A river border is an international boundary that follows a watercourse instead of a line on land, and some of the world’s best-known frontiers work this way: the Rio Grande carries the US–Mexico border for about 2,020 km, the Danube divides a string of European countries from Slovakia to Ukraine, the Amur and Ussuri divide Russia from China, and the Mekong forms more than half of the 1,845 km Laos–Thailand border. Unless a treaty says otherwise, international law places the line on the thalweg, the middle of the main navigable channel; some agreements use the median line between the banks instead. Rivers are chosen because they are visible and easy to describe, but they move, braid, flood and build islands, so many river borders have needed commissions, swaps or courts to keep it working. The clearest way to see how is to travel four of them from end to end.
El Paso and Ciudad Juárez: Where the Rio Grande Takes Over
The US–Mexico border is 3,145 km long, and most of its western half is ruler lines across the Sonoran and Chihuahuan deserts. At El Paso, Texas, and Ciudad Juárez, Chihuahua, it meets the Rio Grande (the Río Bravo del Norte in Mexico) and follows it all the way to the Gulf of Mexico. The 1848 Treaty of Guadalupe Hidalgo put the line “along the deepest channel”, which sounded precise until the river kept shifting south, most sharply after floods in 1862, 1864 and 1865. By 1873 about 2.4 km² of Mexican land around El Paso had ended up on the US bank. This Chamizal dispute took until the Chamizal Treaty of 1963 to settle, and that treaty returned 630 acres (about 2.5 km²) of south El Paso to Mexico. The Rio Grande Rectification Treaty of 1933 then straightened the river through the El Paso–Juárez valley, and between 1935 and 1938 each country ceded an equal area to the other.
Ojinaga to Brownsville: Bancos on the Lower Rio Grande
Downstream, the Río Conchos, a major tributary, joins from Mexico near Ojinaga, about 310 km southeast of El Paso; the Pecos and Devils rivers arrive from Texas near Amistad Reservoir. Nearer the coast the river meanders across the subtropical Lower Rio Grande Valley, cutting off loops of land called bancos. Under the Banco Convention of 1905, the border follows the new channel and the stranded loop changes country. Such adjustments were made on 37 different dates between 1910 and 1976, always for small, uninhabited parcels of 1 to 646 acres. The International Boundary and Water Commission, founded in 1889, runs the system, and the Boundary Treaty of 1970 settled all remaining boundary disputes along the river. At the end of the line, the Brownsville & Matamoros International Bridge of 1910 still carries cars, and the river reaches the Gulf in a small sandy delta, so depleted that a sandbar blocked its mouth in 2001 and 2002.
Devín Gate to Esztergom: The Danube Between Slovakia and Hungary
The Danube runs 2,850 km from Germany to the Black Sea and passes through or along ten countries. Its middle course begins at the Devín Gate on the Austria–Slovakia border near Bratislava. Downstream of Bratislava, past the Gabčíkovo dam and locks, the river becomes the frontier between Slovakia and Hungary, with towns facing each other across the water: Komárno and Komárom, then Štúrovo and Esztergom. Komárom was once a single town on both banks; after the Treaty of Trianon in 1920 the new Czechoslovak border cut it in half, and the Elisabeth Bridge between the two halves became a border crossing.
The Mária Valéria Bridge between Esztergom and Štúrovo tells the story in one structure. About 500 m long, it opened on 28 September 1895, was damaged in 1919, and was blown up by retreating German troops on 26 December 1944. Decades of mistrust between communist Hungary and Czechoslovakia kept it in ruins until it reopened on 11 October 2001, half-funded by a €10 million EU grant. Both countries are now in the Schengen Area, so crossing it involves little more than a walk.
Vukovar: The Islands Croatia and Serbia Still Dispute
Past Budapest, the Danube becomes the border between Croatia and Serbia, and here the question of where the line sits in the water is still open. Serbia holds that the border follows the thalweg and centreline of the river. Croatia argues that it follows the boundaries of the cadastral municipalities along the banks, which trace the Danube’s 19th-century course before engineering straightened it. Along a 140 km stretch the two lines diverge, and the disputed area is reported at up to 140 km². The quarrel began in 1947, when authorities in Vukovar refused to hand four river islands to Vojvodina, and it remains unresolved; the line actually controlled mostly matches Serbia’s claim.
