Geoscience Reference
In-Depth Information
4. A meandering river and oxbow lakes in a remote part of New Guinea
The meander cut-off creating an oxbow lake is one way in which a channel makes an abrupt
change of course, a characteristic of some alluvial rivers that is generally referred to as
'avulsion'. It is a natural process by which flow diverts out of an established channel into
a new permanent course on the adjacent floodplain, a change in course that can present a
major threat to human activities. Rapid, frequent, and often significant avulsions have typ-
ified many rivers on the Indo-Gangetic plains of South Asia. In India, the Kosi River has
migrated about 100 kilometres westward in the last 200 years, and the Gandak River has
moved about 80 kilometres to the east over the last 5,000 years. The lower Indus River in
Pakistan also has a history of major avulsions. Why a river suddenly avulses is not under-
stood completely, but earthquakes play a part on the Indo-Gangetic plains.
Sometimes an avulsion can result in a channel being left dry, but on other occasions the
channel becomes split, creating a river that flows in multiple channels. These multi-channel
rivers are called 'anastomosing' or 'anabranching' rivers. At first sight, an anastomosing
river can easily be confused with a braided river, which has a roughly comparable pattern.
A braided river has multiple flows within a single channel, whereas an anastomosing river
has multiple interconnected channels. Nonetheless, debates about the differences continue
and numerous classifications of channel pattern are used. Misunderstandings can also arise
when river flow is high or low. At high discharge, a braided river with submerged bars may
look like a single-thread channel, and at low discharge an anastomosing river may carry
water in a single main channel only, so appearing as a single-channel river.
