Question

what is call River system? From where the three major long river of the Himalaya raise?

Answer 1

A river along with its tributaries are called River System

The 3 major long rivers of Himalaya :
1)Indus - rises in Tibet , near mansarowar lake
2)Ganga- Ganga emerges from mountains on to the plains in Haridwar. (gangotri glacier)
3)Brahmaputra- rises in Tibet (East of mansarowar lake)

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Answer 2

the rivers which flow into the river mouth (ocean,lakes,etc) . a water boundary which forms due to the flowing water is also known as the drinage system also known as the river system
the rivers of the nothern india and also the ganga  that come from the himalaya raise are very long