Autogenic Formation of Bimodal Grain Size Distributions in Rivers and Its Contribution to Gravel-Sand Transitions

Riverbeds often fine downstream, with 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 relative paucity (gap) around the range 1–5 mm. There is no general 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 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.