A mouthpiece is the place where a river empties into a lake , a Wed or ocean.

The mouth can take the form of an estuary or delta. The word of mouth (s) is sometimes used as in the case of the mouths of the Rhone. A mouth area is bounded by a stream undergoing the effect of tides.

The mouths are usually places of high biodiversity because they are places where freshwater and salt combine to form water more or less brackish water and where nutrients and sediments carried by the river are watered down and rush. A mouthpiece is often prolonged and under water by a zone of sediment or submarine canyon.