An official website of the United States government
Abstract. River deltas are sites of sediment accumulation along thecoastline that form critical biological habitats, host megacities, andcontain significant quantities of hydrocarbons. Despite their importance, wedo not know which factors most significantly promote sediment accumulationand dominate delta formation. To investigate this issue, we present a globaldataset of 5399 coastal rivers and data on eight environmental variables.Of these rivers, 40 % (n=2174) have geomorphic deltas defined eitherby a protrusion from the regional shoreline, a distributary channel network,or both. Globally, coastlines average one delta forevery ∼300 km of shoreline, but there are hotspots of delta formation, for examplein Southeast Asia where there is one delta per 100 km of shoreline. Ouranalysis shows that the likelihood of a river to form a delta increases withincreasing water discharge, sediment discharge, and drainage basin area. Onthe other hand, delta likelihood decreases with increasing wave height andtidal range. Delta likelihood has a non-monotonic relationship withreceiving-basin slope: it decreases with steeper slopes, but for slopes >0.006 delta likelihood increases. This reflects differentcontrols on delta formation on active versus passive margins. Sedimentconcentration and recent sea level change do not affect delta likelihood. Alogistic regression shows that water discharge, sediment discharge, waveheight, and tidal range are most important for delta formation. The logisticregression correctly predicts delta formation 74 % of the time. Our globalanalysis illustrates that delta formation and morphology represent a balancebetween constructive and destructive forces, and this framework may helppredict tipping points at which deltas rapidly shift morphologies.
more »
« less
First‐Order River Delta Morphology Is Explained by the Sediment Flux Balance From Rivers, Waves, and Tides
Abstract We present a novel quantitative test of a 50‐year‐old hypothesis which asserts that river delta morphology is determined by the balance between river and marine influence. We define three metrics to capture the first‐order morphology of deltas (shoreline roughness, number of distributary channel mouths, and presence/absence of spits), and use a recently developed sediment flux framework to quantify the river‐marine influence. Through analysis of simulated and field deltas we quantitatively demonstrate the relationship between sediment flux balance and delta morphology and show that the flux balance accounts for at least 35% of the variance in the number of distributary channel mouths and 42% of the variance in the shoreline roughness for real‐world and simulated deltas. We identify a tipping point in the flux balance where wave influence halts distributary channel formation and show how this explains morphological transitions in real world deltas.
more »
« less
- Award ID(s):
- 1812019
- PAR ID:
- 10380744
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 49
- Issue:
- 22
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract River deltas are home to large populations and can be composed of complex channel networks which convey flows of matter to the shoreline. Knowledge of flow within individual channels is needed to quantify the distribution of discharge across the delta, and thus its sustainability over time. Due to a lack of field measurements at the local channel scale, researchers leverage remote sensing data to estimate the partitioning of flow. We compare data from 15 river deltas to discharge partitioning estimates based on channel network graphs derived from remote sensing imagery. We quantify errors in the common width‐based method and test alternative partitioning techniques to find that width‐based discharge partitioning is universally applicable, suggesting that absent any site‐specific information, discharge partitioning by average channel width is an appropriate approach. We also provide networks, streamflow measurements, and flux partitioning estimates for 28 delta networks as the Discharge In Distributary NeTworks (DIDNT) dataset.more » « less
-
Abstract Deltas exhibit spatially and temporally variable subsidence, including vertical displacement due to movement along fault planes. Faulting‐induced subsidence perturbs delta‐surface gradients, potentially causing distributary networks to shift sediment dispersal within the landscape. Sediment dispersal restricted to part of the landscape could hinder billion‐dollar investments aiming to restore delta land, making faulting‐induced subsidence a potentially significant, yet unconstrained hazard to these projects. In this study, we modeled a range of displacement events in disparate deltaic environments, and observe that a channelized connection with the displaced area determines whether a distributary network reorganizes. When this connection exists, the magnitude of distributary network reorganization is predicted by a ratio relating dimensions of faulting‐induced subsidence and channel geometry. We use this ratio to extend results to real‐world deltas and assess hazards to deltaic‐land building projects.more » « less
-
Abstract. We investigate the interaction of fluvial and non-fluvial sedimentation on the channel morphology and kinematics of an experimental river delta. We compare two deltas: one that evolved with a proxy for non-fluvial (“marsh”) sedimentation (treatment experiment) and one that evolved without the proxy (control). We show that the addition of the non-fluvial sediment proxy alters the delta's channel morphology and kinematics. Notably, the flow outside the channels is significantly reduced in the treatment experiment, and the channels are deeper (as a function of radial distance from the source) and longer. We also find that both the control and treatment channels narrow as they approach the shoreline, though the narrowing is more pronounced in the control compared to the treatment. Interestingly, the channel beds in the treatment experiment often exist below sea level in the terrestrial portion of the delta top, creating a ∼ 0.7 m reach of steady, non-uniform backwater flow. However, in the control experiment, the channel beds generally exist at or above relative sea level, creating channel movement resembling morphodynamic backwater kinematics and topographic flow expansions. Differences between channel and far-field aggradation produce a longer channel in-filling timescale for the treatment compared to the control, suggesting that the channel avulsions triggered by a peak in channel sedimentation occur less frequently in the treatment experiment. Despite this difference, the basin-wide timescale of lateral channel mobility remains similar. Ultimately, non-fluvial sedimentation on the delta top plays a key role in the channel morphology and kinematics of an experimental river delta, producing channels which are more analogous to channels in global river deltas and which cannot be produced solely by increasing cohesion in an experimental river delta.more » « less
-
Abstract It is widely recognized that waves inhibit river mouth progradation and reduce the avulsion timescale of deltaic channels. Nevertheless, those effects may not apply to downdrift‐deflected channels. In this study, we developed a coupled model to explore the effects of wave climate asymmetry and alongshore sediment bypassing on shoreline‐channel morphodynamics. The shoreline position and channel trajectory are simulated using a “shoreline” module which drives the evolution of the river profile in a “channel” module by updating the position of river mouth boundary, whereas the channel module provides the sediment load to river mouth for the “shoreline” module. The numerical results show that regional alongshore sediment transport driven by an asymmetric wave climate can enhance the progradation of deltaic channels if sediment bypassing of the river mouth is limited, which is different from the common assumption that waves inhibit delta progradation. As such, waves can have a trade‐off effect on river mouth progradation that can further influence riverbed aggradation and channel avulsion. This trade‐off effect of waves is dictated by the net alongshore sediment transport, sediment bypassing at the river mouth, and wave diffusivity. Based on the numerical results, we further propose a dimensionless parameter that includes fluvial and alongshore sediment supply relative to wave diffusivity to predict the progradation and aggradation rates and avulsion timescale of deltaic channels. The improved understanding of progradation, aggradation, and avulsion timescale of deltaic channels has important implications for engineering and predicting deltaic wetland creation, particularly under changing water and sediment input to deltaic systems.more » « less
