Coastal Louisiana has lost about 5,000 km2 of wetlands over the past century and concern exists whether remaining wetlands will persist while facing some of the world's highest rates of relative sea-level rise (RSLR). Here we analyse an unprecedented data set derived from 274 rod surface-elevation table-marker horizon stations, to determine present-day surface-elevation change, vertical accretion and shallow subsidence rates. Comparison of vertical accretion rates with RSLR rates at the land surface (present-day RSLR rates are 12±8 mm per year) shows that 65% of wetlands in the Mississippi Delta (SE Louisiana) may keep pace with RSLR, whereas 58% of the sites in the Chenier Plain (SW Louisiana) do not, rendering much of this area highly vulnerable to RLSR. At least 60% of the total subsidence rate occurs within the uppermost 5–10 m, which may account for the higher vulnerability of coastal Louisiana wetlands compared to their counterparts elsewhere.

Fluvial channels encounter a backwater reach when they approach a standing body of water, and recent studies have shown that the transition from normal flow to backwater-influenced flow is associated with sediment mass extraction through deposition. Here we test the hypothesis that systematic changes in the geometry of channel-belt deposits and sedimentary architecture occur across this transition, using data from the late Holocene Mississippi (southern USA) and Rhine (The Netherlands) fluvio-deltaic systems. We use the estimated backwater length and average channel width as characteristic length scales to non-dimensionalize the downstream trends in channel-belt width for these systems. The collapsed data follow similar trends, suggesting that the observed variations in channel-belt geometry and fluvio-deltaic stratigraphy are tied to the location of the backwater transition zone. These findings suggest a unifying hydraulic control on fluvio-deltaic channel belts and provide a new framework for predicting and understanding the properties of ancient rivers in the coastal zone.

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Coastal Louisiana has experienced catastrophic rates of wetland loss over the past century, equivalent in area to the state of Delaware. Land subsidence in the absence of rapid accretion is one of the key drivers of wetland loss. Accurate subsidence data should therefore form the basis for estimates of and adaptations to Louisiana's future. Recently, Jankowski et al. (2017) determined subsidence rates at 274 sites along the Louisiana coast. Based on these data we present a new subsidence map and calculate that, on average, coastal Louisiana is subsiding at 9 ± 1 mm yr −1 .