Rivers are freshwater bodies that primarily form due to surface precipitation runoff. When rain falls or snow melts, the water either infiltrates into the ground or collects into small puddles that grow in size and become rivers. Due to the action of gravity, these bodies of water naturally flow downstream through various channels before they eventually empty into the sea.
Rivers typically rise from elevated regions, such as mountains, highlands and hills, where they flow at a rapid rate when the terrain steeply slopes down. Although the majority of river sources come from runoff, some rivers are fed by water stored underground in rock crevices and pores. This process occurs when rivers have enough force to incise into the saturated layer of the water table.