River deltas form where sediment-laden rivers meet standing water. Channels bifurcate when flow velocity drops and sediment is deposited. Avulsion — sudden channel switching — occurs when a main channel raises its bed above the floodplain and a flood breach creates a steeper path. The Mississippi has avulsed 5 times in 5,000 years.
Galloway's ternary diagram classifies deltas by dominant process: fluvially-dominated (elongate, bird-foot like Mississippi), wave-dominated (arcuate, like Nile), or tide-dominated (funnel-shaped, like Ganges-Brahmaputra). The balance between river energy, wave reworking, and tidal prism determines the final shape.
Channel networks exhibit fractal geometry similar to diffusion-limited aggregation (DLA). Hack's law: L ∝ A^0.6 (channel length scales with drainage area). Horton's laws describe bifurcation ratios (~3-5) and length ratios (~2) across stream orders. Self-organized criticality drives the branching hierarchy.