313 river basins cross a national border. Together they drain 47% of the world's land outside Antarctica and are home to 45% of its people. One map of the rivers, the basins and the countries that share them, with an honest look at why nobody quite agrees how long the longest river is.
Rivers do not stop at borders. Rain that falls in the Ethiopian highlands reaches the Mediterranean through Sudan and Egypt; water from the Black Forest flows past Vienna, Bratislava, Budapest and Belgrade on its way to the Black Sea. A river basin is all the land that drains to one river, and when that land lies in more than one country, everything upstream matters to everyone downstream.
This study puts the world's shared basins on one map, using the Transboundary Freshwater Dispute Database (TFDD) at Oregon State University, which keeps the register of international river basins. It adds the rivers themselves from Natural Earth, measured end to end, and the share of each country's water that flows in from its neighbours from the UN Food and Agriculture Organization's AQUASTAT. Click a basin, a river or a country.
Most shared basins are shared by just two countries: 235 of the 313. Only 18 are shared by five or more. At the top, the Danube basin reaches into 19 countries, from Germany and Austria to Ukraine and Moldova, though several of them hold only a sliver of it. The Nile and the Niger are shared by 11 each, the Congo by 10 and the Rhine and Meuse by 9.
The basins shared by most countries
Each bar is one basin, split by each country's share of its area. Countries only; three disputed areas in the Nile basin and two in the Ganges basin are left out of the count. Hover a segment.
Source: TFDD 2024 register (riparian countries) and 2018 basin country units (areas).
The bars show how lopsided sharing can be. Brazil holds 63% of the Amazon basin; the Democratic Republic of the Congo 62% of the Congo's. The Danube is the most evenly divided: Romania has the largest piece, only 29%. Counting basins the other way round, Russia shares more international basins than any other country (33), then United States (25) and Chile (21), many of them short rivers that cross a long border.
How many countries share each basin
Number of international basins by number of countries sharing them.
For 32 countries, at least 99% of the land lies in a shared basin: almost every drop that falls on Hungary, Paraguay, Rwanda or Laos also belongs, in a sense, to someone else. Shared basins hold 3.6 billion people, and the most crowded are in Asia: the Ganges, Brahmaputra and Meghna basin alone holds 849 million, more than the Nile (291 million) and the Indus (245 million) together.
The most populous shared basins
People living in each basin, 2022 (TFDD estimates), with the number of countries sharing it.
Which river is longest sounds like a question with an answer. It is not, quite. A river's length is usually taken from its mouth to its most distant source, and every part of that is a choice. Which headwater counts as the source? Where does a river end when it reaches the sea through an estuary or a maze of delta channels? And how finely do you measure a line that wiggles at every scale, from great bends down to single meanders?
To make the comparison fair I measured every river system the same way, on one map: the Natural Earth 1:10 million river lines. I joined the 4,224 lines into a network, found each system's mouth, and followed it back to the most distant source. On that map the Nile is the longest system at 6,151 km, then the Yangtze (5,320 km), the Amazon (5,230 km) and the Mississippi-Missouri (5,125 km).
The longest river systems on one map
Mouth to most distant source along the Natural Earth 1:10m lines (bar), the same on the coarser 1:50m lines (tick), and a published length for comparison where one is given below (diamond). Hover a bar for the rivers along the path.
Measured lengths: my network analysis of Natural Earth river centrelines (public domain). Published lengths: Encyclopaedia Britannica, see the table below.
Every one of these is shorter than the figures in an atlas, and that is the point. A map at 1:10 million smooths out the bends, so a river traced on it is shorter than the same river traced on a detailed survey. Measure the same paths on Natural Earth's coarser 1:50 million lines and they shrink again: the Nile to 5,682 km, the Yangtze to 4,699 km. Simplify the lines further and the shrinking continues; there is no scale at which it settles down.
The coarser the map, the shorter the river
Length of the same mouth-to-source paths as the lines are simplified, as a share of their length on the 1:10m line as drawn (Douglas and Peucker; tolerance in degrees). Hover a point for kilometres.
