Borders & How They're Defined

The border between the United States and Canada is the longest international land border in the world, running roughly 8,891 kilometers (5,525 miles) when the Alaska–Yukon/British Columbia segment is included, according to the CIA World Factbook. Much of it, especially across the Great Plains, follows the 49th parallel north — a pure geometric border with no natural feature underneath it — established by a series of 19th-century treaties, most notably the 1818 Treaty and the 1846 Oregon Treaty. Other stretches follow natural features instead, including long segments through the Great Lakes and the St. Lawrence River. This mixed approach — geometric where convenient, natural where a clear feature already existed — is common for borders negotiated through diplomacy rather than drawn purely by an outside colonial power.

Many of Africa's modern national borders were fixed at or shortly after the 1884–85 Berlin Conference, where European colonial powers divided territory among themselves largely using straight lines on a map, with little to no reference to the hundreds of existing ethnic, linguistic, and kingdom boundaries already on the ground. This is the textbook case of a superimposed border. Because these lines often split single ethnic groups across two or more modern countries — or joined historically rival groups inside one — they remain a widely cited contributing factor in a number of post-independence regional and ethnic conflicts across the continent. This is a case where the legal validity of a border (which international law generally treats as fixed once independence is achieved, a principle called uti possidetis juris) and its underlying cultural fit are two separate questions — a border can be entirely settled in international law while still cutting across communities that predate it by centuries.

Rivers make intuitive natural borders, but they present a genuine technical complication: rivers move. Over decades, a meandering river can gradually shift its channel through erosion and sediment deposit — a process called accretion when it's slow and gradual, or avulsion when a river suddenly jumps to a new channel, often during a major flood. International law generally treats gradual accretion as shifting the border along with the river, but treats sudden avulsion as leaving the border fixed at the river's old channel, which can strand a piece of one country's territory on the "wrong" side of the new river course. This exact situation has produced real, documented border anomalies along the Rio Grande (US–Mexico) and the Mississippi River (between individual US states), and is one practical reason many modern river borders are instead fixed using a permanent set of GPS coordinates rather than "the river" as a moving reference.