|
This is a list of the longest rivers on Earth. It includes river systems over 1,000 kilometers. ==Definition of length== There are many factors, such as the source, the identification or the definition of the mouth, and the scale of measurement〔for more on this, see coastline paradox〕 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 (see also coastline paradox). 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.〔(【引用サイトリンク】publisher= Brazilian National Institute for Space Research )〕 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. 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 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. 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 .〔 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「List of rivers by length」の詳細全文を読む スポンサード リンク
|