Mountain streams comprise between 60 and 80% of total global river length. A primary trait of these streams are step-pool sequences. Whether the spacing of these features are regularly or randomly spaced is matter of debate. Erikson and Turowski resolve longstanding tension between the two competing models. Using data on natural, experimental, and numerically simulated step-pool sequences, they demonstrate that step-pool spacing rarely matches either assumption, but rather exists within a continuum. The findings of Erikson and Turowski challenge existing step-pool formation models, emphasize that efforts to replicate or install step-pools are fundamentally misguided when assuming regularity or randomness, and offer a potential alternative.

This research letter presents a conceptual model and data from physical experiments for a new mechanism of sediment entrainment on the seafloor during the huge co-seismic motion imposed by the large subduction earthquakes. This new mechanism introduces the concept of sediment entrainment being due to the motion of the sediment bed instead caused my movement of the water above. Furthermore, by identifying the sedimentary fingerprint of megathrust ruptures with high tsunamigenic potential, the authors propose a new approach to constraining the seismic and tsunami hazard in subduction zones.