Every cell drains to its lowest neighbour. Flow piling up downstream erodes the bed faster, which captures more flow; gentle uplift renews the land.
Tap to raise a hill and watch rivers reroute. The same ?seed= always gives the same run.
LITStream-power erosion and drainage-network models (Rodríguez-Iturbe & Rinaldo). The landscape must stay finite, or the page throws.
AMBERParameters are illustrative.