Morphodynamics and large wood dispersal in braided rivers

Abstract

Distinctive wood dispersal patterns are observed in spatially complex, highly dynamic braided rivers, but limited quantitative information is available for these large systems. In this work, physical modelling was used to investigate wood deposition patterns, remobilisation processes and long-term wood budget and to explore the influence of flow regime, channel morphology, wood supply and piece shape on wood dynamics. Wood dispersal was simulated by adding cohorts of dowels to self-formed, fully mobile braided networks. Laser surveys and digital imagery were used to reconstruct the downstream and vertical distribution of wood and the size and time evolution of jams. Wood storage volumes showed a threshold behaviour with wood supply rate. Logs tended to form small accumulations, but complex piece shape (presence of a root wad), low discharge and high wood supply supported the formation of large jams. Logs with roots were deposited at low relative elevation and short distance from input point. Piece mobility showed a complex relationship with discharge because flow stage influences both network conveyance and the availability of highly retentive sites (bar apex areas). Finally, flood magnitude was identified as the primary driver of wood remobilization during rapid flow pulses, with log shape and elevation playing a secondary stabilising role. Over a longer time scale, bed reworking caused intense turnover of stored wood, but large jams forming only under high wood supply conditions exhibited higher persistence over time.