Rock type as a driver of drainage network reorganizations in the Amazon region

Drainage networks are responsible for water, sediment, and nutrient fluxes across landscapes. In the Amazon region, the freshwater system also houses an unparalleled aquatic biodiversity. Therefore, understanding the origins of drainage network reorganizations across variable spatial and temporal scales is fundamental for various disciplines. To elucidate the evolution of river systems in continent interiors, it is necessary to constrain the mechanisms that regulate the upstream propagation of base-level changes.  Features such as drainage captures, knickpoints, paleochannel and paleovalleys, wind gaps, and asymmetric drainage divides are widespread and have long been observed in the Amazon region. These features are typically thought to result from climate change or intraplate tectonics whereas the influence of rock type as a trigger is largely overlooked. In this study, we link the spatial patterns of landscape transience to lithologic variations in the Amazon Craton. Before their confluence with the Amazon River, the largest left-margin tributary systems exit the Amazon Craton and cross sedimentary rocks of the Amazon Sedimentary Basin (ASB). This lithologic transition is marked by an expressive escarpment formed over resistant sandstones of the basal units of the ASB. Through quantitative geomorphologic analysis of the topography, drainage divides, and rivers, we investigate if this sharp lithologic transition contributed to the observed patterns of drainage rearrangement. The results revealed that rivers with larger drainage areas, lower mean elevations, and that flow shorter distances over the resistant rocks systematically capture neighboring basins. We suggest that this pattern produces the observed ‘ladder-like’ topography where the smallest basins are perched at higher elevations and vulnerable to river captures and divide migrations. We argue that, as tributaries draining the shield respond to downcutting and/or base-level fall of the Amazon River, bedrock incision and knickpoint propagation are differentially slowed down by the resistant rocks according to their incision capacity, generating the observed systematic landscape transience. The observed widespread and systematic distribution of geomorphic transients suggests that lithology could be an important autogenic control of drainage network rearrangement in the Amazon region as well as in other post-orogenic landscapes. The protracted exhumation of resistant rocks in cratons and continental interiors offers an exceptional natural laboratory for studying landscape dynamics associated with rock type.