I am puzzled by your question in the use of the word you.
The main river of Bangladesh is the Brahmaputra, which becomes the Jamuna River as soon as it enters Bangladesh. It then becomes the Padma River when the Ganges joins it some 72kms west of Dhaka. It then becomes the Padma-Meghna river when they join at Chandpur.
the source is the start of a river
It is a tributary.
Qua main
Narayanganj is a city in Bangladesh
If you mean the actual name of a river in Germany, the main river is The Rhine, althought hey have many rivers. the translation of the word "river" in German is "Fluss"
There is no root word of river. It itself is a root word.
The tributary flowed mostly into the delta, not the main channel of the river. The Missouri River is the longest tributary of the Mississippi river.
river in Hindi is called "NADI"
There is no English word for a river of ice.
The Maori word for river is "awa".
The Sanskrit word for river is "nadi."
"un fleuve" (masc.) is a river in French, but a main river which runs into the sea or into the ocean. A river that runs into another one (and not directly into the sea) is called "une rivière" (fem.), a word that has the same root as the English 'river'.