Page 37 - Fundamentals of Geomorphology

20 INTRODUCING LANDFORMS AND LANDSCAPES

Box 1.3

THRESHOLDS

A threshold separates different states of a system. It change in an external variable. A prime example is the
marks some kind of transition in the behaviour, oper- response of a geomorphic system to climatic change.
ation, or state of a system. Everyday examples abound. Climate is the external variable. If, say, runoff were
Water in a boiling kettle crosses a temperature thresh- to increase beyond a critical level, then the geomor-
old in changing from a liquid to a gas. Similarly, ice phic system might suddenly respond by reorganizing
taken out of a refrigerator and placed upon a table itself into a new state. No change in an external vari-
in a room with an air temperature of 10 C will melt able is required for a geomorphic system to cross an
◦
because a temperature threshold has been crossed. In internal threshold. Rather, some chance fluctuation
both examples, the huge differences in state – liquid in an internal variable within a geomorphic system
water to water vapour, and solid water to liquid water – may take a system across an internal threshold and
may result from tiny changes of temperature. Many lead to its reorganization. This appears to happen
geomorphic processes operate only after the crossing in some river channels where an initial disturbance
of a threshold. Landslides, for instance, require a criti- by, say, overgrazing in the river catchment triggers
cal slope angle, all other factors being constant, before a complex response in the river channel: a compli-
they occur. Stanley A. Schumm (1979) made a power- cated pattern of erosion and deposition occurs with
ful distinction between external and internal system phases of alluviation and downcutting taking place
thresholds. A geomorphic system will not cross an concurrently in different parts of the channel system
external threshold unless it is forced to do so by a (see below).

environmental changes or random internal fluctuations change is seen as a simple response to an altered input.
that cause the crossing of internal thresholds (Box 1.3), It shows that landscape dynamics may involve abrupt
a landscape will respond in a complex manner (Schumm and discontinuous behaviour involving flips between
1979). A stream, for instance, if it should be forced away quasi-stable states as system thresholds are crossed.
from a steady state, will adjust to the change. However, The latest views on landscape stability (or lack of it)
the nature of the adjustment may vary in different parts come from the field of dynamic systems theory,
of the stream and at different times. Douglas Creek in which embraces the buzzwords complexity and chaos.
western Colorado, USA, was subject to overgrazing dur- The argument runs that steady states in the landscape
ing the ‘cowboy era’ (Womack and Schumm 1977). It may be rare because landscapes are inherently unsta-
has been cutting into its channel bed since about 1882. ble. This is because any process that reinforces itself
The manner of incision has been complex, with discon- keeps the system changing through a positive feed-
tinuous episodes of downcutting interrupted by phases back circuit and readily disrupts any balance obtain-
of deposition, and with the erosion–deposition sequence ing in a steady state. This idea is formalized as an
varying from one cross-section to another. Trees have ‘instability principle’, which recognizes that, in many
been used to date terraces at several locations. The ter- landscapes, accidental deviations from a ‘balanced’ con-
races are unpaired (p. 236), which is not what would dition tend to be self-reinforcing (Scheidegger 1983).
be expected from a classic case of river incision, and This explains why cirques tend to grow, sinkholes
they are discontinuous in a downstream direction. This increase in size, and longitudinal mountain valley profiles
kind of study serves to dispel for ever the simplistic become stepped. The intrinsic instability of landscapes
cause-and-effect view of landscape evolution in which is borne out by mathematical analyses that point to the