Why do so many borders follow rivers and mountains, and why are some ruler-straight?

A river or a mountain ridge makes a very convenient border, which is why so many borders follow them. These are what geographers call natural borders: geographical features that get in the way of travel and transport. Often a political border is simply the formalization of such an old, natural obstacle. A wide river or a high ridge slows armies, and so a border along it is easier to defend. Rivers have a second advantage: if you want a precise line, you can draw it along the thalweg, the deepest line of the river. Mountain borders often run along a drainage divide.

But nature never draws a border by itself. A border may follow a river or a mountain range, yet the feature does not automatically define it, even when it is a serious barrier. Borders are made by people, through warfare, colonization or agreement, and people sometimes ignore the landscape altogether. These are geometric borders: straight lines or arcs, drawn regardless of the physical and cultural features of the area. They are often found around states that grew out of colonial holdings, in North America, Africa and the Middle East. The Canada–United States border follows the 49th parallel for roughly 3,500 kilometres, from Lake of the Woods to the Pacific Ocean.

There is a pattern in the maps. Technology has made it cheaper to cross rivers and mountains, so natural barriers matter less than they used to. As a result, borders formalized more recently, in Africa or the Americas, conform less to natural borders than the very old borders of Europe or Asia do. Next time you look at a map, ask a simple question of each border: was this line found in the landscape, or drawn on top of it?