Natural fluvial connection across watersheds

Topographically defined drainage boundaries typically enclose natural drainage systems, which eradicate surface-water transfer across drainage divides. With this ingrained perception, cases of inter-watershed fluvial connection were overlooked or regarded as “geographic errors”. For the first time, we discuss different scenarios that may be viewed as inter-watershed fluvial connections in a global scale, collect their occurrences, assess the formative and evolutionary processes, and quantify their drainage and channel properties. By categorizing all cases into three primary classes, we demonstrate that, despite their relative rarity, these occurrences are not random phenomena. Instead, they embody a comparatively unique assemblage of scenarios governed by established hydrogeomorphic theories and understanding. Reported and known cases are found to develop as an intermediate stage of river piracy. They may also form in various avulsive settings, including alluvial fan, inland delta, anabranching river, and sand-silt bed river, as well as in headwater topography under the impact of Late Pleistocene glaciers. The wide variety of their morphologic outcomes, underlying processes, and environmental impacts through inter-watershed dispersal of water, sediment, nutrients, and species highlights the diversity and complexity of natural fluvial systems, calling for the inclusion of inter-watershed connections in future assessments of Earth's surface dynamics.