Who Shares the World's Rivers? International River Basins, River Lengths Measured on One Map, and the Countries that Depend on Water from Upstream

Rivers cross borders, and the land that drains to them often lies in several countries. Using the register of international river basins kept by the Transboundary Freshwater Dispute Database (TFDD), I map the 313 basins shared by two or more countries and the share of each held by each country. They cover 47.1% of the world's land outside Antarctica and held 3.6 billion people in 2022, 45.4% of the world's population. Most are shared by two countries (235), but the Danube basin reaches 19 countries and the Nile and the Niger 11 each. To compare the rivers themselves on equal terms, I join the Natural Earth 1:10 million river centrelines into a network and measure every system from its mouth to its most distant source. On that map the Nile is the longest system (6,151 km), followed by the Yangtze (5,320 km) and the Amazon (5,230 km). These lengths are 81.7% to 92.5% of the figures in a standard reference work, and they shrink further on coarser lines, which shows why the ranking of the longest rivers depends on choices of source, mouth and scale rather than on the rivers alone. Finally, the FAO AQUASTAT dependency ratio shows that for 38 of 191 countries more than half of the renewable water originates abroad, led by deltas and dry lowlands such as Egypt (96.9%) and Turkmenistan (97.0%). The study is descriptive: sharing a basin says nothing by itself about cooperation or conflict. Interactive version: https://maps.tristanhotham.com/shared-rivers/. Data and code: https://github.com/Tristan-Hotham/shared-rivers.