Interplay of river and tidal forcings promotes loops in coastal channel networks

Global coastlines and their dense populations have an uncertain future due to increased flooding, storms, and human modification. The distributary channel networks of deltas and marshes that plumb these coastlines present diverse architectures, including well-studied dendritic topologies. However, the quasi-stable loops that are frequent in many coastal networks have not yet been explained. We present a model for self-organizing networks inspired by vascular biophysics to show that loops emerge when the relative forcings between rivers and tides are comparable, resulting in interplay between processes at short timescales relative to network evolution. Using field data and satellite imaging, we confirm this control on 21 natural networks. Our comparison provides the first evidence that hydrodynamic fluctuations promote loop formation in geophysical systems.