Coherent Motion of Channel Threads in the Braided Brahmaputra-Jamuna River

Abstract

Braided river channels are formed by multiple channel threads that often shift laterally through either abrupt avulsion or more coherent migration over time. Several approaches have been developed to quantify the planform change of braided rivers, but predicting this change remains challenging due to the intricate structure of the channel network and the strong sensitivity of planform morphology to water stage. To address these challenges, we developed an approach to test whether individual channel threads move laterally across annual timescales. We then test whether a curvature-driven model for the migration of meandering rivers can similarly explain the motion of braided channel threads. As a case study, we analyze the migration pattern of the Brahmaputra-Jamuna River, whose discharge is strongly seasonal due to the monsoonal hydroclimate. We characterize planform morphology by selecting low-stage water masks derived from Landsat images. We find that from 2001 to 2021, ∼43% of the total length of channel threads migrated coherently, at an average rate equivalent to ∼30% of the local width of the channel thread per year. Migration rate is weakly related to channel-thread width. In three examples, the migration rate is closely related to the channel-thread curvature, and the curvature-driven model successfully describes their migration. By connecting the motion of channel threads with their planform geometry, the analysis suggests that the seemingly chaotic motion of braided rivers is somewhat organized over decadal timescales, indicating the potential for better hazard and flood predictions for communities living within and along these river channels.