Impact of Tectonic Uplift in Paro Chu Basin, Bhutan

The Paro Chu, originating in the southern slope of the Jomolhari Peak of the Bhutan Himalaya, is a right-bank tributary of the Wong Chu. It passes through a wide variety of rock beds, which have differential status of metamorphism. The drainage basin shows the impact of variable rock characteristics and tectonic movements on stream network, valley-form and channel morphology. Morphometric studies of stream networks with testing of Horton’s Laws of stream number and stream length have revealed the deviation of streams from the Laws in relation to litho-characteristics and tectonic influences. The Asymmetry factor (AF) and Transverse topographic symmetry factor (TTSF) also indicate the tilting of the basin landscape. The valley-form significantly changes in different reaches of the Paro Chu. After the initial gorge segment, the course of the channel drains through a flat floodplain at Paro Town and beyond, but the lower course again passes through a narrow and deep gorge right up to the confluence. The stream gradient changes distinctly in the three valley forms. The braided reaches of the stream mark the impact of structural dismemberments under tectonic influence and the carrying of excessive quantity of sediments as related to the shearing of rocks in the immediate upstream of the braided parts. The variation of hypsometric integrals clearly reveals differential erosion under the impact of tectonic uplift and variable resistances of the rocks composing the drainage basin.