Kosi is one of Ganga's largest tributaries and in turn is fed by 7 tributaries - Sun Koshi, Dudh Koshi, Indravati, Tama Kosi, Likhu, Arun, and Tamar. These tributaries encircle Mt Everest from all sides and are fed by world's highest glaciers. The river drains a total area of 69,300 sqkm upto its confluence with Ganga. The river basin stretches 54,000 sq.kms.
The river is known as the "The Sorrow of Bihar". It is highly prone to flooding and has shifted course by 150 kms in the last 250 years. This unstable nature of the river is due to the heavy silt that it and its tributaries carry from the mountain ranges. The Kosi yields the highest silt in the world - 19 cum/ha/year. This is due to the large scale erosion and land slides in the upper regions of the Himalayas
Bihar has faced regular floods due to the River Kosi. The worst floods were of 1998 and 2007 which affected 13.47 million and 14.45 million people.
The Bihar government had constructed embankments along the river Kosi to contain its flood waters and to prevent its shifting course. It has affected over 338 villages which are trapped within the embankments. These 800, 000 villagers live in constant fear of floods and being trapped during high floods. It has meant the floodwaters outside the embankments are not able to drain into the river causing widespread water logging.
The flood prone area of the state has increased from 25 lakh hectares (1952) to 68.8 lakh hectares (1994).
Considering the vulnerability of the river Kosi's basin, the melting glaciers will acerbate the problem a hundredfold. The melting glaciers will release much more water than normal in a short time thus creating a flood situation. And over the year the discharges will tend to decrease.
The meteorological and hydrological time series studies between 1947 and 1993 have shown an increase in temperature and rainfall (precipitation). The hydrological tests have also indicated an overall decrease in the discharge of Kosi and its tributaries.
The monsoonal storms and flooding in the Himalayas will increase in severity and frequency due to climate change. This will alter the area's erosion, river discharge and sediment patterns. This will directly impact the hydropower reservoirs along the river and the new and planned constructions. The increased sedimentation will directly affected the agricultural lands and the irrigation canals and streams. This will lead to an over deterioration in cropping patterns in the region.
With the embankments all along the river course, the villagers inside the embankments and around them will be highly vulnerable to the flood waters and frequent breaches in the embankment walls.
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