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Definition of length The length of a river is not always easy to calculate. It depends on the identification of the source, the identification of the mouth, and the precise measurement of the river length between source and mouth. As a result, the length measurements of many rivers are only approximations. In particular, there has for long been disagreement whether the Amazon or the Nile is the world's longest river. The source of a river may be hard to determine because a river typically has many tributaries. Among the many sources, the one that is farthest away from the mouth is considered as the source of the river, thus giving a maximal river length. In practice, the tributary with the farthest source is not always the one given the name of the river. For example, the farthest source of the Mississippi River system is the source of the Missouri River; but a different tributary is identified as the Mississippi. When the river is measured from mouth to farthest source, it is called the Mississippi-Missouri. Also, it is hard to state exactly where a river begins as very often rivers are formed by seasonal streams, swamps, or changing lakes. In this article, length means the length of the river system, including all tributaries. The mouth of a river may be hard to determine in cases where the river has a large estuary that gradually widens and opens into the ocean; examples are the Amazon River and the Saint Lawrence River. Some rivers do not have a mouth, instead they evaporate, and the exact point where the river ends will vary seasonally. The length of a river between source and mouth may be hard to determine because of a lack of precise maps. In these cases, the measured length of a river will depend on the scale of the map on which the measurement is based; in general, due to the fractal quality of a river, the larger the scale, the longer the resulting length measurement. This issue was discovered by Lewis Fry Richardson and also applies when measuring borders between countries and coastlines. Ideally, length measurements should be based on maps that are of a large enough scale to show the width of the river. Even when precise maps are available, the length measurement is not always clear. A river may have multiple arms. It may depend on whether the center or the edge of the river is taken as reference point. It may not be clear how to measure the length through a lake: this may also vary by season. These points make it difficult, if not impossible, to get a precise measurement of the length of a river or stream. List of rivers longer than 1000 km One should take the aforementioned discussion into account when using the data in the following table. For most rivers, different sources provide conflicting information on the length of a river system. The information in different sources is between parentheses. Notes Nile The Virunga Volcanoes started to form in the Miocene period, blocking the African Rift Valley. Before that, the water which now accumulates in Lake Tanganyika drained northwards into the Albert Nile, which would make the Nile somewhere around 1500 km = 900 miles longer, with its longest headwater in Zambia. If this blockage did not happen until after the Messinian salinity crisis started, add a hundred miles or more for the added northern extension of the Nile flowing across the dry bed of the Mediterranean Sea during the Messinian salinity crisis. Amazon-Congo The Amazon basin formerly drained westwards into the Pacific Ocean, until the Andes rose and reversed the drainage. The Congo basin is completely surrounded by high land, except for its long narrow exit valley past Kinshasa, including waterfalls around Manyanga. That gives the impression that most of the Congo basin was formerly on a much higher land level and that it was rejuvenated by much of its lower course being removed. In Permian and early Triassic times Africa and South America were close against each other with no sea between (see continental drift and plate tectonics), and the Congo probably drained into the Amazon basin and eventually into the Pacific. Including part of its course that was completely lost when the South Atlantic opened, its total course may have been anything up to approximately 12000 km = 7200 miles long. See also | ||||||||||
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