Why such a long 1000 km rerouting is needed I do not know. But, I do not think, a diversion of water will affect the present Brahmaputra flow to India and Bangladesh. I had some hydrological data before that I lost. I found another one which I am posting with a minimal editing. -@bluesky-
"(1) Very small proportion of water in transboundary rivers of India come from Tibet".
"Tibet is climatologically a barren cold plateau. It receives between
4 to 12 inches of rainfall depending on time and place. Water efflux in these rivers ,from Tibet, is very low and you could cross Brahmaputra in Tibet without getting your knees wet in most of the places, and in winter when Brahmaputra enters India it is practically dry. This is not just a theoretical. It is supported by hydrological data".
"Brahmaputra enters India just after Tsela Dzong. After this point it it goes into
16.8m/km steep fall before entering India and creating a gorge in the process. Of the
19,800 m3/sec average discharge of Brahmaputra, water from China account for less than
500 m3/sec. Thus there is not much water for China to divert, and it would hardly affect river flow of Brahmaputra".
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Reading and almost understanding the content of hydrological data, I am of the opinion that water diversion by China will not harm India and BD. It is because 500m3/sec is very small comparing to 19,800m3/sec.
Moreover, China will have to use a part of this 500m3/sec water to produce hydro-electricity which will power the pumps to lift up the northbound water. Power stations will certainly be built in the sloped gorge (Tibet to Arunachal) where the gradient is 16.8m per 1 km. It means, at least a small part of this 500m3/sec water from Tibet will also come down to Arunachal after producing power.
I think, the Chinese construction will cause only a loss of 300m3/sec of water for India and BD. It will certainly cause no effect in BD, because in order to get a uniform volume of water throughout the year, China will have to build reservoir in the north of Himalaya. So, this withdrawal will cause less havoc of floods during monsoon and more water during winter dry season. Bd has nothing to worry about.