Geoscience Reference
In-Depth Information
River hierarchies
One interesting aspect of rivers is that they seem to be organized hierarchically. When
viewed from an aircraft or on a map, rivers form distinct networks like the branches of a
tree. Small tributary channels join together to form larger channels which in turn merge to
form still larger rivers. This progressive increase in river size is often described using a nu-
merical ordering scheme in which the smallest stream is called first order, the union of two
first-order channels produces a second-order river, the union of two second-order channels
produces a third-order river, and so on. Stream order only increases when two channels of
the same rank merge. Very large rivers, such as the Nile and Mississippi, are tenth-order
rivers; the Amazon twelfth order.
Each river drains an area of land that is proportional to its size. This area is known by sev-
eral different terms: drainage basin, river basin, or catchment ('watershed' is also used in
American English, but this word means the drainage divide between two adjacent basins in
British English). In the same way that a river network is made up of a hierarchy of low-or-
der rivers nested within higher-order rivers, their drainage basins also fit together to form a
nested hierarchy. In other words, smaller units are repeating elements nested within larger
units. All of these units are linked by flows of water, sediment, and energy.
Recognizing rivers as being made up of a series of units that are arranged hierarchically
provides a potent framework in which to study the patterns and processes associated with
rivers. At the largest scale, the entire river basin can be studied. Within the basin, at pro-
gressively smaller scales, a researcher may focus on a particular segment of a river between
tributaries, a reach within a segment, and so on all the way down to a small patch of sand
grains on the river bed. This hierarchical approach also emphasizes that processes oper-
ating at the upper levels of the hierarchy exert considerable influence over features lower
down in the hierarchy, but not the other way around. At the river basin scale, important
factors are climate, geology, vegetation, and topography. These factors have an influence
at all lesser scales, down to the small patch of sand grains. That patch of sand also comes
under other local influences, such as ripples in the flowing water, but these small variations
in the current have a negligible impact on the drainage basin as a whole.
There is an appropriate timescale associated with related spatial scales and these too can be
arranged into a hierarchy. Generally, the larger the spatial scale, the slower the processes
and rates of change. Changes in climate and geology, for instance, occur on lengthy times-
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