Water is often taken for granted, if you’re lucky enough to have it coming out of taps. Yet it lies at the heart of national security. Controlling water means having control over a key resource that keeps an economy running and stable. Water supports jobs, businesses and livelihoods. When it is managed well, countries’ economies are stronger and more secure. Read more: Worrying insights from UN's first-ever assessment of water security in Africa I’m an academic specialist in the field of trans-boundary rivers and national security. This field of research studies the clash between the legal concept of sovereign equality (that all countries are equal under international law), and rights associated with river flows and border demarcations.Disputes over rivers, from the Chobe and Orange rivers in southern Africa to the Nile in the north, show that being able to access water and control water sources can determine social stability, migration, investment and even international relations.How changing rivers create border disputesA good example is the small island in the Chobe River between Botswana and Namibia. This island is named Kasikili in Botswana and Sedudu in Namibia. Who owned the island became an important question after Namibia became independent and took the case to the International Court of Justice in 1996, claiming that the island was its territory because it always had been.The court ruled against Namibia, citing the international norm that recognises the thalweg of the river as the actual border. Read more: Crisis proofing South Africa's water security The thalweg is the deepest part of a river channel, but in fast-changing rivers, this deepest point can change over time, even after a single big flood. In this case, the island was on the Botswana side of the thalweg and therefore belonged to Botswana. This legal demarcation can be highly contested, especially when determining which country has access to minerals contained in rivers and the sea (border demarcation extends out to the oceans at estuaries). Another example is the 2,200 kilometre Orange River, the longest river in South Africa. It passes through four countries – Lesotho, South Africa, Botswana and Namibia – and in 1890, the border between Namibia and South Africa was defined as running along the Namibia bank of the river (the high‑water mark) rather than the thalweg (centre line). Read more: Why Africans must join forces to protect scarce water resources The reason that this particular border demarcation ignores the international legal norm of using the thalweg dates back to the colonial era. At that time, hostilities existed between Britain and Germany. The British colonial authorities in the Cape Colony believed that a permanent German settlement could be prevented by denying then German South-West Africa access to reliable water flows from the Orange River. The driver of this decision was national security and perceptions of threat. In my research, I’ve seen this type of thing quite often. How borders in water affect a country’s securityControl of water creates security, of which there are many forms. Control over floodwaters creates security from being flooded and drowned. Flood control usually involves building one or more dams to reduce the size of the biggest floods.Controlling captured water in dams means that during drought, society will still have a water supply and be able to continue with business as usual. Control over water therefore generates security upon which society can start to flourish. It is also a highly contested issue, as we see in the Nile River. At 6,650km long, it is one of the world’s longest rivers, draining 10% of the entire African continent. There are 11 riparian states – Burundi, Democratic Republic of Congo, Egypt, Eritrea, Ethiopia, Kenya, Rwanda, South Sudan, Sudan, Tanzania and Uganda – which share the Nile and the Grand Ethiopian Renaissance Dam is the latest of many points of contention. Read more: Ethiopia's future is tied to water -- a vital yet threatened resource in a changing climate Egypt claims sovereign rights over the Nile. But this clashes with the sovereign rights of Ethiopia, which has constructed the dam. Egypt has accused Ethiopia of having captured the resource. The matter is legally complicated and politically fraught. Water can also make a country, city or town more secure. In 2018, Cape Town in South Africa approached a Day Zero crisis that made international headlines. The city was faced with the reality of literally running out of water. This was because the water resources of many local rivers had been captured through a series of inter-basin transfers, and together with drought, made the city vulnerable. Today, the city administration has developed a long-term strategy that includes the recovery of water from waste, and the development of seawater desalination capabilities at utility scale. National security depends on water securityPeople are migratory in nature, so they will naturally move from areas of low security to areas of greater security. Migrating people bring capital with them – human skills and financial resources. They flow from areas of low security to areas of higher security. Water management needs to be based on the natural flow of humans. Internal movement of people in South Africa was once controlled by a policy called influx control. This was rejected as a violation of human rights and lay at the heart of the armed struggle for liberation. Read more: Woman-headed households in rural South Africa need water, sanitation and energy to fight hunger – G20 could help Over the past four decades, the population has almost trebled, leading to uncontrolled migration from rural areas to cities. This has overwhelmed infrastructure, while water supply and sanitation services have lagged behind, creating a new form of national security crisis.Capital is needed to create jobs and bring about social stability in a population shaped by migration. But investors are increasingly unwilling to flow into areas overwhelmed by migration that outstrips water supply and sanitation services.In this way, we can see that national security depends on water security, because social stability and economic wellbeing are directly linked to the flow of people with skills and capital. Understanding water as a national security risk is key to the policy reforms needed to create conditions in which humans can flourish. Capital always flows to places where people can thrive, and water policy needs to be aligned to this simple fact.Anthony Turton is retired, but in the past he has received funding from the Water Research Commission, the Swedish Foreign Ministry, Green Cross International and the FAO.