Rivers are natural drainage channels. They collect runoff from precipitation and groundwater and discharge it into oceans or lakes. The main watercourse is determined by its water volume, length, or highest source. River size is determined by three factors: discharge, length, and drainage basin.
The global annual runoff into the oceans (38,500âÂÂ44,200 km<sup>3</sup>/year) is dominated by runoff into the South Atlantic from eastern South America, into the western Pacific from east Asia, and into the Indian Ocean from India, and southeast Asia. Depending on the estimate, the Amazon alone accounts for 15âÂÂ18% of the annual freshwater runoff into the oceans.
Water discharge is expressed as an average in mó/s and as an average value over a period of one year or more (MQ), or as an average annual discharge (Q) in kmó. For large rivers, these values are not only difficult to measure (see â ADCP), but they also vary considerably from year to year. The very large variation between low and high flow is often not apparent from the average runoff volume.
The discharge values shown in the tables below are m/sec. To convert to annual km/year, multiply by 0.031536.