The dense populations that inhabit global coastlines have an uncertain future due to increased flooding, storms, and human modification. The channel networks of deltas and marshes that plumb these coastlines present diverse architectures, including well‐studied dendritic topologies. However, the quasi‐stable loops that exist in nearly all 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 hydrodynamic forcings at short time scales relative to network evolution. Using field data and satellite imaging, we confirm this control on 21 field networks. Our comparison provides compelling evidence that hydrodynamic fluctuations are capable of stabilizing loops in geophysical systems. Plain Language Summary: How do coastal channel networks form, and what are their stable configurations? The answers to these questions are essential for predicting how coasts will change under growing human and climatic stresses like sea level rise, river damming, or river engineering. Loops exist in most coastal networks, yet physical controls on the initiation and stability of this fundamental network feature are unknown. We use a model of channel network evolution to show that loops form only when there is interplay between distinct flow patterns. Field data also show that river‐tidal interplay coincides with the presence or absence of loops in networks on coasts. This link between network structure and flow patterns can indicate a network's equilibrium state or inform how it might react to changing conditions. Key Points: No theory exists to explain the presence of quasi‐stable loops in coastal channel networksA simple model that includes interaction between river and periodic tidal forcing produces loops at equilibriumAn analysis of 21 coastal channel networks generally agrees with the simulations [ABSTRACT FROM AUTHOR]