Autogenic Formation of Bimodal Grain Size Distributions in Rivers and its Contribution to Gravel-sand Transitions

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Riverbeds fine downstream and often have a gravel-bedded reach, a relatively abrupt gravel-sand transition (GST), and a sand-bedded reach. Underlying this behavior, bed grain size distributions are often bimodal, with a gap around the range 1-5 mm. Despite several viable theories, there is no morphodynamic model capable of producing the grain size gap and gravel-sand transition autogenically from a unimodal sediment supply. Here we use a one-dimensional morphodynamic model including size-specific bedload and suspended load transport, to show that bimodality readily evolves autogenically even under unimodal sediment feed. A GST forms when we include in the simulation a floodplain width that abruptly increases at some point. Upstream of the transition, non-gap gravel ceases to move and gap sediment is preferentially transported. At the transition, non-gap sand rapidly deposits from suspension, enhancing gap sediment mobility and diluting its presence on the bed.