Geology Reference
In-Depth Information
Alpine
Rhine
Middle
Rhine
High Rhine
Upper Rhine
Lower Rhine
3,000
3,000
Rhenish
Slate
Mountains
Upper Rhine
Graben
1,000
2,500
Lowlands
500
2,000
300
100
1,500
50
1,000
30
10
500
5
0
3
0
200
400
600
Distance (km)
800
1,000
1,200
Figure 9.4
Long-profile of the River Rhine, shown on an arithmetic height scale (dashed line) and logarithmic height
scale (solid line).
Source:
After Ahnert (1998, 174)
Grade
, as defined by J. Hoover Mackin (1948), is
a state of a river system in which controlling variables
and baselevel are constant:
Drainage basins and river channel
networks
A river system can be considered as a network in which
nodes
(stream tips and stream junctions) are joined
by
links
(streams). Stream segments or links are the
basic units of stream networks.
Stream order
is used
to denote the hierarchical relationship between stream
segments and allows drainage basins to be classified
according to size. Stream order is a basic property of
stream networks because it relates to the relative dis-
charge of a channel segment. Several stream-ordering
systems exist, the most commonly used being those
devised by Arthur N. Strahler and by Ronald L. Shreve
(Figure 9.5). In
Strahler's ordering system
, a stream
segment with no tributaries that flows from the stream
source is denoted as a first-order segment. A second-
order segment is created by joining two first-order
segments, a third-order segment by joining two second-
order segments, and so on. There is no increase in order
when a segment of one order is joined by another of
A graded stream is one in which, over a period of years, slope is
delicately adjusted to provide, with available discharge and with
prevailing channel characteristics, just the velocity required for
the transportation of the load provided by the drainage basin.
The graded stream is a system in equilibrium; its diagnostic
characteristic is that any change in any of the controlling fac-
tors will cause a displacement of the equilibrium in a direction
that will tend to absorb the effect of the change.
(Mackin 1948, 471)
If the baselevel changes, then streams adjust their grade
by changing their channel slope (through aggradation
or degradation), or by changing their channel pattern,
width, or roughness. However, as the controlling vari-
ables usually change more frequently than the time taken
for the channel properties to respond, a graded stream
displays a quasi-equilibrium rather than a true steady
state.
