Wood jam mobility in a morphologically active river in northern Chilean Patagonia

Abstract

Wood is crucial in river systems, influencing both ecological and physical dynamics. Understanding its behaviour in relation to fluvial morphology is essential for effective watershed management, especially after extreme events. The tendency of wood to accumulate into wood jams (WJs) adds complexity, as these dynamics remain challenging to assess. This study explored the relationship between channel morphological changes and WJ mobility in the Blanco River, Chile, which was affected by a volcanic eruption in 2008–2009. The methodological approach involved utilising multi-temporal drone surveys conducted in 2018, 2019 and 2023, over a river segment with an area of 29.5 ha and a length of 2.2 km. First, the characterisation of morphological units, quantification of geomorphic changes, and classification of morphodynamic mechanisms were accomplished using orthophotos and robust Digital Terrain Models (DTM) of Difference (DoD). Then, WJ abundance, spatial arrangement and mobility were derived from their manual delineation over 3 years. Finally, the characteristics of missing, newly formed, and persistent WJs were further analysed in relation to the morphodynamics. The results revealed that the river remains highly active, even 10–15 years post-eruption. From 2018 to 2023, at least 1.43 × 10⁵ m³ of sediment was displaced, yielding a value of 1.21 × 10⁵ m³ km⁻² year⁻¹. Most of this displacement was attributed to floodplain erosion from the channel's lateral shifts. WJs covered up to 10% of the study area annually. Mobility rates were 68% and 78% for the periods 2018–2019 and 2019–2023, respectively. In areas with no geomorphic changes, larger, less compact WJs were more likely to persist. However, persistent WJs were also associated with bank erosion and sediment deposition over bars. In contrast, missing or newly formed WJs tended to be smaller, more compact and were primarily linked to significant erosional and depositional processes.