Can countries use cross-border rivers to pressure neighbouring countries?

Question

For example, can Sudan block/divert the Nile to exert political pressure on Egypt? (The Nile is a major river that flows through Sudan into Egypt.)

I can't find any results via Google on this. The closest is the Berlin Rules on Water Resources, which says for example that:

The document requires that nations take appropriate steps to sustain and manage water resources, in conjunction with other resources, and minimize environmental harm. In addition to setting out various regulations for nations to follow with respect to water within their boundaries and water they may share, it regulates behavior in wartime, including damage to water installations such as dams and dikes. Nations are not permitted to take action that may result in a shortage of life-sustaining water for civilians, unless a nation being invaded is compelled by military emergency to disable its own water supply, or that may cause undue ecological damage. Poisoning water necessary for survival is in all cases forbidden.

Well and good, but the rules appear to be set by the International Law Association, which does not appear to have any authority in international relations; it is not even an arm of the United Nations.

Answer 1

Yes

You asked if countries can "pressure" other countries with river management. Of course they can. There is a wide range of possible behaviour that is allowed but not required between nations, and how that is used can be considered "pressure." Take, for instance, EU members using the thread of cutting subsidies to pressure other EU members. That's pressure if one takes unconditional subsidies for granted, not pressure if one assumes that subsidies are the expression of shared (or no longer shared) interests. Likewise, countries don't have to grant visas or visa-free travel, they don't have to allow trade. At an extreme, look what the US is doing to Iran.

As to the specific, the Convention on the Law of the Non-navigational Uses of International Watercourses talks at length about consultation and dispute resolution when the use changes, with the clear implication that change is normal.

In war, an intentional attack on a dam or other water infrastructure may meet the requirements of military necessity and proportionality. But probably not. Simply taking what one needs from upriver is no intentional attack, however.

Answer 2

Blocking or diverting rivers is not an easy task, being outright impossible if the river is big. The water still has to go anywhere, and if it doesn't flow to your neighbour country it will flood yours. Retain enough water and your dam will break. With a small river it's an easier task, but the smaller it is, the more negligible its blocking will be to the other country. Country A is at odds with country B, so country A decides to block a river to pressure country B. It takes $2b and 12 years to build a dam to block the river. Is it worthy?

Dams are very expensive, and they take a lot of effort to build. By the time it is finished the spat could be over. Dams are built to provide your own country with water for human consumption, irrigation or power generation, not as tools of political pressure over neighbours.

That doesn't mean that they are not instruments of very serious political and diplomatic conflicts. Building a dam upstream of a river affects its flow downstream, with a lot of ecological and economical implications. If the dam is used for irrigation, a good part of the river waters won't ever flow down, effectively reducing the amount of water available to downstream neighbours. This is a matter of serious concern, since if the water level drops too much it may cause serious disruptions there. Egypt is threatening to attack Ethiopia, while Iraq shows serious concerns about Turkey's dams on the Tigris - the difference in tone being caused by the relative forces among the incumbents.

But neither Ethiopia nor Turkey are building this dams with the purpose of "applying pressure to neighbour countries", and once built is very complicated to use them that way - even if you can cut the waters downstream with a dam, it's not advisable to do so. First, it's internationally condemned, due to its heavy ecological, economical and public health impact everywhere down the dam (certainly including parts of your own country); and second, if a dam fills too fast it can collapse.

So, while you can reduce the flow of the river, you can't halt it, which greatly reduces its usefulness as a tool for political pressure. While the reduced flow has an impact downstream, it is not near as critical as total lack of water would be, and unless the dam is completely dry and you are starting to fill it, you can't keep this reduced flow downstream too much time before your dam threatens overflowing.

Answer 3

Perhaps the most (in)famous case of intentional pressure of this kind was the Jordan River Headwater Diversion Plan.

In 1964, Arab League officially adopted the plan to divert the major tributaries of Jordan River - Hasbani and Banias. From the same link:

Syria began its part of the overall Arab diversion plan with the construction of the Banias to Yarmouk canal in 1965, with financing from Egypt and Saudi Arabia. Once completed, the diversion of the flow would have transported the water into a dam at Mukhaiba for use by Jordan and Syria and prevent the water from reaching the Sea of Galilee. Lebanon also started a canal to divert the waters of the Hasbani, whose source is in Lebanon, into the Banias. The Hasbani and Banias diversion works would have had the effect of reducing the capacity of the Israeli carrier from the Sea of Galilee (Kinneret) by about 35%.

Ultimately, this led to Israeli air strikes and was one of the major factors of the Six-day war and later annexation of Golan Heights. (One of the affected tributaries, the Banias River, flows within Golan Heights).

Answer 4

The Indus Water Treaty signed in 1960 between India and Pakistan gives Pakistan rights over the three rivers Indus, Jhelum & Chenab. Time and again whenever the situation between the two countries worsens, India threatens to revisit the treaty or revoke the provisions of the treaty by diverting the water from these rivers to the Indian states of Punjab, Haryana and Rajasthan for various uses

According to Wikipedia article Indus Waters Treaty

In 2016 after a terrorist attack in URI India threatened to revoke the treaty

In the aftermath of the 2016 Uri attack, India threatened to revoke the Indus Waters Treaty. Prime Minister Narendra Modi declared, "blood and water cannot flow together.

Again in 2019 after an attack in Pulwama, India again stressed upon revisiting the provisions of the treaty

In the aftermath of the 2019 Pulwama attack, then Minister of Roads and Water Resources Nitin Gadkari stated that all water presently flowing into Pakistan in the three eastern rivers will be diverted to Punjab, Haryana and Rajasthan for various uses.

Pakistan has a disadvantage in that all the rivers of the Indus river system flow from India to Pakistan, hence India being in a strategically advantageous position does exert pressure on Pakistan.

More reference

1. Modi-Government Quietly Working To Revoke The Indus Water Treaty With Pakistan – Reports
2. Blood and water cannot flow together: PM Modi at Indus Water Treaty meeting

Answer 5

They can try. But...

Where is the water going to go? Diverting that amount of water in a direction away from another nation would be a huge effort. Building a dam would only cut the water flow temporarily.

Also, in parts of the world where water is in short supply, cutting off a nation's water source is considered an act of war, as it is an attack on that nation's people that threatens their very survival.

If Sudan were to try to divert the Nile river away from Egypt, they could expect an immediate visit by the Egyptian military, which is considerably more powerful than theirs.