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.

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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.