Abstract: The Hequ-Machang reach of the upper Yellow River in the Gannan Tibetan Autonomous Prefecture, a typical plateau braided reach, is characterized by a wide-shallow channel and its significant bed deformation and frequent bank erosion, threatening the safety of local infrastructure. Based on flow and sediment data, remote sensing images, and other multi-source datasets, new calculation methods are developed to quantify the channel width swept by mainstream migrating and the degree of channel planform braiding. We examine the process of river regime adjustment that occurred in the years of 2006-2021, focusing on a discussion on the impacts of flow-sediment regime. Results show the main channel in this reach manifested an equal shifting frequency in both left and right directions, with the average annual migration width of 60.6 m/a. However, at certain typical cross-sections, the width exceeded 100 m/a with the mainstream scouring the locations of the proposed bank revetment projects, which needs a special attention. The reach featured a moderate braiding intensity (an average braiding index = 1.9), but significant longitudinal variations occurred with stronger braiding intensity in the upper and lower sections. Hydrodynamic conditions and sediment concentration are two major factors that affect the main channel migration and braiding intensity, respectively; their variations are reflected using certain combined flow-sediment indexes (e.g., fluvial erosion intensity and incoming sediment coefficient). These indexes should also be suitable for revealing overall river regime adjustments in plateau braided reaches.

Key words: braided reach, river regime adjustment, main channel migration, braiding index, upper Yellow River