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Functional characteristics of braided rivers with very high elevations (>3400 m) located in the Qinghai-Tibet Plateau, China provide unique insight into fully understanding global braided river functioning, but have not yet been explored because of their remote locations and harsh physiographic conditions. In this study, we unveiled these characteristics at the reach spatial scale over three decades in one of these braided rivers, the Upper Lancang River (ULR). Using Google Earth Engine resources and a specific selection of Landsat images obtained between 1989 and 2018 during the dry season (November to early May when water levels were low and comparable over years), we analyzed water area, extracted flowing and non-flowing channels, active channel widths (unvegetated bars and water channels), and calculated lateral shifting rates of the main channel for 13 sub-periods. We also developed an empirical method for estimating vegetated areas in the selected years. We found that (1) this braided system is partly controlled by peak flows and flood durations and the braiding activity evolved through time according to braided planform characteristics, (2) braiding intensity is high during low discharges, suggesting that the ULR is a very well-connected braided system with groundwater, but we observed higher braiding activity in spring than in fall, suggesting other factors than groundwater recharge also control non-flowing channel occurrence, and (3) the ULR supports a vegetation mosaic that is dynamic at a multi-year scale with period of declines that can be caused by scouring and rapid growth in an optimal recruitment window. These findings lead to our interpretation of vegetation behavior and surficial water – groundwater interactions, which needs future field validation. • The ULR is a well-connected braided system with sufficient sediment supply due to less vegetation cover and lower surface roughness. • Vegetation dynamics of the ULR is driven by channel morphodynamics and scouring as well as optimal recruitment windows. • A new method of combining UAV data with the concurrent Landsat image to assess channel network and vegetation areas in Landsat images is developed. • Using NDWI values obtained from GEE to assess daily discharges that cannot be measured directly and detect major channel shifts has great advantages for future applications. [ABSTRACT FROM AUTHOR]

Large and complex braided channels (>1 km wide) in the Source Region of the Yangtze River (SRYR) on the Qinghai-Tibet Plateau, China, are characterized by a unique alpine environment with very high elevations and glacier sources. Yet, little is known about the morphological complexity and changes of large braided channels owing to the scarcity of hydrological and topographical data. In this study, we provided the first insight into these issues using compiled long-term (several decades) climatic and suspended sediment data, and Landsat images of braided channels in two nested headwater watersheds (i.e., Tuotuo and Tongtian river watersheds) in the SRYR. Analysis of annual and monthly mean temperature, precipitation, and water discharges illustrated the nature of the hydrological regime and their response to glacier melting in the two watersheds. Examination of annual mean suspended sediment loads and concentrations (C) led to two sediment rating curves and generally similar C values between the two watersheds. Among a suite of morphological indices for braided rivers, braiding intensity may be quantified using four different ones. After comparing the indices, we showed that they had similar ability of characterizing braiding patterns and adopted branch count index (BI T3) for further analysis. We found that BI T3 was linearly correlated with active valley width (W R) during both flood and dry seasons for the 17 selected braided reaches distributed within the two nested watersheds. In addition, braiding density, defined as the ratio of BI T3 to W R , remained approximately unchanged irrespective of elevations and locations of these reaches, indicating clearly that morphological structures of braided channels are spatially uniform in the SRYR. Along the main channels of the two nested watersheds over the entire study period, BI T3 always reached the maximum as R W , defined as the ratio of water body to exposed active riverbed area, was around 0.43. Furthermore, R W was always positively correlated with the associated water discharge (Q). Variable degrees of braiding adjustment in response to changes of hydrological regime were represented by hysteresis loops between BI T3 and Q. These results revealed relatively homogeneous morphologic properties of braided channels across the two nested watersheds and provided an important benchmark for future morphodynamic research on braided rivers in the SRYR. • Changes of morphological structures of braided rivers are better captured using braiding intensity and active valley width • Braided rivers in the source region of Yangtze River (SYR) demonstrate morphological similarity across space • Variable morphological responses to discharge changes only occur in smaller watersheds • Increased glacier melting has had limited impact on sediment transport and morphological structures of braided rivers in SRYR [ABSTRACT FROM AUTHOR]