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It is known as the mouth of the river, or the river's mouth.
I am assuming you mean the rivers and rivulets that flow int a bigger river and contribute to its flow. In that case they are called tributaries. If you mean what are the rivers called when they flow into the sea, then they are called delta's or river mouths.
The place where the two rivers join to flow together is called a confluence.
it is called the tumber river
old river
The term for the place where a tide meets a river current is called the "tidal bore." This phenomenon occurs when the rising tide enters a river channel, causing the water to flow upstream against the current.
This is called a waterfall
Lava flow.
The place where the river begins is called a source.
The direction that a river flows is called the flow or the drainages. Water generally flows downhill or from a higher elevation to a lower one.
It is called the mouth of the river.
A river usually ends by flowing into an ocean, a lake or a bigger river. The place where the river flows out into a bigger body of water is called the "mouth" of the river.