Sediment deposition, distributary channels, and delta lobe switching
River deltas form where rivers deposit sediment into standing water. As lobes prograde into the sea, the channel gradient decreases until a steeper path — often during floods — triggers an avulsion, switching the active distributary to a new lobe. The Mississippi delta has switched lobes roughly every 1,000–1,500 years, creating the bird-foot morphology. Delta type (river-dominated, wave-dominated, tide-dominated) depends on the ratio of river flux to marine energy.