The map also has to decide where a river begins and ends. Natural Earth's Amazon follows the Ucayali all the way into the Andes of southern Peru, to 15.3°S, close to where most accounts put its source. What shortens it most is the other end: the map's coastline opens into the Amazon's vast estuary at 52.7°W, so the river meets the sea well before the open ocean. Its Nile follows the White Nile and Lake Victoria up the Kagera to a source in Burundi. Choose the Blue Nile instead and the Nile system is 4,708 km on the same map. Choose the Mississippi's own source rather than the Missouri's, and the Mississippi is 3,181 km rather than 5,125.
| River | Published | Source of the figure | This map |
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Published lengths have the same problems, solved in different ways. Encyclopaedia Britannica gives the Nile as about 6,650 km and the Amazon as 6,400 km, but measurements that take the Amazon's mouth through the Pará estuary and its source in the far Peruvian Andes have made it the longer of the two. There is no referee: no body decides river lengths, and the two rivers are close enough that the choices, not the rivers, decide the ranking.
The Nile and the Amazon are close enough that the choices, not the rivers, decide which is longer.
A basin can be shared by many countries while its main river touches only a few. Counting the countries that a river itself flows through or along, the Danube leads: 9 on the Natural Earth lines, from Germany to Ukraine. The usual count is ten, and the missing one shows the limits of the map: Moldova's stretch of Danube bank is so short that Natural Earth's outline of Moldova stops 1.7 km from the river line. Next come the Mekong and the Rhine (6 each).
Rivers that cross or line the most countries
Named rivers on the Natural Earth 1:10m lines; a country counts if the river runs through it for at least 1 km or along its border for at least 5 km. Hover a bar.
Sharing a basin matters most to the countries at the bottom of it. AQUASTAT's dependency ratio is the share of a country's renewable water that comes from outside its borders, in rivers that flow in and groundwater that seeps across. For 38 of the 191 countries with a value, more than half of their water starts somewhere else.
The countries most dependent on water from abroad
Dependency ratio, FAO AQUASTAT, values for the period 2013 to 2017.
Source: FAO AQUASTAT. Kuwait and Bahrain have no rivers; their small renewable supply is counted as groundwater from outside.
The list reads like a map of the world's great deltas and dry lowlands. Egypt (97%) and Sudan (96%) live on the Nile; Turkmenistan (97%) and Uzbekistan (80%) on the Amu Darya; Bangladesh (91%) at the foot of the Ganges and Brahmaputra; Hungary (94%), Romania (80%) and the Netherlands (88%) at the lower ends of the Danube and the Rhine. Switch the map to "Water from abroad" to see them all.
Basins. The TFDD register lists 313 international basins in its 2024 version, with the countries that share each and their area and population. The polygons on the map are the TFDD's 2018 spatial data for 310 of them (the 3 basins added since, the Loa, the Allahein and the Ems, are counted but not drawn), split into 812 basin country units, which give each country's share. I count sovereign countries: the TFDD also lists disputed areas such as Abyei or Aksai Chin as riparians, and I leave those out of the count but show them in the panels. The West Bank is counted as Palestine and French Guiana as France. Population is the TFDD's estimate for 2022; the world total is the World Bank's.
Land. The share of land is the TFDD basin area over the land area of the Natural Earth 1:10m outlines outside Antarctica (134.5 million km²). The share of each country in shared basins compares its basin country units with its Natural Earth area, so small differences in outline can push it slightly over or under; I cap it at 100%.
Rivers. The river lines are Natural Earth's 1:10 million river and lake centrelines (the edition with stroke weights), 4,224 lines in all. I split and joined them into a network, left out canals and two channels that link separate rivers (the Casiquiare between the Orinoco and the Amazon, and the Kovzha along the Volga to Baltic waterway), and found mouths at the sea or at five terminal lakes such as the Aral Sea and Lake Chad. Each system's length is the network distance from its mouth to its most distant source. Countries along a river are found by sampling its line every kilometre. These are map lengths, consistently measured, not survey lengths.
Water from abroad. The AQUASTAT dependency ratio for 191 countries, as downloaded from AQUASTAT in 2017 and published in a public teaching repository; the values are for 2013 to 2017. AQUASTAT revises its estimates rarely and slowly, but newer values may differ.
What this is not. Sharing a basin is not the same as sharing water fairly, or fighting over it. The TFDD also records treaties, river basin organisations and disputes, which this study does not map. Basins hide groundwater aquifers, which have borders of their own, and the population and area figures are estimates. The river lengths are those of one generalised map, and the published lengths are quoted only to show how they differ.