  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.
