Decadal-scale morphological adjustment of a lowland tropical river

Compared with temperate regions, much less is known about the dynamics of tropical river systems. Tropical rivers are typically characterised by pronounced seasonal changes in precipitation, large sediment loads and high rates of lateral channel migration across often very low-gradient and densely populated floodplains. Understanding the controls on channel migration or change is integral to our ability to fully predict and build resilience against flood risk and wider river-related hazards. Here, we analyse channel and confluence migration over the last ~40years along a ~85km reach of the Cagayan River and one of its tributaries, the Pinacanauan de Ilagan (Luzon, Philippines) using optical satellite imagery captured during this period. Combining this with spatial variations in channel pattern, valley width and new bed material grain size data, we demonstrate that sediment transport and deposition are key drivers of the observed tropical channel morphodynamics in this region. The high sediment supply generated in the catchment headwaters (by mass-wasting of hillslopes triggered especially in typhoons) results in high aggradation rates and channel widening on the lower gradient alluvial plain. We suggest that this aggradation enhances local confluence and lateral channel migration rates, which can reach >300m per decade, and that lateral migration rates of tropical rivers are typically greater than those of temperate rivers. Channel morphodynamics have implications for how to best manage these types of tropical river systems, where hard bank protection structures may result in a complex geomorphic response and flood risk mapping may need to include assessment of sensitivity to varying channel position and topography.