Aspects of erosion and sedimentation in the Nepalese Himalaya: highland-lowland relations

Nepal is a mountainous country, except for the nearly level lowlands in the south, representing less than 20 % of the total territory. Several irrigation schemes operate in the southern lowlands, increasing not only the extent of cultivated land, but also the production of rice, the staple and cash crop of the rural population. But the condition of the irrigated fields in the Terai and the Dun valley regions (e.g. Chitwan valley) is deteriorating because of the use of sediment-loaded irrigation water. Irrigation is carried out through diverting river water, which flows from the mountains and brings lots of suspended sediments. Yearly deposition of sediments in the agricultural fields gradually lowers the fertility status of the soils, deteriorates soil physical properties and eventually results in lower yields. The sedimentation issue in the Chitwan valley raises three main research questions: (1) Where do the sediments come from? (2) Which processes are involved in sediment production? (3) To what extent human activities play a role? In order to investigate these questions and search for answers, three study areas were selected: (1) the watershed of Langtang Khola in the High Himal and High Mountain regions, (2) the watershed of Likhu Khola in the Middle Mountain region, and (3) the Chitwan valley in the lowlands (Dun Valleys). In the High Himal and High Mountain regions, climate plays an important role. Freezing and thawing, ice and snow avalanches and debris slides contribute to the physical disintegration of bedrocks and bring debris down the steep slopes. Glaciers help in further disintegration of rocks by their grinding effect. They also collect dust during dry season. Melting water from the glacier ice during summer months brings lots of suspended sediments (< 2 mm) to the river system. Since river gradient is high, the suspended sediments are easily transported. Sediment production in the upper watershed areas by mass movements and glacier activity is essentially controlled by natural processes, without human intervention.