How is topographic simplicity maintained in ephemeral dryland channels?

Abstract

The lack of topographic complexity in ephemeral dryland channels, despite large variations in hydraulic controls and sediment texture, presents an unexplained paradox that limits understanding of their long-term evolution. In dryland basins, spatially and temporally discontinuous channel flow transports and sorts sediment along the bed intermittently and irregularly. The cumulative effect of these processes counterintuitively produces simple topography, manifest in straight longitudinal profiles and symmetrical cross sections, in contrast with perennial channels. This paper presents numerical modeling experiments based on field measurements to investigate dryland channel topographic development through the responses of bed-material flux and net sediment storage to variations in channel hydrology. We show that spatially variable flow creates and subsequently destroys incipient topography along ephemeral reaches, and that large flood events above a threshold overcome hydraulic and grain-size controls to dampen fluctuations in longitudinal sediment flux through a smoothing of the incipient channel bar forms. The results provide a physical explanation for emergent topographic simplicity in ephemeral dryland channels despite higher variability in streamflow and sedimentary characteristics compared to perennial systems.