A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, rivers flow into the ground and become dry at the end of their courses without reaching another body of water. There are many factors, such as the source, the identification or the definition of the mouth, and the scale of measurement of the river length between source and mouth, that determine the precise meaning of “river length”.
As a result, the length measurements of many rivers are only approximations. Even when detailed maps are available, the length measurement is not always clear. A river may have multiple channels, or anabranches. The length may depend on whether the center or the edge of the river is measured. It may not be clear how to measure the length through a lake. Seasonal and annual changes may alter both rivers and lakes.
Other factors that can change the length of a river include cycles of erosion and flooding, dams, levees, and channelization. In addition, the length of meanders can change significantly over time due to natural or artificial cutoffs, when a new channel cuts across a narrow strip of land, bypassing a large river bend. These points make it difficult, if not impossible, to get an accurate measurement of the length of a river. The varying accuracy and precision also make it difficult to make length comparisons between different rivers without a degree of uncertainty.
Here are the top 20 longest rivers in the world.
| Rank | River | Location | Length |
| 1. | Nile – White Nile – Kagera – Nyabarongo – Mwogo – Rukarara | Africa | 6,650 km (4,130 miles) |
| 2. | Amazon – Ucayali – Tambo – Ene – Mantaro | South America | 6,575 km (4,086 miles) |
| 3. | Yangtze – Jinsha – Tongtian – Ulan Moron | Asia (China) | 6,300 km (3,917 miles) |
| 4. | Mississippi – Missouri – Jefferson – Beaverhead – Red Rock – Hell Roaring | North America | 6,275 km (3,902 miles) |
| 5. | Yenisei – Angara – Selenge – Ider | Asia | 5,539 km (3,445 miles) |
| 6. | Yellow River | Asia (China) | 5,464 km (3,395 miles) |
| 7. | Ob – Irtysh | Asia | 5,410 km (3,365 miles) |
| 8. | Río de la Plata – Paraná – Rio Grande | South America | 4,880 km (3,030 miles) |
| 9. | Congo – Chambeshi | Africa | 4,700 km (2,922 miles) |
| 10. | Amur – Argun – Kherlen | Asia | 4,444 km (2,763 miles) |
| 11. | Lena | Asia (Russia) | 4,400 km (2,736 miles) |
| 12. | Mekong | Asia | 4,350 km (2,705 miles) |
| 13. | Mackenzie – Slave – Peace – Finlay | North America (Canada) | 4,241 km (2,637 miles) |
| 14. | Niger | Africa | 4,200 km (2,611 miles) |
| 15. | Brahmaputra – Yarlung Tsangpo | Asia | 3,969 km (2,466 miles) |
| 16. | Murray – Darling – Culgoa – Balonne – Condamine | Australia | 3,672 km (2,282 miles) |
| 17. | Tocantins – Araguaia | South America (Brazil) | 3,650 km (2,270 miles) |
| 18. | Volga | Asia (Russia) | 3,645 km (2,266 miles) |
| 19. | Indus – Sênggê Zangbo | Asia | 3,610 km (2,250 miles) |
| 20. | Shatt al-Arab – Euphrates – Murat | Asia | 3,596 km (2,236 miles) |
