Geoscience Reference
In-Depth Information
viewed as points on a continuum of flow regime types. This is because, for example, during
an extended wet period lasting several years, an ephemeral river may exhibit the character-
istics of a seasonal river, while during dry periods, the wet season flow of a seasonal river
may be absent or more intermittent, making it appear more like an ephemeral river.
How long is a river?
Measuring the length of a river is more complicated than it sounds. Measurements and es-
timates for the length of the world's rivers vary greatly depending on all sorts of factors, in-
cluding the season of the year, the abilities of the cartographer, and the quality of his equip-
ment, as well as decisions about what exactly is measured. In theory, the exercise should
be straightforward: determine the position of the source, identify the mouth, and accurately
measure the length of the river between the two. Finding the mouth is usually clear-cut. Its
exact location is commonly defined as the intersection between the central line of the river
and a line drawn between the two sides of the outlet.
Determining the exact location of the source is often more difficult. Searches for the source
of particular rivers in remote and inaccessible regions have intrigued and inspired explorers
for centuries, and continue to do so even today.
Disagreements about the true source of many rivers have been a continuous feature of this
history of exploration. In one sense, a mission to find 'the' source of a river is destined to
be a matter of conjecture simply because most rivers typically have many tributaries and
hence numerous sources. For most authorities, the source that is farthest away from the
mouth is considered to be 'the' source of the river, thus giving a maximum river length.
But, unsurprisingly, differences arise as to the farthest source.
Another complicating factor is whether or not to include tributaries that have been given
different names. In practice, the series of decisions made about the inclusion or exclusion
of tributaries is probably the major element of a quest to find the source of a river, and these
decisions represent one of the central reasons why not all measurements for a particular
river agree. Take the Mekong as an example. Everyone acknowledges that the river origin-
ates on the Tibetan Plateau, but where exactly is open to debate. Candidates for its source
include glaciers on the Guozongmucha Mountain, Lasaigongma Mountain, Zhanarigen
Mountain, Chajiarima Mountain, and Mount Jifu. Others include Rup-sa La Pass, Lungmo
Pass, and Lake Zhaxiqiwa. Given the number of designated sources, perhaps it is not sur-
prising that the Mekong is variously referred to as the ninth longest and the twelfth longest
