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 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.
For the purpose of determining maximum length a river's "true source" is considered to be the source of whichever tributary is farthest from the mouth. This tributary may or may not have the same name as the main stem river. For example, the source of the Mississippi River is normally said to be Lake Itasca in the U.S. state of Minnesota, but the most distant source in the Mississippi system is that of the Jefferson River in the state of Montana, a tributary of the Missouri River which in turn is a tributary of the Mississippi. When the Mississippi is measured from mouth to this farthest source, it is called the Mississippi-Missouri-Jefferson. Furthermore, it is sometimes hard to state exactly where a river begins, especially rivers that are formed by ephemeral streams, swamps, or changing lakes. In this article, length means the length of the longest continuous river channel in a given river system, regardless of name.
The mouth of a river is hard to determine in cases where the river has a large estuary that gradually widens and opens into the ocean; examples are the River Plate and the Saint Lawrence River. Some rivers, like the Okavango, Humboldt, and Kern do not have a mouth; instead they dwindle to very low water volume and eventually evaporate, or sink into an aquifer, or get diverted for agriculture. The exact point where these rivers end will vary seasonally.
The source of some rivers starting in farming areas can be difficult to determine, if the river is formed by the confluence of several farm field drainage ditches which only contain water after rain. Similarly, in rivers starting in a chalk area, such as the Chilterns in south England, the length of the upper course which is dry varies with how high the water table is, which varies with the weather: see winterbourne (stream).
The length of a river between source and mouth may be hard to determine due to issues of map scale. Small scale maps (those showing large areas) tend to generalize, or "smooth" lines more than large scale maps (those showing small areas). According to the generally accepted ideal, length measurements should be based on maps that are of a large enough scale to show the width of the river, and the path measured is the path a small boat would take down the middle of the river. 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.
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