Being a senior member and the moderator of this forum, its a pity if you are not aware that it is not the origin rather the catchment area which feeds a river the most principle criterion. An in the case of all the rivers and their tributaries (with the exception of Brahmaputra) listed in the above picture have the bulk of their catchment in either India or Nepal. Moreover the terrain and geographical barriers ain't that easy for China or anyone in that case to try, block and divert the river water with ease and little expense.
I guess this clears most of the doubts.
You don't seem to know much about the kinetic energy of a river and just how important that is regarding flow, you reduce the flow you ultimately slowly kill the river. The sheer elevations from Tibet give the tributaries their flow, this steadily reduces as the waters go on to merge and form the Ganges
This is what the Bhote Kosi looks like
before it gets into Nepal, you can see the power of the river (outside dry season).
The Ganges is already experiencing slower flow and increased salinity, which left unchecked it makes the water unfit for human use. Some of the biggest culprits in this case is the building of dams and reservoirs, something China can do, something which free Tibet movements have picked up on and keep on telling India.
If the flow is impeded further, this increases the already stressed Ganges river.
As for catchment that is indeed correct, catchment does form a good part of the Ganges river, but where does the water for that catchment come from? It's from large-scale groundwater systems. The groundwater levels in the Ganges basin have declined over the years. Research papers from the respected journal 'Water Resources Rsearch' stated that basin is falling by 15-20mm every year. Staying on Ganges basin experts have stated that at least 60% of the groundwater has "excessive" levels of arsenic and salt, nature geoscience published the findings.
Now the Ganges is heavily dependent on the summer monsoon rains which provide 85% of India's per annum precipitation. Studies have shown that India is experiencing a reduction in average rainfall during the monsoon season, and also extreme rain events where we see a reduction in length of 'active rain periods' and an increase in break periods. There's also the trend of rain falling heavy on a shorter number of days, with a reduction of longer moderate rain days. This doesn't bode well for the future for the catchment area, an area already under stress, and
if (big if) interfered higher upstream by the Chinese will mean worse things.
I will agree that it is not easy to conduct activities which can damage a neighbour's water supply, so hence why we haven't seen much in the way from India or China in this regard.
Here is an article on why India will not do much to Pakistan's rivers;
The social media is aflame with elated stories, tall stories really, about India’s retaliation for Uri by turning off the spigot of the three western rivers of the Indus basin that flow unhindered into Pakistan and sustain its agriculture.
Prime Minister Narendra Modi lent much credence to this when he said that “blood and water cannot flow together”. But the truth is that the flow of blood can be stopped,
but water will continue to flow.
The Indus river system has a total drainage area exceeding 11,165,000 sq km. Its estimated annual flow is about 207 km, the world’s 21st largest river in annual flow. It’s also Pakistan’s sole means of sustenance. The World Bank brokered the Indus Waters Treaty between India and Pakistan after many years of intense negotiations. It was designed to allocate the Indus river basin waters after the 1947 Partition. The British had constructed a complex canal system to irrigate undivided Punjab. Partition left a large part of this network in Pakistan, but the headwork dams remained in India,
fuelling much insecurity among Pakistan’s Punjabi landowning elite. The IWT was mainly intended to address these Pakistani fears. Jawaharlal Nehru and Field Marshal Ayub Khan signed the treaty in Karachi on September 19, 1960.
Under this agreement, control over three “eastern” rivers — Beas, Ravi and Sutlej — was given to India, while that over three “western” rivers — Indus, Chenab and Jhelum — went to Pakistan. Since Pakistan’s rivers flow through India,
the treaty allowed India to use them for irrigation, transport and power generation, laying down precise regulations for Indian building projects along the way. The treaty was a result of the Pakistani fear that, since the source rivers of the Indus basin were in India, it could potentially create droughts and famines in Pakistan, specially at times of war. Since the treaty’s ratification in 1960, India and Pakistan fought three wars, but the flow of water was not hampered even for a single day.
On the face of it, the pact seems generous to Pakistan as it gives the lower riparian state 80 per cent of the western rivers’ water. The reality, however, is that IWT makes a virtue out of necessity, as it is the region’s geography that decides this, rather than any altruism. The Kashmir Valley is just 100 km wide at its maximum and 15,520.3 sq km in area. While the Himalayas divide the Valley from Ladakh, the Pir Panjal range, that encloses the Valley from west and south, separates it from the great plains of northern India.
