Rivers are flows of water that connect a source (a spring, snowmelt, etc.) to lower ground, often an ocean or sea. Fictional rivers may contain rapids, waterfalls, and islands. Portions of rivers may pass through lakes, canyons, or completely underground. They may be spanned by bridges and fords. There may be ports, ferries, dams, and fishing towns along a rivers’ length. They may be important for travel routes or trade routes within a region.

Rivers always move from high areas to low areas, eventually meeting the sea at a river delta. As they make this journey other rivers and streams flow into them, so a river is usually larger and carries more water at its end compared to its’ beginning (unless the river completely evaporates or is absorbed by dry ground before reaching its’ destination). Some rivers bifurcate (split into two as they seek low ground), but this is very rare. When designing rivers for fictional worlds (and in particularly maps) it may be best to think of them as a thin tree, with many small branches (streams and tributaries) feeding into a central trunk (the main river) until it reaches the ground (the sea or ocean).