Flexi Says: The source is where the river starts usually in mountains while the mouth of the river is where it enters a larger body of water such as a lake or an ocean. Most rivers have a number of tributaries that meet to form larger streams that make up the main river. Where the land has little slope there may be meanders. The lands on the sides of the river where the overflow goes during flooding are the floodplains. The river meets a lake or the sea at its mouth and a delta may form from the buildup of sediment deposited by the river.