Remote sensing technology allows us to recognize manifestations of regional thresholds, especially in the spatial characteristics of process agents. For example, a change in river channel pattern over a short distance reflects a threshold alteration in the physical controls of discharge and/or sediment. It is, therefore, a valuable indication of conditions as they exist. However, we probably will have difficulty determining whether the systemic parameters are now close to threshold conditions at which a different change will occur. This, of course, is a temporal and magnitude problem which is difficult to solve from the spatial characteristics

Ritter, D. F.

English

NASA Technical Reports Server

REGIONAL LANDFORM THRESHOLDS
Dale F. Ritter
Department of Geology
Southern Illinois University
Carbondale, IL 62901
One of the more significant trends in geomorphic thinking is the con-
tinuing development of the threshold concept. The importance of the
concept stems from its premise that boundary conditions exist within
geomorphic systems beyond which different processes will prevail or
existing processes will change in magnitude or rate. Identification
of the critical (threshold) conditions in terms of real parameters
would allow geomorphologists to provide extremely valuable informa-
tion to disciplines such as land management or environmental planning.
In light of this, most studies involving thresholds have concentrated
on establishing the conditions at which process change manifests itself
through local erosion or deposition or by a change in the character of
the process agent (rivers, glaciers, etc.). The emphasis on process
thresholds seems to be prevalent regardless of whether the change is
induced by internal or external factors. These studies are very
important, but they require long and tedious collection of data.
The application of space technology to the understanding of process
thresholds is an appealing goal of megageomorphology, but it may be
extremely difficult to achieve because of spatial, magnitude and
temporal problems. For example, changes in process attendant to the
onset or decline of glaciation and the rise and fall of sea level are
certainly large enough in magnitude to qualify as mega-thresholds.
However, the time required to reach the threshold condition is probably
well beyond the feasibility of continuous space monitoring. Other
events such as the large dust storms viewed from space reveal processes
that operate on a large scale. Clearly, these events represent the
crossing of a threshold boundary of wind erosion, but whether they can
be considered as a mega-threshold depends on factors such as recovery
time. After the event has occurred the system may quickly revert to
its original process condition of rate and magnitude. Thus, the
threshold crossing may be only a minor aberration of process in a
temporal sense, even though it was large in magnitude and affected a
vast area.
Remote sensing technology allows us to recognize manifestations of
regional thresholds, especially in the spatial characteristics of pro-
cess agents. For example, a change in river channel pattern over a
short distance reflects a threshold alteration in the physical controls
of discharge and/or sediment. It is, therefore, a valuable indication
of conditions as they exist. However, we probably will have difficulty
determining whether the systemic parameters are now close to threshold
conditions at which a different change will occur. This, of course, is
a temporal and magnitude problem which is difficult to solve from the
spatial characteristics. Certainly we can tell where threshold conditions
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https://ntrs.nasa.gov/search.jsp?R=19850024049 2020-03-20T18:11:57+00:00Z
have been exceeded. Paleochannels and exhumed regional landscapes are
testimony to mega-thresholds, and to the fact that part of the surfi-
cial environment is not adjusted to present conditions. However, it
is difficult to ascertain from space when the mega-threshold was
passed and at what parameter limits.
Despite these problems, the threshold concept should not be abandoned
in megageomorphology. Another approach to the threshold concept might
be to consider whether large geomorphic features also respond to exter-
nal changes such as climatic variation or tectonic activity. In other
words, do we have large scale landform thresholds whereby changes in
external controls require significant adjustments in the character of
the features themselves? Assuming that landform properties represent
an accommodation between process and form, it might follow that process
thresholds are forerunners of changes in the "equilibirum" character
of landforms. If this scenario is viable, it suggests that continous
monitoring of landforms in regions experiencing climatic change or
tectonic activity could yield important insights as to how and why land-
forms change. The converse may also be true. In addition, identifica-
tion of the conditions that produce landform thresholds and the extent
and nature of the response could also be important in disciplines con-
cerned with the environment. Certainly the planned development of large
communities in areas having landforms prone to threshold response would
