Spatial distribution of denudation processes in Bhutan

Bhutan, being a Himalayan mountain country, extends ca. 305 km from west to east and 145 km from south to north, covering an area of 47,000 km². Bhutanese Himalaya mountains are divided into three latitudinal belts from south to north: the sub-Himalayas or Himalayan foothills (up to 1,500 m in altitude), the inner Himalayas or middle ranges with flat valleys (from 1,500 to 4,500 m), and the Great Himalayas of high mountains (about 4 500 m). The Black Mountain Range, stretching longitudinally, divides this country into the eastern and western parts. The belt system of landforms is closely related to the geological structure. In the north, metamorphic and crystalline rocks (mainly gneisses and granites) dominate, while in the south there are sedimentary rocks associated with the molasse-like sediments in the Siwalik range. The distribution of precipitation from approx. 500 mm in the north to 5000 mm in the south of the country relates to this geological and morphological belt system. The main rivers flow longitudinally and have perpendicular tributaries. The digital elevation model of Bhutan was analysed geomorphometrically and, on this basis, places with high potential mechanical denudation were identified. The most evident manifestation of this denudation are numerous landslides occurring on the valley slopes. These landslides, especially in the monsoons period, deliver enormous amounts of landslide material to the valley bottoms, and most often directly to the river channels. The landslide material in the form of fluvial material is further transported along the rivers and deposited in the foreland of the Himalayas. After the summer monsoon period, water samples were collected in selected rivers. The collected water samples were subjected to hydrochemical analyzes. The studied river waters in the middle part of the country are characterized by low mineralization. The obtained results allow for a preliminary characterization of the spatial diversification of the denudation potential of Bhutan's river waters. Referring to the physiographic division of Bhutan, it can be confirmed that the middle zones, meridional ridges and valleys of this country are characterized by low chemical denudation and high mechanical denudation, while the southern tips of the country Front Hills and Piedmont are dominated by accumulation processes due to a significant reduction in river gradient.