The border that followed a river

Two provinces agreed in 1908 that their boundary was the main channel of the river. The agreement was precise, the map was signed, and the river has since moved four hundred metres.

Rivers in flat country migrate. The outer bank of a bend erodes, the inner bank gains sediment, and over decades the whole curve walks sideways; occasionally a flood cuts across a loop and abandons it entirely. A boundary defined by the channel therefore moves with it, which is convenient for navigation and inconvenient for everything else. Farms changed province without anyone moving. A village found its school in one jurisdiction and its clinic in another.

International practice distinguishes two cases, and the distinction is old. Gradual change by erosion and deposition moves the boundary; sudden change by a flood cutting a new course does not. The reasoning is practical rather than elegant: slow change is noticed late and accommodated as it happens, while a sudden jump would transfer whole settlements overnight.

The modern solution is to replace the description with coordinates. A boundary fixed at a set of points stays where it was put, and the river is free to wander. This settles the legal question and creates a cartographic one: for most of its length the border no longer follows any visible feature, so it cannot be seen on the ground and has to be surveyed to be known.

That trade is now the normal one. Boundaries that follow features are legible and unstable; boundaries defined by numbers are stable and invisible. Which is preferable depends on whether the main risk is a dispute about where the line is or a dispute about whether it should have moved.