Comparison of Flow Dynamics and Bifurcation Angles in Tributary and Distributary Channel Networks

The dynamics of channel mouth bifurcations on river deltas can be understood using theory developed in tributary channel networks. Bifurcations in groundwater-fed tributary networks have been shown to evolve dependent on diffusive ground water flow patterns directly adjacent to the channel network, producing a critical angle of 72°. We test the hypothesis that bifurcation angles in distributary channel networks are likewise dictated by a diffusive external flow field, in this case the shallow surface water surrounding the subaqueous portion of distributary channels in a deltaic setting. We measured 25 unique distributary bifurcations in an experimental delta and 197 bifurcations in 10 natural deltas, yielding a mean angle of 70.4° ± 2.6° (95% confidence interval) for natural field-scale deltas and a mean angle of 68.3° ± 8.7° for the experimental delta, consistent with the theoretical prediction. Further analysis shows that angles cluster around the critical angle over small measurement length-scales relative to channel width, even at the moment that channel bifurcations initiate. Distributary channel bifurcations are important features in both modern systems, where the channels control water, sediment, and nutrient routing, and in river delta stratigraphy, where the channel networks can dictate large-scale stratigraphic heterogeneity. Although distributary networks do not mirror tributary networks perfectly, the similar control and expression of bifurcation angles suggests that additional morphodynamic insight may be gained from further comparative study