Geoscience Reference
In-Depth Information
Figure 16.9 Stream ordering.
Stream ordering
classifies the hierarchy of channels in a drain-
age basin. Stream order changes only where two
streams of the same order merge. A positive rela-
tionship exists between stream size and stream
order, with streams of progressively higher order
portrayed by progressively thicker lines on the
diagram.
Stream
order
1
2
3
4
Number of
streams
16
5
2
1
1
2
3
1
2
2
1
3
1
1
1
4
3
4
2
2
1
2
2
1
1
1
1
2
1
1
1
1
1
any drainage network can be arranged into a hierarchy
based on their size. Stream ordering is useful because it
provides a good relative measure of a stream's place in
a basin hierarchy, which is typically a function of how
many tributaries occur in any given watershed. Order-
ing the streams of any basin is a simple task, as shown in
Figure 16.9. An important rule to remember when calculat-
ing stream order is that it changes whenever two streams
of the
same order
join at their confluence. The smallest
tributaries of a basin are first-order streams. At the conflu-
ence of the first-order tributaries, a second-order stream
forms. Subsequently, two second-order streams join to
form a third-order stream.
It is tempting to think that the ordering of a stream
changes
whenever
it meets another stream, regardless of the
second stream's order. For example, Figure 16.9 shows sev-
eral places where first-order streams flow into streams of a
higher order, such as a second- or third-order stream. You
can also see a confluence where a second-order stream meets
a third-order stream. Remember that stream order changes
only
at the confluence of streams of the same order. For
example, just because a first-order stream meets a second-
order stream that does not mean the second-order stream
becomes a third-order stream. It does not; rather, it remains
a second-order stream. The second-order stream becomes a
third- order stream
only
where it joins another second-order
stream.
A last point to make about stream ordering is that a positive
relationship exists between increased stream order and stream
size. In other words, you can expect a second-order stream in a
drainage basin to be larger than a first-order stream. Similarly,
a third-order stream is bigger than a second-order stream. This
increase occurs because a stream of a higher order contains the
combined flow of the lower ones. Thus, higher-order streams
logically contain more water than lower-order streams. In
Figure 16.9, this geographical relationship is portrayed by using
progressively thicker lines to represent streams of progressively
higher order.
The Rhine River
The next section of the chapter focuses on hydraulic geometry
and channel flow. You can preview this material by going to the
Geo Media Library
and accessing
The Rhine River
. This video
follows the course of the famous Rhine River in Europe from its
origin in the Swiss Alps to where it meets the Atlantic Ocean in
Holland. As you watch the video, notice the various landscapes
through which the Rhine flows. Listen for the technical terms,
such as
graded stream, rapids,
and
valley
, and try to understand
their meaning, as they will be discussed in this text. As you con-
tinue through the chapter, keep this video in mind as an excellent
example of a stream. Once you complete the exercise, be sure
to answer the questions at the end to test your understanding of
this concept.
