The Monterey East system is formed by large‐scale sediment waves deposited as a result of flows stripped from the deeply incised Monterey fan valley (Monterey Channel) at the apex of the Shepard Meander. The system is dissected by a linear series of steps that take the form of scour‐shaped depressions ranging from 3·5 to 4·5 km in width, 3 to 6 km in length and from 80 to 200 m in depth. These giant scours are aligned downstream from a breech in the levee on the southern side of the Shepard Meander. The floor of the breech is only 150 m above the floor of the Monterey fan valley but more than 100 m below the levee crests resulting in significant flow stripping. Numerical modeling suggests that the steps in the Monterey East system were created by Froude‐supercritical turbidity currents stripped from the main flow in the Monterey channel itself. Froude‐supercritical flow over an erodible bed can be subject to an instability that gives rise to the formation of cyclic steps, i.e. trains of upstream‐migrating steps bounded upstream and downstream by hydraulic jumps in the flow above them. The flow that creates these steps may be net‐erosional or net‐depositional. In the former case it gives rise to trains of scours such as those in the Monterey East system, and in the latter case it gives rise to the familiar trains of upstream‐migrating sediment waves commonly seen on submarine levees. The Monterey East system provides a unique opportunity to introduce the concept of cyclic steps in the submarine environment to study processes that might result in channel initiation on modern submarine fans.

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Advances in acoustic imaging of submarine canyons and channels have provided accurate renderings of sea-floor geomorphology. Still, a fundamental understanding of channel inception, evolution, sediment transport and the nature of the currents traversing these channels remains elusive. Herein, Autonomous Underwater Vehicle technology developed by the Monterey Bay Aquarium Research Institute provides high-resolution perspectives of the geomorphology and shallow stratigraphy of the San Mateo canyon-channel system, which is located on a tectonically active slope offshore of southern California. The channel comprises a series of crescent-shaped bedforms in its thalweg. Numerical modelling is combined with interpretations of sea-floor and shallow subsurface stratigraphic imagery to demonstrate that these bedforms are likely to be cyclic steps. Submarine cyclic steps compose a morphodynamic feature characterized by a cyclic series of long-wave, upstream-migrating bedforms. The bedforms are cyclic steps if each bedform in the series is bounded by a hydraulic jump in an overriding turbidity current, which is Froude-supercritical over the lee side of the bedform and Froude-subcritical over the stoss side. Numerical modelling and seismic-reflection imagery support an interpretation of weakly asymmetrical to near-symmetrical aggradation of predominantly fine-grained net-depositional cyclic steps. The dominant mode of San Mateo channel maintenance during the Holocene is interpreted to be thalweg reworking into aggrading cyclic steps by dilute turbidity currents. Numerical modelling also suggests that an incipient, protoSan Mateo channel comprises a series of relatively coarse-grained net-erosional cyclic steps, which nucleated out of sea-floor perturbations across the tectonically active lower slope. Thus, the interaction between turbiditycurrent processes and sea-floor perturbations appears to be fundamentally important to channel initiation, particularly in high-gradient systems. Offshore of southern California, and in analogous deep-water basins, channel inception, filling and maintenance are hypothesized to be strongly linked to the development of morphodynamic instability manifested as cyclic steps.

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Upper-flow-regime bedforms and their role in the evolution of marine and lacustrine deltas are not well understood. Wave-like undulations on delta foresets are by far the most commonly reported bedforms on deltas and it will take time before many of these features get identified as upper-flow-regime bedforms. This study aims at: (1) Providing a summary of our knowledge to date on deltaic bedforms emplaced by sediment gravity flows; (2) illustrating that these features are most likely transitional upper-flow-regime bedforms; and (3) using field case studies of two markedly different deltas in order to examine their role in the evolution of deltas. The study combines numerical analysis with digital elevation models, outcrop, borehole, and high-resolution seismic data. The Mazzarrà river delta in the Gulf of Patti, Italy, is selected to show that upper-flow-regime bedforms in gullies can be linked to the onset, growth, and evolution of marine deltas via processes of gully initiation, filling, and maintenance. Ice-marginal lacustrine deltas in Germany are selected as they illustrate the importance of unconfined upper-flow-regime bedforms in the onset and evolution of distinct delta morphologies under different lake-level trends.