What happens to a border when the river that marks it moves?

Sometimes it moves with the river, and sometimes it stays exactly where it was. Rivers shift their beds, so the law has had to decide when a border line should follow the water and when it should stay put.

The starting rule is the thalweg. Under international law it is taken to be the middle of the main navigable channel, and it is the default legal presumption for a river boundary between states. The idea suits boats: ships can follow the waterway while staying inside their own country. But the thalweg is not painted on the riverbed. Notable arguments over where it lies include the Shatt al-Arab between Iraq and Iran, the Danube between Serbia and Croatia, and Kasikili/Sedudu Island, a dispute between Namibia and Botswana that the International Court of Justice settled in 1999.

The second rule is about speed. A river can change gradually, as it deposits sediment, or suddenly, by avulsion, when the flow jumps out of its channel into a new course. The law of accretion says that a river border follows the river only when it moves gradually. If it jumps, the border stays where it was.

The case that tested this most famously is the Chamizal dispute, between El Paso in Texas and Ciudad Juárez in Mexico. After a remarkable flood, the Rio Grande shifted dramatically southward in 1864, and land that had been south of the river ended up on its north bank. Was that gradual or sudden? Mexico said the border had never moved. The United States said the shift was sudden erosion. They argued for decades. Americans controlled the land, but Mexico never gave up its claim, and in 1967 the United States finally handed it over. The settlement also provided for a concrete channel, so that the river could never blur the line again.