar is ~180 km long and 150 km wide,
and is believed to be one of the largest
river-built alluvial fans in the world1.
This alluvial cone preserved the evidences
of lateral channel shifting of the
Kosi river in the form of many distinct
palaeochannels, suggesting a dynamic
river system. Through the first maps prepared
by British surveyor, C. C. English
(1779), the Kosi course could be traced
since AD 1731. These maps indicated that
in the 18th century Kosi flowed near
Purnea town and then gradually moved
westward, shifting its course from east of
Purnea to west of Supaul, i.e. a shift of
about 120 km in about 275 years.
The river cuts across the Himalayas
and the Shiwalik ranges carrying huge
quantities of sediment. High rainfall (mean
1451.8 mm/yr) leads to extensive soil
erosion and landslides in its upper catchment.
The silt load of the river is one of
the highest in the Indian sub-continent,
to the tune of 80 million tonnes/yr. On
reaching the plains, high aggradations of
river bed and sediment bed load offer
enough resistance to the water, forcing
the river...