The Negro River in the Anavilhanas Archipelago: Streamflow and geomorphology of a complex anabranching system in the Amazon

Abstract

The evolution of the fluvial landscape is the result of controls and processes that can be observed by the adjustments a river makes to its hydrological regime. The Negro River is the sixth-largest river in the world by water volume and contains two complex anabranching systems, the Anavilhanas and Mariuá archipelagos, and despite its importance, there is a lack of hydrological survey data on its multichannel reaches. The objective of this study was to analyse the fluvial dynamics of the lower Negro River and the morphodynamics of the Anavilhanas Archipelago channels in the Amazon basin. The analysed set of fluviometric surveys and cartographic data indicated that the average water velocity and depth of the channels were the most sensitive variables for water discharge variability. In addition, the reduction in the width/depth ratio of the channels in the Anavilhanas region is associated with an increased number of islands, but not with a fluvial transport capacity increase. These landforms play an important role in modulating flood wave dynamics in the lower reach of the Negro River. Seasonal water storage occurs in the channels and lakes of the archipelago due to the backwater effect caused by the Amazon River, which is the main control in the spatiotemporal variation of water discharge along the Anavilhanas Archipelago.