Long-term hydro-sedimentary dynamics of the Ucayali River (Amazon Basin) revealed through combined observations, remote sensing, and SWAT-Amazon modelling

The Amazon basin is undergoing increasing environmental changes, potentially approaching a climatic tipping point in the coming decades. Understanding how these changes affect water and sediment fluxes is key for constraining large-scale biogeochemical cycles, yet conventional hydrological networks lack the spatial and temporal resolution required to accurately quantify hydro-sedimentary budgets. To address this limitation, we develop an integrated, physically constrained framework combining long-term observations, remote sensing, and hydrological–hydraulic modelling (SWAT-Amazon) to quantify multi-decadal hydro-sedimentary budgets and investigate how floodplain inundation controls sediment dynamics in large Amazonian rivers. Focusing on the Ucayali River, a major foreland tributary of the Amazon, this study provides the first detailed, long-term hydro-sedimentary budgets for the Upper Amazon, distinguishing fine sediment fluxes from sand loads. Results reveal a previously undocumented floodplain-controlled sand sedimentation process: during high waters, large floodplain water storage (up to 19.1 [15.3, 22.9] km3, ∼ 38 % of discharge) reduces main-channel transport capacity, capturing up to 14 % [10 %, 20 %] of the sand flux at peak discharge, while recycling during recession contributes 22 % of the total suspended load at the basin outlet. This dual control partially decouples sediment transport from water discharge. The Andean Ucayali exports 455 [410, 500] × 106 t yr−1 of suspended sediment (40 % sand), of which 36 % is trapped within the floodplain, predominantly as sand (65 % of total deposition). The river delivers 290 [235, 345] × 106 t yr−1 to the Amazon River (26 % sand), making it the dominant sediment source among the Andean foreland tributaries. Uncertainty analysis combining Sobol indices and GLUE simulations shows that, despite substantial equifinality among secondary floodplain parameters, sediment fluxes and associated trapping and recycling fractions remain stable across all behavioural simulations. Budget accuracy is therefore controlled by long-term, multi-variable, multi-source observations rather than by parameter calibration or model structure alone. These findings demonstrate that floodplains control hydro-sedimentary fluxes in large river systems and act as dynamic regulators of sediment transport, storage, and recycling, with major implications for biogeochemical cycles.