Tributary channel networks formed by depositional processes

Understanding the detailed structure of landscape topography is important when assessing risks in coastal plain areas susceptible to the combined effects of fluvial, pluvial and coastal flooding. Key to this analysis is the identification and characterization of drainage basins that control surface water flow, but the factors controlling the formation and evolution of drainages in low-relief coastal plains is not well known. Here, we analyse the distribution and morphology of coastal drainage networks using a compilation of airborne lidar covering the entirety of the Gulf of Mexico coastal plain between the Rio Grande and Mississippi rivers. We observe that the dendritic drainage basins that govern the coastal landscape have boundaries that are initially set and controlled by sinuous alluvial ridges defining previous courses of modern rivers that were abandoned through the process of channel avulsion. These depositional ridges form topographic highs on an otherwise low-relief coastal plain and define the initial extent and occurrence of the coastal drainages. While the basin boundaries are formed by depositional processes, they exhibit geometric scaling characteristics similar to basins interpreted to have evolved through erosion alone. This work presents evidence for the creation and evolution of erosional dendritic channel networks within depositional environments with broad implications for understanding floodplain channelization, partitioning and routing of sediment and water across low-relief landscapes, and timescales and mechanisms of landscape evolution.