Learner's definition of RIVER
a large natural flow of water that crosses an area of land and goes into an ocean, a lake, etc.
see color picture on this page;
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]
a large flow of something
— often + of
sell (someone) down the river
up the river
US, old-fashioned + informal