River Deltas and Sea-Level Rise

Abstract

Future sea-level rise poses an existential threat for many river deltas, yet quantifying the effect of sea-level changes on these coastal landforms remains a challenge. Sea-level changes have been slow compared to other coastal processes during the instrumental record, such that our knowledge comes primarily from models, experiments, and the geologic record. Here we review the current state of science on river delta response to sea-level change, including models and observations from the Holocene until 2300 CE. We report on improvements in the detection and modeling of past and future regional sea-level change, including a better understanding of the underlying processes and sources of uncertainty. We also see significant improvements in morphodynamic delta models. Still, substantial uncertainties remain, notably on present and future subsidence rates in and near deltas. Observations of delta submergence and land loss due to modern sea-level rise also remain elusive, posing major challenges to model validation. ▪ There are large differences in the initiation time and subsequent delta progradation during the Holocene, likely from different sea-level and sediment supply histories. ▪ Modern deltas are larger and will face faster sea-level rise than during their Holocene growth, making them susceptible to forced transgression. ▪ Regional sea-level projections have been much improved in the past decade and now also isolate dominant sources of uncertainty, such as the Antarctic ice sheet. ▪ Vertical land motion in deltas can be the dominant source of relative sea-level change and the dominant source of uncertainty; limited observations complicate projections. ▪ River deltas globally might lose 5% (∼35,000 km2) of their surface area by 2100 and 50% by 2300 due to relative sea-level rise under a high-emission scenario. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 51 is May 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

River Delta Morphotypes Emerge From Multiscale Characterization of Shorelines

“…Network information may disaggregate the relatively large river‐tide morphotypes and the tide morphotypes, with a possible separation of the valley‐confined Ob and Yenisei deltas from estuarine systems such as the Kolyma, Ganges Brahmaputra, and Colorado. This further sub‐division of deltas may also be able to yield insight into the influence of other controls on delta morphology including grain size (Caldwell & Edmonds, 2014), valley confinement, cold region processes, or sea level history (Nienhuis et al., 2023; Overeem et al., 2022). Interestingly, no systematic signature of near‐shore sea‐ice, permafrost, or river‐ice was detected on shoreline structure (Lauzon et al., 2019; Overeem et al., 2022; Piliouras et al., 2021), except for a lack of wave influenced Arctic systems which may relate to the short wind fetch present due to sea ice (Barnhart et al., 2014) or the presence of a shallow subaqueous ramp dampening wave runup and breakup at the subaerial shoreline (Overeem et al., 2022).…”

“…Deltas are particularly vulnerable to climate change due to their low relief, coastal proximity, and large populations (Edmonds et al., 2020; Hoitink et al., 2020). It is therefore critical to understand how sea level rise and changing riverine sediment loads will impact these systems (Chadwick et al., 2020; Nienhuis et al., 2023) and toward this goal, developing a quantitative framework that links the driving forces forming deltas to delta morphology and function is imperative.…”

Holocene relative sea-level changes along the Caribbean and Pacific coasts of northwestern South America

“…In addition, exploring postglacial RSL change informs us about modern coastal morphodynamics. Any RSL framework would imply either transgressive (intermediate-field) or regressive (far-field) conditions during the Holocene with implications for contemporary coastal change (Shadrick et al, 2022; Nienhuis et al, 2023).…”

Microbial Necromass, Lignin, and Glycoproteins for Determining and Optimizing Blue Carbon Formation

“…Coastal wetland ecosystems cover only 0.5% of the sea floor, yet they account for ∼50% of the C burial in the global ocean. , The sources of soil organic matter (SOM) in coastal wetlands are complex and can stem from both autochthonous (produced within the wetlands) and allochthonous (transported from the uplands and marines) sources . Coastal wetlands have specific soil conditions that differ from other terrestrial ecosystems, such as low oxygen, high salinity, fluctuating hydrological, and redox conditions. , With the projected 5% reduction in the areas of river delta by the end of the 21st century, coastal wetlands are expected to undergo seawater intrusion, altered hydrological conditions, increase in salinity levels, and changes in vegetation composition . These threats can alter the quantity and source of SOM in coastal wetland soils and potentially affect their ecological functions and resilience.…”

“…3 Coastal wetlands have specific soil conditions that differ from other terrestrial ecosystems, such as low oxygen, high salinity, fluctuating hydrological, and redox conditions. 4,5 With the projected 5% reduction in the areas of river delta by the end of the 21st century, 6 coastal wetlands are expected to undergo seawater intrusion, altered hydrological conditions, increase in salinity levels, and changes in vegetation composition. 7 These threats can alter the quantity and source of SOM in coastal wetland soils and potentially affect their ecological functions and resilience.…”