UNCHANNELIZED FLOW DICTATING BIFURCATION ANGLE IN DISTRIBUTARY CHANNEL NETWORKS

Distributary channel bifurcations on river deltas are important features in both modern systems, where the channels control water, sediment, and nutrient routing, and in ancient deltas within the stratigraphic record, where the channels dictate the large-scale stratigraphic heterogeneity. Distributary network structure has long been thought to be controlled by flow velocity, grain size, and channel aspect ratio where the channel enters the basin. We show that theory originally developed for tributary networks fed by groundwater seepage can explain the bifurcation angle of distributary bifurcations. Interestingly, bifurcations in groundwater-fed tributary networks have been shown to evolve dependent on the diffusive flow patterns around the channel network. These networks possess a characteristic bifurcation angle of 72°, due to Laplacian flow (nabla²h² =0, where h is water surface elevation) in the groundwater flow field near tributary channel tips. We develop and test the hypothesis that bifurcation angles in distributary channel networks are likewise dictated by the external flow field, in this case the shallow surface water surrounding the subaqueous portion of distributary channel tips in a deltaic setting. We measured 130 unique distributary bifurcations in two experimental deltas and in 10 natural deltas, yielding a mean angle of 70.35°±2.59° (95% confidence interval), in line with the theoretical prediction. This similarity implies that flow outside of the distributary channel network is also Laplacian, which we use scaling arguments to justify. We conclude that the dynamics of the unchannelized flow control bifurcation formation in distributary networks.