The rivers merge in Mukoodal are
Both the rivers merge together in Bangladesh before they merge into Arabin Sea.
Streams often merge with one another to become rivers.
The Gila and Colorado rivers merge near Yuma Arizona.
An estuary.
When two rivers meet it is called confluence. It applies to place where two or more bodies of water merge. There are 10 major confluence around the world.
Estuary
A junction. I live in a place called Grand Junction where three rivers merge.
The Missouri and the Ohio rivers are the two major rivers that merge into the Mississippi.
It forms an oxbow lake.
The rivers Swale, Ure, Nidd and Wharfe all join the River Ouse in North Yorkshire. In South Yorkshire there are the Rivers Don, Rother and Sheaf. In West Yorkshire is the River Calder. They all merge to form the River Humber.
Don't know if I am answering your question correctly. But to my understanding we call them "sub-rivers" or "tributaries" which will merge into the main rivers.
Near present day North Platte, Nebraska.