Learner's definition of RIVER
1 : a large natural flow of water that crosses an area of land and goes into an ocean, a lake, etc.
the Mississippi River
The raft is too small to use on this part of the river.
They have a house on the river. [=on land next to the river]
Large boats came up/down the river. [=in the opposite/same direction that the river is flowing]
the mouth of the river = the river's mouth = the river mouth [=the place where the river enters the ocean]
2 : a large flow of something — often + of
sell (someone) down the river
up the riverUS, old-fashioned + informal