Every Drop Has an Address

One peak, three oceans

Triple divides are ordinary — most are just a hilltop where three streams start. One is not ordinary. On Triple Divide Peak in Glacier National Park, Montana, the Continental Divide of the Americas crosses the Laurentian Divide, and the summit becomes the hydrological apex of North America.

• Snowmelt off the southwest face becomes Pacific Creek, joins the Columbia, and reaches the Pacific near Astoria, Oregon.
• Off the southeast face it becomes Atlantic Creek, joins the Missouri and then the Mississippi, and arrives in the Gulf of Mexico — some 4,000 km of river later.
• Off the north face it becomes Hudson Bay Creek, runs through Saint Mary Lake into the Saskatchewan and the Nelson, and empties into Hudson Bay.
• Whether that third route counts as a third ocean depends on who classifies Hudson Bay. The International Hydrographic Organization puts it in the Arctic; that is why Canada's Snow Dome, 400 km north, has a rival claim to the same title.

The eight largest addresses

Basin areas are wildly unequal. The Amazon alone gathers 6.2 million square kilometres — about 44% of South America — and pushes roughly 209,000 cubic metres of water into the Atlantic every second, close to a fifth of all river discharge on Earth. These eight basins sum to 28.7 million km² between them.

Teal = drains to an ocean. Rust = drains to nowhere.

The fifth that never arrives

An endorheic basin is a basin with no exit: rain falls, rivers run, and the water leaves only by evaporating. The MERIT-Plus hydrography puts that land at 18.81% of the ice-free surface, spread across 1,708 merged basins.

A divide is found one cell at a time

Nobody surveys a watershed boundary by walking it. HydroSHEDS takes the elevation model the Shuttle Radar Topography Mission built in 2000, cuts the land into 15 arc-second cells — roughly 500 m at the equator — and asks each cell one question: which of my eight neighbours is lowest? Follow the chain of answers and you have a river. The cells where two chains part company are the divide.

Each square drains to its steepest downhill neighbour; arrow weight follows accumulated flow. The rust line is where the two networks separate — a divide, drawn without anyone measuring a ridge.

• The same method produces HydroBASINS, the nested watershed layer most global water models now run on, coded with the Pfafstetter scheme so any basin's upstream area is a number lookup.
• It also means divides move when the elevation model improves. The Great Divide Basin's boundary, and which side an overflow would run to, are questions of centimetres.

Basins do not care about borders

Because water follows terrain and states follow everything else, most of the inhabited world lives in a basin it shares. The UN's Transboundary Waters Assessment Programme counted the overlap.

• Those shared basins carry roughly 22,000 km³ of discharge a year — the great majority of the world's accessible surface freshwater.
• A divide is therefore a treaty line nobody negotiated. Upstream of it you set the terms; downstream of it you receive them.

What 41% of a country flushes onto one shelf

The Mississippi–Missouri basin drains 41% of the contiguous United States — parts of 31 states and two Canadian provinces — through a single mouth. Everything the basin loses to its rivers arrives at the same few hundred kilometres of Louisiana coast.