River

List of rivers by length

View of the River Nile, from a cruise boat, between Lu xor and Aswan in Egypt.

The length of a river can be very hard to calculate. There are many factors, such as the source, the identification or the definition of the mouth, and the scale of measurement[1] 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. In particular, there has long been disagreement as to whether the Nile or the Amazon is the world's longest river. The Nile has traditionally been considered longer, but in recent years some Brazilian and Peruvian studies have suggested that the Amazon is longer by measuring the river plus the adjacent Pará estuary and the longest connecting tidal canal

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. For example, due to 18 cutoffs created between 1766 and 1885 the length of the Mississippi River from Cairo, Illinois, to New Orleans, Louisiana, was reduced by 218 miles (351 km)

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 makes it difficult to make length comparisons between different rivers without a degree of uncertainty.