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 Fundamentals of Fluvial Geomorphology and Channel Processes
2.1.1.4 Thresholds
Geomorphic thresholds may be thought of as the straw that broke the camel's back.
In the fluvial system this means that progressive change in one variable may eventually result
in an abrupt change in the system. If a river erodes a few grains of soil from the toe of the
river bank, no particular response will be noticed. If that continues with no deposition to
balance the loss, the bank may eventually fail abruptly and dramatically due to undermining.
The amount of flow impinging along a bank may vary considerably with no apparent effect
on the stabilization; however, at some critical point the bank material will begin to move and
disastrous consequences can result.
In these examples the change was a gradual erosion of a few grains of soil and a
variability of stream velocity, both which could be considered to be within the natural system.
This type of threshold would be called an intrinsic threshold. Perhaps the threshold was
exceeded due to an earthquake or caused by an ill-planned bank stabilization project. These
would be called an extrinsic threshold. The planner must be aware of geomorphic thresholds,
and the effect that their project may have in causing the system to exceed the threshold.
Channel systems have a measure of elasticity that enables change to be absorbed by
a shift in equilibrium. The amount of change a system can absorb before that natural
equilibrium is disturbed depends on the sensitivity of the system, and if the system is near a
threshold condition, a minor change may result in a dramatic response.
2.1.1.5 Time
We all have been exposed to the geologists view of time. The Paleozoic Era ended
only 248 million years ago, the Mesozoic Era ended only 65 million years ago, and so on.
Fortunately, we do not have to concern ourselves with that terminology. An aquatic
biologist may be concerned with the duration of an insect life stage, only a few hours or days.
What we should be aware of is that the geologist temporal perspective is much broader than
the temporal perspective of the engineer, and the biologist perspective may be a narrowly
focused time scale. Neither profession is good nor bad because of the temporal perspective;
just remember the background of people or the literature with which you are working.
Geomorphologists usually refer to three time scales in working with rivers: 1)
geologic time, 2) modern time, and 3) present time. Geologic time is usually expressed in
thousands or millions of years and in this time scale only major geologic activity would be
significant. Formation of mountain ranges, changes in sea level, and climate change would
be significant in this time scale. The modern time scale describes a period of tens of years to
several hundred years, and has been called the graded time scale (Schumm and Lichty, 1965).
During this period a river may adjust to a balanced condition, adjusting to watershed water
and sediment discharge. The present time is considered a shorter period, perhaps one year
to ten years. No fixed rules govern these definitions. Design of a major project may require
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