Changed hydrological regimes, sea-level rise, and accelerated subsidence are all putting river deltas at risk across the globe. Deltas may respond to these stressors through the mechanism of avulsion. Decades of delta avulsion studies have resulted in conflicting hypotheses that avulsion frequency and location are upstream (water and sediment discharge) or downstream (backwater and sea-level rise) controlled. In this study, we use Delft3D morphodynamic simulations to investigate the main controls over delta avulsion. Avulsion timing and location were recorded in six scenarios modelled over a 400-year period with varying alluvial slopes upstream of a delta slope break (1.13x10-4 to 3.04x10-3) within a range representative global deltas. We measure several independent morphometric variables including avulsion length, delta lobe width, channel width at avulsion, delta topset slope and sediment load. Correlating these variables with the avulsion timescales observed in our model shows that avulsion timescale is mostly controlled by sediment load, which in turn is controlled by the alluvial slope upstream of a delta slope break. With higher stream power index in steeper alluvial slopes, more sediment can be carried within a channel, resulting in more frequent avulsions. Our results are consistent with the avulsion timescale derived from an analytical solution, 19 natural deltas and downscaled physical laboratory deltas. These results help mitigate delta avulsion risk by focusing management efforts on variables that primarily control avulsion in a river delta, but also induce further debate over whether sea-level rise may, or may not, trigger more avulsions in river deltas.

River deltas are under external stress from sea-level rise, subsidence, and decreases in sediment and water discharges caused by anthropogenic activity. Naturally, delta channels respond to these stressors by avulsing and bifurcating. Avulsion involves an abrupt change of channel course that changes the locus of sediment deposition. Bifurcation occurs in the most seaward parts of river deltas where channels divide due to mouth bar deposition. However, how avulsion (top-down) and bifurcation (bottom-up) processes interact in river deltas is poorly understood. We conducted a suite of morphodynamic numerical model experiments using six scenarios with different slopes, selected within the range observed in natural deltas, upstream from the delta apex. The experiments allow us to understand the internal (autogenic) interaction of avulsion and bifurcation in the absence of external (allogenic) forcing. We find that topset slope (Stopset) primarily controls the avulsion timescale (Ta) with Ta = 0.3Stopset-1.18 (R2 = 69%; p < 0.05). Avulsion and bifurcation are shown to occur simultaneously based on the non-unimodal distribution of dimensionless island sizes created in our model, even though these are mechanistically different processes. Comparing our findings to natural deltas, we find consistent avulsion timescale-topset slope (Ta-Stopset) relationships. Our findings show how the delta topset slope serves as the first order control of the avulsion timescale, and how avulsion and bifurcation interact throughout delta building processes. This interaction is significant due to their direct impact on coastal and inland hazards that arise from rapid geomorphic change and flooding on densely populated deltas.