Will India be able to prevent the Indus River and its two tributaries from flowing into Pakistan?
It's a question on many minds, after India scrapped a major water management agreement between the two countries for six rivers in the Indus Basin, following a horrific attack in Indian-administered Kashmir on Tuesday.
The 1960 Indus Waters Treaty survived two wars between these nuclear rivals and was seen as an example of transboundary water management.
The cancellation of that agreement is one of several steps India has taken against Pakistan, accusing it of supporting cross-border terrorism, a charge Islamabad categorically denies.
He also retaliated with reciprocal measures against Delhi and said he would "consider stopping the river's flow as a declaration of war."
The treaty allocated the three eastern rivers, Ravi, Beas and Sutlej, from the Indus basin to India, while 80 percent of the three western rivers, Indus, Jhelum and Chenab, were allocated to Pakistan.
The agreement was signed after more than a decade of negotiations and disagreement between the two countries over the division of the Indus and its tributaries, following the split into two countries in 1947.
Disputes have flared in the past as Pakistan has opposed some of India's hydropower and water infrastructure projects, claiming they would reduce river flow and violate the agreement.
(More than 80 percent of Pakistan's agriculture and about a third of its hydropower depend on waters from the Indus Basin.)
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India, meanwhile, has sought a review and modification of the agreement, citing that irrigation and drinking water needs have changed due to climate change.
Over the years, Pakistan and India have pursued opposing legal paths under an agreement brokered by the World Bank.
But this is the first time that any party has announced its complete annulment, and it is particularly notable that it was done by the upstream country, India, which gives it a geographical advantage.
But what does this annulment really mean?
Can India stop or divert waters from the Indus Basin, depriving Pakistan of its lifeblood?
And is she even capable of such a thing?
Experts say it is almost impossible for India to retain tens of billions of cubic meters of water from western rivers during periods of high water levels.
It has neither the massive storage infrastructure nor the large canals necessary to divert such a large amount of water.
"The infrastructure that India has is mostly on-river hydropower that doesn't require massive storage," says Himanshu Thakkar, a regional water resources expert at the South Asia Network of Dams, Rivers and People.
Such hydroelectric power plants use the power of flowing water to turn turbines and generate electricity, without retaining huge amounts of water.
Indian experts say inadequate infrastructure has prevented India from fully utilizing even 20 percent of its share of the Jhelum, Chenab and Indus waters under the treaty, a key reason cited for building storage structures, which Pakistan opposes citing the treaty's provisions.
Experts say India can now modify existing infrastructure or build new ones to retain or divert more water without informing Pakistan.
"Unlike in the past, India will now not be required to share project documentation with Pakistan," says Thakar.
But challenges such as difficult terrain and protests within India over some of its projects meant that construction of water infrastructure in the basin was not proceeding quickly enough.
After the 2016 extremist attacks in Kashmir, An official from India's Ministry of Water Resources told the BBC that it will accelerate the construction of several dams and water storage projects in the Indus basin.
Although there is no official information on the status of these projects, sources say that progress has been very limited.
Some experts say that if India starts controlling the flow with its existing and potential infrastructure, then Pakistan could start feeling the consequences during the dry season, when water availability is at its lowest anyway.
"The bigger concern is what will happen during the dry season."
"When flows across the basin are lower, storage is more important, and timing is critical," wrote Hassan F. Khan, assistant professor of urban environmental policy and environmental studies at Tufts University, in Don's letter.
"In that situation, the absence of restrictions from the agreement could be felt more strongly."
The agreement requires India to share hydropower data with Pakistan, which is crucial for flood forecasting and planning for irrigation, hydropower and drinking water.
Pradip Kumar Saksena, India's former commissioner for the Indus Waters Treaty for more than six years, told the news agency Press Trust of India that the country can now stop sharing flood data with Pakistan.
The region is hit by damaging floods during the monsoon season, which begins in June and lasts until September.
But Pakistani authorities said India already shares very limited hydrological data.
"India was sharing only about 40 percent of the data even before this latest announcement," Shiraz Memon, Pakistan's former additional commissioner for the Indus Waters Treaty, tells BBC Urdu.
Another problem, which arises whenever there is tension over water in a region, is that an upstream country can turn water into a "weapon" against a downstream country.
This is often called a "water bomb," when upstream land temporarily holds back water and then suddenly releases it, without warning, causing massive damage downstream.
Could India do it?
Experts say India would first risk flooding its own territory because its dams are far from the Pakistani border.
However, it could now release sludge from its reservoirs without warning, potentially causing damage downstream in Pakistan.
Himalayan rivers such as the Indus carry high levels of silt, which quickly accumulates in dams and barrages.
A sudden release of this sludge can cause significant damage downstream.
There is also a bigger picture: India is located downstream from China in the Brahmaputra basin, and the Indus rises in Tibet.
In 2016, after India warned that "blood and water cannot flow together" after an extremist attack in Kashmir, China blocked a tributary of the Yarlung Tsangpo, which becomes the Brahmaputra in northeastern India, as part of a hydroelectric project.
After building several hydroelectric dams in Tibet, China has approved what will become the world's largest dam in the lower reaches of the Yarlung Tsangpo.
Beijing claims there will be minimal environmental impact, but India fears it could give China significant control over the river's flow.
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