Which river forms much of the border between the USA and Mexico?

The Rio Grande, called the Rio Bravo del Norte on the Mexican side - the same river with two names, which is itself a fair summary of the relationship. It runs about 3,000 kilometres, and roughly two thirds of the US-Mexico border follows it.

A river makes a poor border because rivers move. The channel shifted repeatedly through the nineteenth and twentieth centuries, leaving parcels of land on the wrong side, and one such parcel at El Paso - the Chamizal - was disputed for a hundred years before being settled in 1963 by concreting the channel into a fixed course and physically relocating the border. Several hundred people had to move.

It is also being used up. Between irrigation and municipal supply, so much water is extracted that the river frequently fails to reach the Gulf of Mexico at all, running dry in stretches of Texas - a border defined by a river that in places is not there. Allocation between the two countries runs on treaties from 1906 and 1944, written when there was more water and far fewer people, and renegotiating them is one of the more difficult items on the bilateral agenda.