the primary path of a river
the main course of a river or stream.
_ The principal channel of the river.
the principal course of a river
The main channel of a river -- as opposed to the streams and smaller rivers that feed into it.
In a drainage, the mainstem is the primary stream or river into which most tributaries flow.
The principle channel of a drainage system into which other smaller streams or rivers flow.
The principle stream or river of a particular basin.
A major stream channel which is joined by numerous tributaries.