Wood dispersal in braided streams: Results from physical modeling

Wood is a key feature of riverine systems, playing a significant role in their morphodynamics and ecology. Wood dynamics have been widely investigated in single-thread streams, but limited information is available about wood transport and deposition in large multithread rivers. In this work, we used a large (3 × 25 m) physical model to provide a quantitative description of wood dispersal processes in braided systems. Deposition patterns were characterized in terms of downstream distribution and accumulation size and linked to wood piece properties (diameter, length, and presence of a rootwad) and flow stage. Statistical analysis of observed wood patterns showed that relative log diameter strongly reduces wood mobility, with travel distance dropping for diameters exceeding 50% of median channel depth. Wood tended to form small, sparse accumulations, with large log jams occurring for large log length, complex piece shape, and moderate flow discharge. Flow stage had a dual effect on mobility as both channel conveyance and the availability of retentive sites increased with discharge for the tested range of flow conditions. Additionally, wood deposition was strongly linked to bed morphology. In particular, 40–60% of transported wood was stored at bar apex, with more than 30% of wood deposited on the first bar downstream of the input point, highlighting the crucial role of local-scale morphology in wood dispersal.