Impacts of human activities on hydrology and morphology of braided channels in Lhasa River of the Qinghai-Tibet Plateau

Abstract:

Among braided rivers developed in the Qinghai-Tibet Plateau of China with very high elevations (>3500 m), the middle and lower reach of the Lhasa River has been affected by comprehensive human activities mainly involving dam construction, urbanization, farming, afforestation, and mining. In this study, we investigated their impacts on hydrology and morphology of the four braided reaches downstream of the two cascaded dams. The study period was divided into 1985-2006 (P1), 2006-2013 (P2), and 2013-2019 (P3), representing the natural and changed flow regimes by the dams. Using available daily discharge data at the two stations within the four reaches, we analyzed dam-induced hydrological alteration based on the Indicators of Hydrologic Alteration and Range of Variability Approach and calculated key discharge proxies. We also selected remotely sensed images in the three periods and compared morphological metrics extracted from them for the four reaches among these periods. We found the attenuated hydrological regime for only two reaches. The total channel width (W_c) and braiding intensity (BI_t) followed different temporal trends among the four reaches. Annual averaged shift rates of the main channel in the four reaches were higher in the short (P2-P3) than in the long (P1-P2 and P1-P3) periods. The longitudinal changes of W_c and the number of channels did not have any identifiable trend in four reaches. By linking these morphological changes to quantified spatial and temporal patterns of various human activities, we revealed that (1) the two dams had insignificant impact on channel morphology, suggesting that the studied braided river has a short relaxation time and (2) the evolutional trajectories of morphological changes in most of the four reaches were similar, suggesting that temporal trends of morphological changes due to complex human activities are not affected by different physiographic settings of the reaches. Continuous exploitation of the valley area requires comprehensive river management strategies for coordinating various human activities.