How much subsidence is enough to change the morphology of river deltas?

Understanding the effect of subsidence on fluviodeltaic morphology is important not only to maintain sustainable coastal cities and habitats but also to interpret the information contained in the stratigraphic record. While tectonic steering in alluvial environments has been investigated, similar studies in fluviodeltaic environments are limited to physical experiments and field observations. We perform numerical experiments with a parcel-based cellular model to analyze the deltaic surface and subsurface responses to regional subsidence. We quantify model results using robust metrics and show that while sediment partitioning and shoreline pattern vary gradually with increasing subsidence rate, channel mobility and stratigraphic connectivity of channel deposits show a threshold transition. Conditions for this transition are captured with a dimensionless filling index β, defined as the ratio between the rates of accommodation creation and sediment supply. A channel-locking mechanism activates when β exceeds 0.6 and is responsible for the threshold transition.