This picturesque and densely settled Valley has an average height of 1,850 metres above sea level but the surrounding Pir Panjal range has an average elevation of 5,000 metres. Thus the Pir Panjal range stands between the Kashmir Valley and the rest of the country, and is an insurmountable barrier that precludes the transfer of water anywhere else. Neither do the contours of the Valley allow for more waters to be stored in any part of it. As the waters can’t be stored or used by diversion elsewhere, it has to keep flowing into Pakistan.
Of the three western rivers “given” to Pakistan, the Indus, which leaves Indian territory near Kargil, flows entirely in Pakistan-controlled territory. The Jhelum originates near Verinag near Anantnag and meanders over 200 km in the Valley before it enters Azad Kashmir. After flowing through Srinagar it fills up the Wular Lake and then traverses past Baramulla and Uri. The hydel projects constructed on it supply most of the Valley’s electricity.
The Chenab, also known as Chandrabhaga, originates in Lahaul Spiti in Himachal Pradesh and flows through the Jammu region into the plains of Pakistani Punjab. In J&K, the Chenab basin covers Kishtwar, Doda, Ramban, Udhampur, Jammu and Rajouri districts.
The catchment of the Chenab is elongated and narrow, and the catchment area mostly in India. But the Chenab runs through deep valleys and the river drops by 24 m per km, imposing physical constraints and huge costs in harnessing it.
The Uri incident fuelled much anger within India and the Modi government, that came to power promising to deter Pakistan-based terror in India by threatening retribution, is now hard pressed to deliver. It is discovering there is a yawning gap between promise and reality. The PM’s pre-election speeches are being played back to taunt him. The Modi government is flailing for options, short of the use of arms. Thus the somewhat exasperated suggestion that the government would take a relook the treaty, though senior Pakistani diplomat Sartaj Aziz has made it clear that this will be tantamount to an act of war.
But a relook is easier said than done. For a start, India will find it very difficult to repudiate IWT as there is no clause in it for any one party to “denounce” (legalese for repudiation) the treaty, making it incumbent under the Vienna Convention on the Law of Treaties for both parties to agree on this. Pakistan will never agree to have a bullet fired into its head. Besides as
Dr Shakil Ahmad Romshoo, head of earth sciences at the geology and geophysics department of the University of Kashmir, says: “Let us assume we stop water supply for the sake of argument. Where would the water go? We do not have infrastructure to store this water. We have not built dams in J&K where we can store water. And being a mountainous state, unlike Tamil Nadu or Karnataka, you cannot move water to another state. So you cannot stop the water technically.”
But even if it can be done, climate change is upon both nations with severe implicit consequences for both, but mostly for Pakistan. The Indus river basin is fed mostly by glacier melt, unlike the Ganga and Brahmaputra basins, that are fed mostly by the monsoons. Since climate change is first affecting the Himalayan glaciers, the water patterns in the Indus basin are already showing changes. Hence Pakistan constantly keeps up a drumbeat of false charges on non-adherence to IWT by India. It has only reinforced its determination to seize J&K in the mistaken belief that it will control the water to its irrigation network. But will there be water for very long?
Widely-referenced estimates indicate a troubling long-term trend for the Indus river basin’s flow. River water provides 80 per cent of all irrigation water for Pakistan’s critical agriculture sector. These water sources are already near their limits, with most water diverted to northern Pakistan’s agricultural regions at the expense of the south. In fact, so much water is diverted from the Indus before it reaches the ocean that seawater has invaded the river channel miles inland.
Based on current projections, the Indus river system is expected to fall below 2,000 flow levels between 2030 and 2050. The drop-off is estimated to be most serious between 2030 and 2040, with a new equilibrium flow of 20 per cent below that of 2,000 reached after 2060. Not only is Pakistan running out of water, it seems it will soon be running out of time. As its founding father poet Allama Iqbal wrote: Watan Ki Fikar Kar Nadan! Musibat Ane Wali Hai/Teri Barbadiyon Ke Mashware Hain Asmanon Mein. (Think of the homeland, O ignorant one! Hard times are coming/ Conspiracies for your destruction are afoot in the heavens.)
https://www.deccanchronicle.com/opinion/op-ed/290916/indus-the-water-flow-cant-be-stopped.html