What is the source of a river called?

All rivers have a starting point where the water begins its flow. This source is called a headwater. The headwater can come from rainfall or snowmelt in mountains, but it can also bubble up from groundwater or form at the edge of a lake or large pond. The other end of a river is called its mouth, where water empties into a larger body of water, such as a lake or ocean. Along the way, rivers may pass through wetlands where plants slow down the water and filter out pollutants.

Source: National Geographic