Iron Gates to Silistra: Serbia, Romania and Bulgaria
Below Belgrade the Danube cuts the Iron Gates gorge, which forms part of the Serbia–Romania border. Romans bridged the river near Drobeta to invade Dacia, and today two hydroelectric dams, Iron Gate I and, about 60 km downstream, Iron Gate II, stand on this border stretch, and their double locks are the last on the Danube. From there the river runs free for more than 860 km to the Black Sea.
For most of that distance it separates Romania from Bulgaria. Crossings are few: the New Europe Bridge between Vidin and Calafat, ferries at Oryahovo–Bechet, Nikopol–Turnu Măgurele and Svishtov–Zimnicea, and the Danube Bridge between Ruse and Giurgiu. Built with Soviet help and opened on 20 June 1954, that 2,223 m road-and-rail bridge was at the time the only bridge the two countries shared; its central span can still lift for tall ships. At Silistra the river turns north into Romania and the border becomes a land line across Dobruja. Both countries became full members of the Schengen Area on 1 January 2025, and border controls at the bridge were due to end by 30 June 2025.
Giurgiulești to the Chilia Arm: Moldova’s 480 Metres
Moldova’s stretch is the shortest of all. At Giurgiulești, where the Prut joins the Danube, Moldova touches the river for only about 480 m, squeezed between Romania and Ukraine near Galați and Reni. That sliver is enough for a port and an oil terminal, completed in 2006. Below it, the Danube and then the Chilia, the northernmost arm of its delta, carry the Romania–Ukraine border all the way to the Black Sea, with the delta’s northern fringe in Ukraine and most of it in Romania. The waterway itself ends at Sulina: since the Rhine–Main–Danube Canal was completed in 1992, ships can travel about 3,500 km from Rotterdam to the Black Sea.
Argun to Khabarovsk: The Amur Between Russia and China
The Russia–China border measures about 4,209 km after final demarcation in the early 2000s, and in the east rivers carry most of it. From the eastern Mongolian tripoint it follows the Argun, then the Amur, which begins where the Argun and Shilka meet at 303 m above sea level. The Treaty of Nerchinsk in 1689 had left the whole Amur valley Chinese; the Treaty of Aigun in 1858 moved more than 600,000 km² north of the river to Russia, and the Convention of Peking in 1860 fixed the boundary on the Amur and Ussuri.
The river widens where the Zeya joins it between Blagoveshchensk and Heihe. A road bridge between the two cities, finished in 2019, began carrying freight on 10 June 2022 and opened to passenger vehicles on 2 January 2025; about 200 trucks a day crossed in 2024. The Amur then receives the Songhua at Tongjiang and turns northeast to Khabarovsk, where the Ussuri joins and the border turns south. Islands caused the worst trouble. China argued for the thalweg rule, under which it claimed some 600 of the rivers’ 700 islands, and in 1969 fighting broke out over Zhenbao (Damansky), a 0.74 km² island in the Ussuri; a 1991 agreement recognised it as Chinese. At the confluence itself, Bolshoy Ussuriysky Island, about 327–350 km² beside Khabarovsk, was split in an agreement fully implemented in 2008, with the western part going to China.
Golden Triangle to Pakse: The Mekong Between Laos and Thailand
The Mekong, 4,909 km long, becomes a border soon after leaving China: about 10 km along the China–Myanmar line and about 100 km between Myanmar and Laos, to the Golden Triangle where Myanmar, Laos and Thailand meet. From there it separates Laos and Thailand. The alignment dates from French pressure on Siam: the Franco-Siamese crisis of 1893 forced Siam to give up claims east of the river, treaties of 1904 and 1907 took Sainyabuli and Champasak on the west bank, and a 1926 convention settled ownership of the shifting islets.
The border leaves the river for a stretch south of the Golden Triangle, rejoins it, and passes Vientiane, the Lao capital, which stands on the river bank facing Thailand. The First Thai–Lao Friendship Bridge, 1,170 m long and funded by Australia at about A$42 million, opened on 8 April 1994 as the first bridge across the lower Mekong, linking Nong Khai with a road to Vientiane about 20 km away. Northwest of Pakse the border finally leaves the Mekong for the Dângrêk Mountains and the Cambodian tripoint.
Seen end to end, each of these rivers shows the same trade-off: a line that is easy to describe and hard to hold still. For more on the first of them, read our complete guide to the Rio Grande; for the river that crosses the most borders, see the Danube River guide; and for the logic behind choosing rivers over surveyed lines, compare natural and artificial borders.