Page 35 - Fundamentals of Geomorphology

18 INTRODUCING LANDFORMS AND LANDSCAPES

Box 1.2

NEGATIVE AND POSITIVE FEEDBACK

Negative feedback is said to occur when a change
in a system sets in motion a sequence of changes
that eventually neutralize the effects of the origi-
nal change, so stabilizing the system. An example
occurs in a drainage basin system, where increased
channel erosion leads to a steepening of valley-
side slopes, which accelerates slope erosion, which
increases stream bed-load, which reduces channel
erosion (Figure 1.9a). The reduced channel erosion
then stimulates a sequence of events that stabilizes
the system and counteracts the effects of the orig-
inal change. Some geomorphic systems also display
positive feedback relationships characterized by an
original change being magnified and the system being
made unstable. An example is an eroding hillslope
where the slope erosion causes a reduction in infil-
tration capacity of water, which increases the amount
of surface runoff, which promotes even more slope Figure 1.9 Feedback relationships in geomorphic
erosion (Figure 1.9b). In short, a ‘vicious circle’ is cre- systems. (a) Negative feedback in a valley-side slope–
ated, and the system, being unstabilized, continues stream system. (b) Positive feedback in an eroding
changing. hillslope system. Details of the relationships are given
in the text.

system is Uranus and its moons. These structures regardedasacomplexbutratherdisorganizedsystem.
may be thought of as simple systems. In geomor- In both the gas and the hillslope mantle, the interac-
phology, a few boulders resting on a talus slope tions are somewhat haphazard and far too numerous
may be thought of as a simple system. The condi- to study individually, so aggregate measures must
tions needed to dislodge the boulders, and their fate be employed (see Huggett 1985, 74–7; Scheidegger
after dislodgement, can be predicted from mechan- 1991, 251–8).
ical laws involving forces, resistances, and equations 3 In a third and later conception of systems, objects
of motion, in much the same way that the motion of are seen to interact strongly with one another to
the planets around the Sun can be predicted from form systems of a complex and organized nature.
Newtonian laws. Most biological and ecological systems are of this
2In a complex but disorganized system, a vast num- kind. Many structures in geomorphology display
ber of objects are seen to interact in a weak and high degrees of regularity and rich connections, and
haphazard way. An example is a gas in a jar. This may be thought of as complexly organized systems.
system might comprise upward of 10 23 molecules A hillslope represented as a process–form system
colliding with each other. In the same way, the count- could be placed into this category. Other examples
less individual particles in a hillslope mantle could be include soils, rivers, and beaches.