Characterizing braided rivers in two nested watersheds in the Source Region of the Yangtze River on the Qinghai-Tibet Plateau

Abstract

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 (BIT3) for further analysis. We found that BIT3 was linearly correlated with active valley width (WR) 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 BIT3 to WR, 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, BIT3 always reached the maximum as RW, defined as the ratio of water body to exposed active riverbed area, was around 0.43. Furthermore, RW 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 BIT3 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.