Back in 2001, the Boon Companion and I drove from the town of Victoria Falls in Zimbabwe to Kasane Airport in Botswana. As we completed the border formalities at the Kazungula Road checkpoint, I was aware that, less than a mile to our north, in the middle of the Zambezi River, was a very strange international border. But there wasn’t going to be anything to see, and we had a plane to catch, so we never made the short detour to visit it. Do I regret that? Only a tiny bit.

At the point on the Zambezi where it’s joined by the Chobe River, four international borders converge—those separating Botswana, Namibia, Zambia and Zimbabwe. All four of these borders were established more than a century ago, during Africa’s colonial era, and they’ve survived without major revisions since then. But, until recently, no-one has been able to say with any certainty what happens where they all meet up in the middle of the Zambezi.

The boundary between Zambia and Zimbabwe was drawn when these countries were the British colonies of Northern and Southern Rhodesia. Its course is a little complicated farther downstream, but in our area of interest it’s very simple—it follows the medium filum (that is, the centre line) of the Zambezi.

The British also drew the line between Botswana (at that time, the British Protectorate of Bechuanaland) and Zimbabwe (Southern Rhodesia). As the border approaches the Zambezi from the south, it follows the line of an old road, which is now marked by a series of white-painted boundary posts.

Namibia’s borders with Zambia and Botswana required a bit of international diplomacy, however. The colony of German South West Africa, which would eventually become Namibia, had negotiated ownership of the Caprivi Strip, a long geographical salient (visible in the map at the head of this post) which gives access to the Zambezi River at its confluence with the Chobe River. Since both Britain and Germany wanted to be able to move boats on these rivers, the borders were deemed to lie along the middle of the main navigable channels. That’s a common solution when international boundaries involve waterways, and it has its own bit of legal jargon—the centre of the navigable channel is called the thalweg, a German word (with now outdated spelling) meaning “valley way”.

The problematic area, where these well-defined boundaries converge, is only a couple of hundred metres long, and a few tens of metres wide. At most map scales it can be treated as a single point (as at the head of the post), and is therefore often referred to as a quadripoint (where four borders meet), the very rare big brother of the common tripoint, which involves only three borders.

But here’s the situation as it pertained when the Boon Companion and I passed by:

The thalweg of the upper Zambezi is very unlikely to align and merge perfectly with the midline of the lower Zambezi. It’s unknown what happens to the Chobe thalweg after its waters emerge into the Zambezi. And the Botswana-Zimbabwe border has only ever been defined up to the Zambezi shoreline—what route does it take into the middle of the river?

To make this a true quadripoint would require an agreement between all four states, resolving these issues. Absent such an agreement, the real state of affairs was going to be that there were two tripoints in the middle of the Zambezi, in one of two configurations:

There could be various kinks and curves involved, but these are the only two topological possibilities. And at the time the Boon Companion and I drove past, no-one knew whether the short international border between these two tripoints was between Botswana and Zambia, or between Namibia and Zimbabwe—just that such a thing almost certainly existed, that it was going to be the only line along which the two countries involved touched each other, and that it was going to be very short indeed.

It took a surprisingly long time for this situation to sort itself out. Back in 1915, Germany had lost control of South West Africa, which was transferred to the administration of South Africa. So for the next 45 years all four territories involved in the “quadripoint” were part of the British Commonwealth, and no-one seems to have paid much attention to the issue. Things began to change in the 1960s, however, as African nations began, one by one, to gain independence from the colonial powers. South Africa became a republic in 1961, retaining control of South West Africa; Zambia gained independence in 1964; a minority white government in Southern Rhodesia made a Unilateral Declaration of Independence (as just plain Rhodesia) in 1965; Bechuanaland became the Republic of Botswana in 1966. And amidst all those events, Botswana and Zambia began running a ferry service across the Zambezi, right through the middle of the unresolved border area.

Both South Africa and Rhodesia claimed that the ferry crossed illegally through their territory, and that it was being used to supply arms to revolutionary forces operating within South West Africa and Rhodesia. Shots were fired, and in 1979 a ferry was sunk by Rhodesian forces.

Things quietened down for a while after that—Rhodesian elections in 1980 led to the formation of the Republic of Zimbabwe; and UN pressure on South Africa was leading to the slow transformation of South West Africa into the independent Republic of Namibia. The ferries continued to run—in 2001, a pair of large pontoon barges were moving 70-tonne loads back and forth across the river, taking care never to drift downstream into incontrovertible Zimbabwean territory.

But political strife returned at the start of the millennium. While the Boon Companion and I were passing through, Botswana and Zambia were already in discussion about replacing the ferries with a bridge. This was a Big Deal, because it had the potential to pull a good chunk of the trade along the African North-South Corridor away from Zimbabwe. So President Robert Mugabe of Zimbabwe opposed the bridge construction, on the grounds that a straight crossing would inevitably encroach on Zimbabwean territory. But, after much to-ing and fro-ing, and with development costs secured, Zambia and Botswana cut the Gordian knot:

We approached Namibia and asked that the bridge pass through their territory and they agreed.

Nonofo Molefi, Botswana Minister of Transport and Communications, 2014

Indeed, Namibia seems to have gone further in their cooperation. The African Development Fund’s Project Appraisal Report for the bridge, dated October 2011, shows how the proposed bridge takes a long upstream curve, so as to avoid encroaching on the Zimbabwean border, which has been drawn as a direct extension of the land border. But the Namibian border with Botswana is shown to abandon the thalweg of the Chobe and tilt upstream, opening up a short midstream section of border between Botswana and Zambia, through which the bridge passes!

I’ve so far found no record of how that agreement was reached, but it seems to be official enough to have found its way into the US Department of State’s Large Scale International Boundaries (LSIB) geospatial dataset, which I’ve used below to update my map and show the course of the bridge, which opened in May 2021.

That 150-metre stretch of river is the only place at which the countries of Zambia and Botswana come into contact with each other. Making it (by one criterion at least) the shortest international border in the world.

By another criterion, it doesn’t quite hold the record—and that’s the topic for another post.