Geomorphic Controls on Floodplain Carbon Sequestration: Linking Flood Dynamics, Channel Migration, and Carbon Sequestration Across Meandering and Braided Rivers

Abstract

Floodplains are integral components of dynamic biogeomorphic river systems, serving as significant storage reservoirs for water, sediment, and carbon across diverse temporal and spatial scales. Previous research suggests that floodplain carbon storage potential varies with river type and planform characteristics (e.g., meandering vs. braiding). Nevertheless, the fundamental mechanisms by which distinct geomorphic floodplain types govern carbon sequestration and downstream export remain poorly understood. This study integrates remote sensing and quantitative geomorphic analyses to evaluate how variations in river and floodplain planform configuration control spatio-temporal patterns of overbank inundation, sediment deposition, and channel migration across two contrasting East African river systems: (1) the braided Sabaki River and (2) the meandering Tana River. The insights gained are then used to discuss potential implications for floodplain carbon storage. We first estimate spatial patterns of flood extent and recurrence using a combination of Sentinel‑1 and Sentinel‑2 data and a simple inundation model. The inundation model delineates potential flooding extents by river stage, cross-referenced with observed spatial flooding patterns. This approach allows us to estimate the potential maximum flooded area per season and identify locations of riverine suspended sediment and associated carbon deposition. We subsequently link the predicted flooded areas with a multi-temporal analysis of river channel migration rates along longitudinal river profiles. Reach-averaged flooded areas and channel migration rates are further compared with measurements of sediment core organic carbon stocks, which are upscaled to the floodplain extent. Preliminary results indicate that floodplains in meandering reaches serve as more effective organic carbon sinks than those in braided reaches. This trend is attributed to the high lateral fluxes and chronic sediment reworking inherent to braided systems, which exhibit higher seasonal reach-averaged channel migration rates of approximately 50 m compared to 20 m in meandering systems, reflecting the lower migration and enhanced burial stability characteristic of meandering reaches. Finally, this integrated framework establishes a mechanistic link between geomorphic regimes, floodplain type, and carbon cycling, providing new constraints on the role of river geomorphology in regulating fluvial carbon pathways and long-term carbon sequestration potential.