Rivers can abruptly shift pathways in rare events called avulsions, which cause devastating floods. The controls on avulsion locations are poorly understood as a result of sparse data on such features. We analyzed nearly 50 years of satellite imagery and documented 113 avulsions across the globe that indicate three distinct controls on avulsion location. Avulsions on fans coincide with valley-confinement change, whereas avulsions on deltas are primarily clustered within the backwater zone, indicating a control by spatial flow deceleration or acceleration during floods. However, 38% of avulsions on deltas occurred upstream of backwater effects. These events occurred in steep, sediment-rich rivers in tropical and desert environments. Our results indicate that avulsion location on deltas is set by the upstream extent of flood-driven erosion, which is typically limited to the backwater zone but can extend far upstream in steep, sediment-laden rivers. Our findings elucidate how avulsion hazards might respond to land use and climate change.
River deltas and their marsh platforms host diverse ecosystems threatened by anthropogenic impacts to coastal areas, such as rising sea levels, subsidence, and leveeing of channels (Ericson et al., 2006). Organic material production, a critical form of sediment accumulation in many river deltas, is the primary driver of marsh platform growth (Nyman et al., 2006), whereas clastic sedimentation via rivers drives deltaic lobe growth (Edmonds et al., 2009). To successfully predict the long-term fate of these ecosystems, the interaction controlling delta and marsh growth must be understood (Paola et al., 2011). While much is known about surface processes in channelized portions of river deltas (
Lowland deltas experience natural diversions in river course known as avulsions. River avulsions pose catastrophic flood hazards and redistribute sediment that is vital for sustaining land in the face of sea‐level rise. Avulsions also affect deltaic stratigraphic architecture and the preservation of sea‐level cycles in the sedimentary record. Here, we present results from an experimental lowland delta with persistent backwater effects and systematic changes in the rates of sea‐level rise and fall. River avulsions repeatedly occurred where and when the river aggraded to a height of nearly half the channel depth, giving rise to a preferential avulsion node within the backwater zone regardless of sea‐level change. As sea‐level rise accelerated, the river responded by avulsing more frequently until reaching a maximum frequency limited by the upstream sediment supply. Experimental results support recent models, field observations, and experiments, and suggest anthropogenic sea‐level rise will introduce more frequent avulsion hazards farther inland than observed in recent history. The experiment also demonstrated that avulsions can occur during sea‐level fall—even within the confines of an incised valley—provided the offshore basin is shallow enough to allow the shoreline to prograde and the river to aggrade. Avulsions create erosional surfaces within stratigraphy that bound beds reflecting the amount of deposition between avulsions. Avulsion‐induced scours overprint erosional surfaces from sea‐level fall, except when the cumulative drop in sea‐level is greater than the channel depth and less than the basin depth. Results imply sea‐level signals outside this range are removed or distorted in delta deposits.
River deltas are home to hundreds of millions of people worldwide and are in danger of sinking due to anthropogenic sea-level rise, land subsidence, and reduced sediment supply. Land loss is commonly forecast by averaging river sediment supply across the entire delta plain to assess whether deposition can keep pace with sea-level rise. However, land loss and deposition vary across the landscape because rivers periodically jump course, rerouting sediment to distinct subregions called delta lobes. Here, we developed a model to forecast land loss that resolves delta lobes and tested the model against a scaled laboratory experiment. Both the model and the experiment show that rivers build land on the active lobe, but the delta incurs gradual land loss on inactive lobes that are cut off from sediment after the river abandons course. The result is a band of terrain along the coast that is usually drowned but is nonetheless a sink for sediment when the lobe is active, leaving less of the total sediment supply available to maintain persistent dry land. Land loss is expected to be more extensive than predicted by classical delta-plain–averaged models. Estimates for eight large deltas worldwide suggest that roughly half of the riverine sediment supply is delivered to terrain that undergoes long periods of submergence. These results draw the sustainability of deltas further into question and provide a framework to plan engineered diversions at a pace that will mitigate land loss in the face of rising sea levels.
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 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) and longer. We also find that the treatment channels have the same width from the entrance to the shoreline, while the control channels get narrower as they approach the shore. 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, nonuniform 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 as 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.
<p>River deltas and the vast marshes that they host are impossible to separate. However, the sediment dynamics of rivers (channel- and lobe-based deposition) and marshes (elevation-based deposition) have not been investigated as a coupled system. We investigate this coupling by comparing a laboratory delta experiment with proxy marsh accumulation to a proxy-less control. The proxy adds just 8% mass to the system but clearly influences delta slopes and mass partitioning. Slopes in the marsh window (elevations around sea level where marsh accumulates) are reduced by 40%, as marsh deposition away from channels smooths topography. While riverine sedimentation continues to be 85% of the deposit in the marsh window, the reduced slopes increase the area in this zone such that 1.3 times more clastic volume is deposited in this window. The area above the marsh window and the mass fraction deposited at these high elevations is correspondingly reduced as if the marshes are &#8220;stealing&#8221; riverine sediment from upstream. Comparing experimental elevation distributions to the field, we show that large deltas might also exhibit this signature. Given that coastal risk is tied to elevation, these findings show that the coupling between marshes and deltas significantly impacts how they should be managed.</p>
<p>Many deltas contain extensive marshes, typically defined as laterally extensive, low energy settings tied to a narrow elevation window around sea level. Biological activity in marshes results in in-situ organic sediment accumulation that has the potential to be stored in the sedimentary record. However, it is unclear how marshes interact with channels that transport the clastic sediment and typically control autogenic stratigraphic architecture. We present results from a physical experiment designed to explore the coupled evolution of marshes and deltas over geologic timescales. In the experiment, deltaic channels self-organized due to constant input rates of water and clastic sediment that experience constant long-term accommodation production through sea-level rise. A low bulk density kaolinite clay was deposited on the delta-top following rules developed by the ecology community for in-situ organic production. The kaolinite clay serves as a proxy for the in-situ organic sediments in overbank regions. As such, the autogenic processes of the clastic transport system, which influence elevation relative to sea-level, also exert a control on the scales of preserved organic-rich strata. We quantify the fraction of the organic sediment proxy in the fluvio-deltaic deposit to define a transfer function between the accumulation of organic sediment and its preservation beneath the morphodynamically active layer. We also use synthetic stratigraphy and images of the preserved strata to characterize the spatial arrangements of organic strata, and the influence of marshes on the resulting arrangement of channel bodies. Initial findings suggest that the thickest seams are located near the mean shoreline but extend significant distances from this location due to autogenic shoreline transgressions and regressions. Quantifying these trends will inform our understanding of how in-situ organic sediment accumulation influences clastic transport systems and the structure of deltaic stratigraphy.</p>
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