A dendritic pattern is a pattern of growth that resembles a tree. The term "dendritic pattern" is especially associated with the pattern water takes as it drains off of land, but dendritic patterns are extremely common throughout nature. Dendritic patterns form when a kind of random motion occurs.
Dendritic patterns are formed from a kind of growth called diffusion-limited aggregation. Diffusion-limited aggregation occurs when resources travel about randomly until they hit each other and join a static structure. This sort of diffusion-based growth occurs in the flow of water and the formation of lightning strikes. Dendritic drainage systems are not the only pattern in which water may form rivers, they are the most common. Dendritic pattern drainage occurs on flat land with steady water flow and soft, non-porous rock that is free of geological fault lines.