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.
10. Amur – Argun
The Amur is the world’s
tenth longest river, forming the border between the Russian Far East
(Outer Manchuria) and Northeastern China (Inner Manchuria). It rises in
the hills of western Manchuria at the confluence of its two major
affluents, the Shilka River and the Ergune River, at an elevation of 303 metres (994 ft). It flows east forming the border between China and Russia, and slowly makes a great arc to the southeast for about 400 kilometers
(250 mi), receiving many tributaries and passing many small towns. At
Huma, it is joined by a major tributary, the Huma River. Afterwards it
continues to flow south until between the cities of Blagoveschensk
(Russia) and Heihe (China), it widens significantly as it is joined by
the Zeya River, one of its most important tributaries.
