Meander hydromorphology of ephemeral streams: Similarities and differences with perennial rivers

The geomorphology, hydrology and processes of ephemeral streams are poorly known and studies on the geomorphic characteristics of ephemeral meandering streams (EMS) are even less investigated. We collected geomorphic data (namely, channel width, sinuosity, wavelength and curvature radius) from 107 EMS reaches in different drylands of the world from GoogleEarth Pro® satellite images. Geomorphology features/processes, such as neck cutoffs, point scrolls and chute cutoffs, were also assessed. The main aim of this paper is to investigate similarities and differences between EMS and perennial counterparts of more humid areas (on which almost all hydrogeomorphic models are based) for the purpose of explaining why meandering rivers form in environments where the lack of vegetation, sporadic flows and a high sediment supply should favour the development of braided rivers. EMS showed both geomorphological similarities and differences with perennial rivers. Point bar chute cutoffs and point bar scrolls are the most common geomorphic features of EMS, suggesting a certain stability of meander behaviour, though within a continuous process of changes towards the most probable form, similarly to perennial streams. The bimodal distribution of curvature ratio values departs substantially from that of perennial rivers that are commonly in the 2-3 range. Data from the rivers in this study indicate that their channel pattern is well described by the same sine generated function used for perennial rivers. Wavelength to channel width ratio and bankfull discharge to streambed gradient of EMS unexpectedly plot in different areas of classical diagrams developed for perennial rivers. We interpret such differences in terms of excess energy expenditure for high sediment load transport. We also propose a new function to discriminate the meandering from the braided pattern of ephemeral streams, and present a conceptual model of meander formation in ephemerals streams based on an autogenic process of bank collapse and bar deposition.