You probably mean the river bed.
tributary
The word is estuary. It means where the river flows into the sea.
The word source means, a place where something has its beginning or is found. eg: one from where a river flows.
un fleuve - a river It's a little more specific than that. The French have two words for river: Riviere: a river that flows into another river. Fleuve: a (usually larger) river that flows directly into a salt water sea or ocean.
The name of the country Zambia has been derived from the name of the Zambezi river. This river flows west from the Kaleni hills to form the southern border of the country.
estuary
It is a tributary.
The Pearl River is named for the pearl-colored shells that were once found along its banks. The name comes from the Chinese word "zhujiang," which means "Pearl River."
The Red river
A Tributary
Connecticut was given its name for one of the rivers that flows through it. The Connecticut River begins in New Hampshire and flows through three other states.
The 'source' of a river.