The geomorphological landscape of the Mariuá Archipelago: An anabranching megacomplex system in the Negro River, Amazon Basin (Brazil)

The Negro River, the Amazon's principal left-bank tributary, exhibits a complex fluvial system of significant ecological and hydrogeomorphological importance. Its middle and lower reaches feature distinctive geomorphological landscapes, notably the Mariuá and Anavilhanas archipelagos, which serve as exemplary sites for investigating large anabranching rivers. This study examines the geomorphology of the Mariuá Archipelago through analysis of water dynamics, fluvial units, and sedimentary characteristics. We evaluated seasonal water level variations, erosion-transport-deposition processes, and grain-size distributions of islands and floodplains, complemented by comprehensive mapping of river units. Volumetric analysis reveals the fundamentally incomplete nature of the Mariuá floodplain, with the degree of incompleteness varying spatially from 30 % in upstream reaches to 60 % downstream, reflecting both temporal progression of filling and interruption of sediment input. The archipelago exemplifies an inherited fluvial landscape in permanent non-equilibrium, maintained by the Negro River's low suspended sediment concentration (3.2 × 10⁶ ton.year⁻¹) and limited stream power. Islands and floodplains comprise predominantly fine sediments (silt, clay, and very fine sand) stabilized by dense vegetation, while sandbars consist of medium to fine sand with occasional granule deposits. Although floodplains function as sediment traps through their levee-depression morphology, current sediment supply cannot explain the archipelago's formation. We propose that the Mariuá Archipelago originated from Japurá River sediments deposited when paleochannels connected these basins, with sediment supply ceasing abruptly around 1000 BP. While this inherited landscape has remained morphologically stable for centuries, recent analysis (2014–2024) reveals a 33 % increase in erosion and 83 % increase in deposition, suggesting the system may be approaching a stability threshold. Climate change and intensifying extreme hydrological events pose significant risks to this delicate equilibrium, potentially triggering irreversible morphological transformations in this unique Amazonian landscape.