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River deltas are built by cycles of lobe growth and abrupt channel shifts, or avulsions, that occur within the backwater zone of coastal rivers. Previous numerical models differ on the origin of backwater-scaled avulsion nodes and their consistency with experimental data. To unify previous work, we developed a numerical model of delta growth that includes backwater hydrodynamics, river mouth progradation, relative sea level rise, variable flow regimes, and cycles of lobe growth, abandonment, and reoccupation. For parameter space applicable to lowland deltas, we found that flow variability is the primary mechanism to cause persistent avulsion nodes by focusing aggradation within the backwater zone. Backwater-scaled avulsion nodes also occur under less likely scenarios of initially uniform bed slopes or during rapid relative sea level rise and marine transgression. Our findings suggest that flow variability is a fundamental control on long-term delta morphodynamics.

Plain Language Summary River deltas are important for farming and drinking water, human populations, and diverse wildlife. Rivers on deltas are unstable and abruptly change course every 10-1,000 years. These channel shifts are necessary for sustaining coastal landscapes and also pose significant hazards. Here we present a mathematical model that shows how rivers require occasional floods, similar to what is observed on natural rivers, to give rise to a predictable location where rivers shift their course. Model simulations without floods produce rivers that change course at random locations, unlike natural rivers. Our findings resolve differences in previous studies about the importance of floods and illustrate that occasional floods are necessary for natural delta growth.