Lowland river sinuosity on Earth and Mars set by the pace of meandering and avulsion

Meandering rivers have shaped the landscapes of Earth and Mars through the development of sinuous and migrating channels. River channel sinuosity reflects an interplay of primary agents including water discharge and sediment supply, information that is archived in the sedimentary record. Here we examine the spatial variability of the sinuosity of 21 lowland rivers on Earth and six ancient river systems on Mars using satellite imagery, and identify a dichotomy in spatial patterns: instead of decreasing downstream as previously suggested, we find that the sinuosity either increases or remains constant approaching the river outlet. We conduct numerical modelling of channel migration to show that these bimodal patterns can be explained as a competition between the timescale required for channels to establish steady-state sinuosity and the avulsion timescale. This highlights the role of varying water discharge on meander development and demonstrates how the planform morphology of modern and ancient fluvial systems may be used to interpret hydrological regimes of river systems, with implications for lowland river migration patterns under future shifting climate regimes. Spatial patterns of channel sinuosity near river outlets reflect the interplay between the channel migration rate and the avulsion timescale, according to sinuosity measurements of lowland rivers on Earth and Mars and channel evolution simulations.