benefit by knowing what changes might be expected in the surface environ-
ment if external factors were altered.
It now seems certain that process thresholds can occur rapidly and often
in response to relatively minor changes in controlling factors. It also
seems clear that landofrms are not permanent and, therefore, must change.
The question, however, is whether they change within the temporal and
spatial framework of the threshold concept. Thus, the suggestion of
landform thresholds producing rapid adjustments to changing external
factors is pure conjecture. Nonetheless, it is conjecture that can be
tested. The reality of landform thresholds producing rapid response
might be ascertained in areas that are presently experiencing climatic
change or tectonic activity. Perhaps an excellent starting point would
be to examine large alluvial fans and pediments in the extreme southern
part of California. Recent acceleration of beach erosion and sea cliff
retreat in this area has been attributed to a reasonably well-documented
climatic change that began in the late 1970s. The suggested climate
variation toward greater storminess and higher annual rainfall is based
on long-term precipitation records and tree-ring analyses, in addition
to varve thicknesses measured in Santa Barbara Basin. These data indi-
cate that significant episodes of increased rainfall occur with a peri-
odicity of about 30-40 years and last for about ten years. It is not
certain how far inland this climatic shift penetrates, but parts of the
Transverse Ranges are apparently being affected. Long-term weather
records should be available farther inland to determine the areal extent
of the climate change and its regional magnitude.
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Assuming that a climatic variation has begun, what questions should we
be asking about geomorphic features, and what information should we
seek that might be provided by remote sensing technology? A few examples
are offered using the climate change - fan adjustment as a model.
i) What geomorphic properties of large fans are best indicators of
climatic change? In general it seems likely that morphometric parameters
should be most fruitful in documenting any landform adjustment. Precisely
which parameters should be employed will probably depend on the resolution
of the sensing system. For example, the well known relationship between
drainage basin area and fan area could easily be measured because the entire
fluvial system can be viewed simultaneously. Depending on sensing resolu-
tion, it may also be possible to detect changes in drainage density in the
source areas and the fan areas, and the ratio of the two. A change in fan
shape might be determined by the relationship between perimeter length and
fan area.
2) To what degree does human activity accelerate or depress the
expected landform adjustments? Fans are commonly the locale of community
development which may prevent adjustment. In contrast, various activities
in the source areas may accelerate fan responses. Comparison of fans in
sparsely inhabited areas, such as Baja, California, to those in highly
developed regions could indicate the effect of human activity.
3) How long does it take for landforms to respond to climatic change?
It may be that fans do not respond to climate variations having the cyclicity
mentioned above; however, even that information would be significant.
4) What magnitude of climate change is needed to produce tangible
modifications in fan character? Perhaps large fans are affected only by
climatic variation having the magnitude of the glacial-interglacial cycle.
If responses do occur, and it can be shown that the climate change is
regional in extent, fans in different parts of the southwest U.S. may
adjust in different ways and with a hierarchy of intensities. Some rela-
tionship might exist between the magnitude of climate change and the rate
and degree of fan modification.
5) Are intrinsic fan thresholds real or do they represent responses
to minor changes in external controls? Certain fan phenomena such as
alternating fanhead trenching and filling are conceivably part of internal
fan mechanics involving changes in channel slope at the fan apex. These
reactions are logically controlled by an intrinsic threshold. It is also
possible, however, that they represent adjustments to minor, periodic shifts
in external controls. Simultaneous responses of the same type occurring on
fans over a wide regional extent may indicate adjustment to external rather
than internal controls.
6) It there a size of fan threshold? It may be possible that the
response or non-response of a fan to minor climatic variation is dependent
on the fan size. Small fans may respond, but larger fans may require greater
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magnitude of climate change or a longer time to reveal a tangible change.
The questions posed above are meant to be examples and are certainly not
the only ones possible. It does seem, however, that modern sensing tech-
nology allows us to raise and possibly answer a variety of questions that
would require enormous effort using more traditional geomorphic approaches.
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Regional landform thresholds

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