Studying rock and sediment samples of Koshi Basin


Samples of rock and sediment from the high and middle altitude mountains of the Koshi River Basin will soon undergo a geochemical analysis at the IIT Kanpur (India) laboratory to understand the contribution of sediment load from different geographic regions.

This is a state-of-the-art study on sediment dynamics in the Koshi River Basin supported by Australian Aid, and conducted by experts from ICIMOD in partnership with IIT Kanpur and Department of Hydrology and Meteorology (DHM), Nepal.

The Koshi Basin has a very high sediment load due to steep topography, young geological formation, and intense rainfall during the monsoon season. However, the exact magnitude and spatial extent of erosion and sediment dynamics are yet to be understood fully. 

Therefore, a team from these partner organizations made a field visit from 24-30 March 2015 to collect rock and sediment samples. The team visited the major tributaries of the Koshi Basin to collect the suspended sediment loads. These included Indrawati, Bhote Koshi, Tama Koshi, Dudh Koshi, Sun Koshi, Arun, and Tamor. Rock samples were also collected from different stretches of high and middle mountains. Altogether 17 water, 17 sediment, and seven rock samples were collected during the field visit. 

The team observed a few large landslides, including the recent one in Jure in Sun Koshi River. The team also interacted with engineers and staff of the Khimti Hydropower Project on the Tamakoshi and discussed the sedimentation problem in settling chambers. The team discussed some possible solutions and agreed to cooperate further in the field. 

The field visit was very useful in understanding how the young geological formations in the Koshi Basin are contributing to large sediment flux in the Koshi River, especially from the lesser and higher Himalayas. The team gained first-hand information on the causative factors of erosional processes on the fragile mountain slopes. The team also observed the widespread road networks, both existing as well as the newly constructed ones, across the middle mountains that have caused significant mass movements and have contributed significant sediment flux into the Koshi and its tributaries.