Martian linear dune gullies are landforms consisting of parallel and often sinuous channels with distinct levees and pit-shaped endings that occur in the mid-latitudes on Mars. Recent observations of ongoing activity link their formation to sliding blocks of CO2 ice in early spring. Here we combine laboratory experiments in which we release CO2-ice blocks on sandy slopes under Martian atmospheric pressure with morphometric observations of linear dune gullies on Russell crater mega dune to test this hypothesis. Our work shows that linear dune gullies are carved by burrowing blocks of CO2 ice that induce explosive ballistic sediment transport, resulting in gullies with all morphological characteristics we observe on Mars; high levees, deep channels, and extreme channel sinuosity. Our findings highlight the specific climatological, topographical, and grain size conditions under which these landforms originate, making it possible to use them as a microclimate proxy on present-day and potentially ancient Mars.

Martian gullies are landforms consisting of an erosional alcove, a channel, and a depositional apron. A significant proportion of Martian gullies at the mid-latitudes is active today. The seasonal sublimation of CO2 ice has been suggested as a driver behind present-day gully activity. However, due to a lack of in-situ observations, the actual processes causing the observed changes remain unresolved. Here, we present results from flume experiments in environmental chambers in which we created CO2-driven granular flows under Martian atmospheric conditions. Our experiments show that under Martian atmospheric pressure, large amounts of granular material can be fluidized by the sublimation of small quantities of CO2 ice in the granular mixture (only 0.5% of the volume fraction of the flow) under slope angles as low as 10°. Dimensionless scaling of the CO2-driven granular flows shows that they are dynamically similar to terrestrial two-phase granular flows, i.e. debris flows and pyroclastic flows. The similarity in flow dynamics explains the similarity in deposit morphology with levees and lobes, supporting the hypothesis that CO2-driven granular flows on Mars are not merely modifying older landforms, but they are actively forming them. This has far-reaching implications for the processes thought to have formed these gullies over time. For other planetary bodies in our solar system, our experimental results suggest that the existence of gully-like landforms is not necessarily evidence for flowing liquids but that they could also be formed or modified by sublimation-driven flow processes.

We present a new metric for braiding intensity to characterize multi-thread systems (e.g., braided and anastomosing rivers) called the Entropic Braiding Index, eBI. This metric is a generalization of the widely used Braiding Index (BI) which is simply the average count of intercepted channels per cross-section. The co-existence of diverse channels (widely different widths and discharges) within river cross-sections distorts the information conveyed by BI, since its value does not reflect the diversity and natural variability of the system. Moreover, the fact that BI is extremely sensitive to resolution (BI increases at higher resolution as smaller scale channels can be resolved) challenges its applicability. eBI, addresses these main drawbacks of BI. eBI is rooted in the concept of Shannon Entropy, and its value can be intuitively interpreted as the equivalent number of equally important (in terms of discharge) channels per cross-section. Thus, if the channels observed in a multi-thread system are all carrying the same amount of discharge, eBI has the same value of BI. On the other hand, if a very dominant channel in terms of discharge co-exists with much smaller channels, eBI would take a value slightly larger than 1 (note that the actual value would depend on the number of small channels and their relative size with respect to the dominant channel). We present a comparative study of BI and eBI for different multi-thread rivers obtained from numerical simulations and remote-sensing data and for different discharge stages. We also provide evidence of the robustness of eBI in contrast to BI when a given river system is studied under different resolutions. Finally, we explore the potential of eBI as a metric to characterize different types of multi-thread systems and their stability.

Deltas and estuaries worldwide face the challenge of capturing sufficient sediment to keep up with relative sea level rise. Knowledge about sediment pathways and fluxes are crucial to combat adverse effects on channel morphology, e.g. erosion which enhances risk of bank collapse and increasing tidal penetration. We constructed sediment budgets which quantify annual changes for the urbanized delta of the Netherlands affected by fluvial and coastal fluxes of sediment, engineering works and dredging and dumping activities. The Rhine-Meuse delta shows a negative sediment budget in recent decades due to anthropogenic intervention. Following a large offshore port expansion, dredging in ports and harbours in the region has doubled in the past five years, likely due to the induced change in net sediment fluxes. In addition, the deeper navigation channels, ports and harbours are trapping siltier sediment than before, changing sediment composition in the mouth. The removal of sediment from the system through dredging is adverse to the necessity for sediment in heavily eroding branches. To allow for sustainable sediment management in the future and to cope with sea level rise, further measurements are required to properly quantify the amount of incoming sediment from the rivers and the seaward boundary and the mechanisms of transport which are key to solving the sediment issues in the delta. The varied response of the branches has important consequences for navigation, ecology and flood safety and management of the sediment in the system will be of pivotal importance in coming decades and for other deltas worldwide.

The Braiding Index (BI), defined as the average count of intercepted channels per cross-section, is a widely used metric for characterizing multi-thread river systems. However, it does not account for the diversity of channels (e.g., in terms of discharge) within different cross-sections, omitting important information related to system complexity. Here we present a modification of BI (the Entropic Braiding Index, eBI) which augments the information content in BI by using Shannon Entropy to encode the diversity of channels in each cross section. eBI is interpreted as the number of “effective channels” per cross-section, allowing a direct comparison with the traditional BI. We demonstrate the superior capabilities of eBI via analysis of synthetic, numerical and field examples. In addition, we show that interrogating cross-sections via the ratio BI/eBI has the potential to quantify channel disparity, differentiate types of multi-thread systems, and inform about cross-section stability to forcing variability (e.g., seasonal flooding).