River deltas consist of multiple lobes that prograde into the ocean. Each active lobe builds up sediment, increasing its super-elevation above neighboring channels. When elevation exceeds a threshold H*, water avulses (abruptly switches) to the lowest adjacent lobe — the shortest path to sea. The Mississippi River avulses every ~1,000 years; without engineering, the Atchafalaya would capture the main channel. Sea level rise affects avulsion frequency and lobe preservation.