Rivers that form borders between countries

A border on a map is often a straight line that somebody drew with a ruler. Out in the landscape there is nothing to see of it. A river is the opposite: visible, lasting and clear even without a single boundary stone. That is why rivers serve as natural borders between countries all over the world. Four well known examples show how differently that can look.

The Rhine: one border, several times over

The Rhine rises in the Swiss Alps and works its way through half of central Europe as a border. First it separates Switzerland from Liechtenstein for around 30 kilometres, then Switzerland from Austria. From Lake Constance onwards it forms a long stretch of the border between Switzerland and Germany.

Further downstream the Rhine marks a large part of the border between France and Germany, before it flows out through the Netherlands into the North Sea. Few other European rivers touch so many countries as a border over such a short run.

The Danube: the most international river in the world

The Danube beats them all. From its source in the Black Forest to the Black Sea it flows through or along ten countries: Germany, Austria, Slovakia, Hungary, Croatia, Serbia, Romania, Bulgaria, Moldova and Ukraine. No other river on Earth touches so many states.

Along many of these stretches it is a border as well. Between Slovakia and Hungary, for instance, or between Romania and Bulgaria, the national boundary follows the water for long distances. The Danube covers around 2,850 kilometres, which makes it the second longest river in Europe after the Volga.

Rio Grande and Mekong: borders on other continents

The principle works everywhere. In North America the Rio Grande, called Río Bravo in Mexico, forms the border between the United States and Mexico for around 2,000 kilometres, from El Paso to the Gulf of Mexico. This was fixed as far back as 1848, running along the deepest channel of the river.

In South East Asia the Mekong takes on the same role. On its way from the highlands of Tibet to the sea it crosses six countries. For one stretch it forms the border between Myanmar and Laos, then for around 920 kilometres the border between Laos and Thailand.

Four border rivers at a glance

These four are among the best known border rivers anywhere:

• Rhine: a border between Switzerland and its neighbours and between France and Germany.
• Danube: touches ten countries, more than any other river in the world.
• Rio Grande: around 2,000 kilometres of border between the United States and Mexico.
• Mekong: a border river between Laos and Thailand and between Myanmar and Laos.

Why river borders are tricky

As handy as a river is as a border, it is not simple. Rivers shift their bed, meander and burst their banks. If the channel wanders, the border may wander with it. That is exactly why treaties often state that the bank does not count, but rather the deepest channel, known in the trade as the thalweg.

The rule sounds technical, yet it is the reason river borders stay stable over decades. It ties the boundary to a clear line in the water, instead of to a bank that can move a little with every flood.

Borders like these are easy to fix in your mind on a map. In CountryRush you practise countries together with their neighbours, and the Daily Trip keeps bringing new regions where a river draws the line.