Page 246 - Fundamentals of Geomorphology

FLUVIAL LANDSCAPES 229

3,000 Alpine High Rhine Upper Rhine Middle Lower Rhine 3,000
Rhine Rhine

Slate
2,500 Constance Falls near Upper Rhine Rhenish Lowlands 1,000
Graben
Lake Rhine Laufenburg Mountains 500
(m) 2,000 Rapids Basel 300 Height
level Karlsruhe
sea 1,500 Mainz Bingen 100 above
above Bonn 50 sea

Height 1,000 Emmerich 30 level

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