Lake-islands: A distinct morphology of river systems

Abstract

Islands are important morphologies in multichannel fluvial systems for maintaining secondary channels, increasing flow efficiency in the widest stretches, and hosting important ecological environments. This study presents a hitherto unclassified, distinct type of fluvial island that occurs in large rivers: the lake-island. We analyzed islands from large rivers, particularly from South American rivers, the Negro (Amazon Basin) and Paraná (La Plata Basin) Rivers. Satellite images were used for identification and morphometric analysis, while morphometric indices were used to compare island morphologies. Sedimentary facies analysis and dating were also conducted to characterize the island deposits. Lake-islands are differentiated from typical fluvial islands by their lenticular morphology, and frontal and lateral levees that protect and isolate the lake or allow it to be partially connected to the channel. Lake-island morphology can be elongated or circular, based on their morphometric indices and their relationship with channel width. Lake-island lengths varied from hundreds of meters to tens of kilometers. They are formed by in-channel deposition on preexisting sand bars or fluvial islands. Their sedimentary characteristics are relatively simple and comprise basal sandy facies capped by finer-grained deposits. The permanence of lake-islands depends on the flow regime and sedimentary load of the fluvial system. The lake-island's age ranges from 101 to 103 years. A comprehensive understanding of the formation processes and dynamics of lake-islands is of fundamental importance in the management of large fluvial systems, and studies of fluvial ecology.