Persistently high bedload flux in ephemeral channels

We present the most comprehensive dataset of bedload transport in ephemeral channels compiled to date. These nine ephemeral channels cover a range of dryland climates and channel types. First, we evaluate these channels and how they compare with each other. Next, we contrast this database with a previously compiled bedload dataset encompassing 92 perennial rivers. While previous studies have identified differences between measured bedload flux in perennial and ephemeral systems, we quantify those differences across a wide range of channel types and shear stress conditions. We find that the ephemeral dataset is statistically distinct, showing greater average transport across flow conditions in normalized shear vs. bedload flux space. Prior researchers have variously attributed these high transport rates to a combination of factors that commonly define ephemeral channels: lack of armoring, mixed sand and gravel, flashy hydrographs, erodible banks, and lack of vegetation. We tested the influence of armoring by comparing transport differences at different transport stages, finding that bed armor contributes to the observed differences, but is not the sole reason. In addition to these previously proposed mechanisms, we add that the abundance of very coarse sand and fine gravels in ephemeral channels provides easily-mobilized but difficult-to-suspend particles.