Assessing functional characteristics of a braided river in the Qinghai-Tibet Plateau, China

Abstract

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.