Steps in Geomorphic Analysis and
Prediction
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Observation
Description
Explanation
Extrapolation aka Prediction
• E.g. “What will be the response of
the river to this restoration action?”
• Specific e.g. “If I build these log
jams at these locations, will it result
in good salmon habitat?”
GK Gilbert’s Method of Research
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Concentrated Observation
Classification and Grouping of Facts
Development of Multiple Hypotheses
by Induction to Explain Observations
• “there is indeed an advantage to
entertaining several at once”, Gilbert,
1886
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Hypothesis Testing (and Revising)
An example from Gilbert
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Variable height of Bonneville
shoreline
Measured elevation at 2
locations
• not the same
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Shoreline not horizontal
Explanation?
• Crustal undulation
• Structural: Folding
• Faulting
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More msmts
• max displacement near lake
center
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Isostatic rebound
Lake Bonneville by GK Gilbert, 1890
Another take on multiple hypotheses
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“The studies of the geologist are peculiarly
complex. It is rare that his problem is a
simple unitary phenomenon explicable by a
simple single cause.
Even when it happens to be so in a given
instance, or at a given stage of work, the
subject is quite sure, if pursued broadly to
grade into some complication or undergo
some transition.
If there any advantages in any field in being
armed with a full panoply of working
hypotheses and in habitually employing
them, it is doubtless the field of the
geologist”
Chamberlin, 1897
Problem with Predictions
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Using modern (today) conditions as the
basis for prediction
• uniformitarianism
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“the present is the key to the past” Geikie,
1905
The assumption that natural laws are
permanent, i.e.; under the same
conditions a given cause will always
produce the same results
• this assumption is required to extrapolate from
the present to the past, and to the future
an approach to landform and landscape interpretation
10 Problems Associated with Using Modern
Conditions as a Basis for Extrapolation
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Three classes of problems:
Time: absolute duration and relative
1. Scale and place
2. Cause and effect
Space: scale and size
3. System response
Location
Convergence (equifinality): production of
similar results from different processes/causes
Divergence: production of different results from
similar processes/causes
Efficiency: variable efficacy/work done by a
process
Multiplicity: multiple explanations
Singularity: natural variability among like things
Sensitivity: susceptibility to change
Complexity: complex response to altered
conditions
Problems of scale and place: Time
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“A means of measuring change” Schumm
Not enough of it (for data collection)
• Records are short; life too short
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Short history
• Short-term records less variable than long-term records
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Present physical systems (e.g. landforms,
structures) are influenced by history
Short time-span studies applied to long time span
problems is tricky
Examples
• Colorado River compact
• Floodplain formation
Law of the River, 1922
Allocated 7.5M acre-feet
To the upper and lower basins
Hurst phenomenon:
Persistence within a record
Ie closely spaced events have
a high degree of autocorrelation
Vertical accretion: short time-scale
Lateral migration: longer time-scale
Has the frequency of small debris flows increased since 1870?
Problems of scale and place:
Space
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“Three dimensional field in which
natural phenomena function and
occur”, Schumm
Complexity increases as resolution
increases
• Observations made at poor resolution
may yield predictions of low accuracy
difficult to extrapolate
from small to large
Problems of scale and place:
Location
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“the site or place at which observations of natural
system are made” Schumm
Can we extrapolate from one location to another?
Response to the same event could be different
from one location to another: e.g deglaciation
• E.g., (Knox, 1983) Eastern US: rivers respond to
deglaciation by lateral shift
• Western US: rivers respond to deglaciation by vertical
change (erosion or deposition)
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Climate during glacial periods
• Here: cooler/wetter
• Other places: cooler/drier; cooler/no change in precip
Problems of cause and effect:
Convergence (Equifinality)
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“different processes and different
causes produce similar effect”
Schumm
Different processes produce similar
looking landforms
E.g., terrace formation (channel
incision)
• Caused by
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Change in base level: sea level rise (marine
terraces)
Tectonics: faulting
Climate change: change in sed
supply/precip
Causes:
Fan deposition and coalescence
Lateral stream planation
Problems of cause and effect:
Divergence
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“the opposite of
convergence;
similar causes and
processes produce
different effects”
Schumm
Effect of melting
ice sheets on sea
level
• Generically, SL rises
• Actually, results are
variable
isostatic uplift: raised shoreline
submergence
partial submergence
submergence
submergence/emergence
Fluvial Divergence
humid
humid
Problems of cause and effect:
Efficiency
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“ratio of the work done to the energy
expended” Schumm
“More” energy expended doesn’t
necessarily result in the most work
done
E.g., Flood effectiveness
• Varies with preceding floods (event
ordering)
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Effectiveness is greater if preceded by a
large event rather than a small one
Antecedent precipitation
Effective Discharge: transports the
most sediment
Maximum sed yield not at max precipitation
Supply-limited
Transport-limited
Problems of cause and effect:
Multiplicity (many)
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“multiple causes acting simultaneously and in
combination to produce a phenomenon”
Schumm
Single explanations are not sufficient in most
cases
Multiple explanations are needed
• Ie multiple working hypotheses
• Each variable provides partial explanation
• Each variable deals with a different aspect of the
phenomenon
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Examples
• Hydraulic geometry
• Discharge: Q=f(drainage area)
w  aQ
b
Problems of systems response:
Singularity
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“the condition or characteristic that
makes one thing different from others”
Schumm
Randomness or unexplained variation in a
data set (Mann, 1970)
Easier to make predictions for a pop’n of
landforms but prediction for a single one
is very difficult
Problems of systems response:
Sensitivity
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“propensity of a system to respond to a
minor external change” Schumm
The change occurs at a threshold
If the system is near a threshold, it is
sensitive
Proximity to the threshold drives
response
Example, incipient motion
Problems of systems response:
Sensitivity
External and internal thresholds:
External e.g.: increase in external variable (slope below)
Internal e.g.: long-term weathering to slope failure (Ennis L. dam)
Schumm and Khan, 1973
Problems of systems response:
Complexity
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Multiple responses to a perturbation
Example of process linkage and complex response
1959 Hebgen Lake
earthquake-induced
landslide
t0, x0
TIME
SPACE
Incision t1, x1 Deposition t1, x2
Incision t2, x2
Locke, 1998
Deposition t2, x3
Incision t3, x3
Deposition t3, x4
Problems of systems response:
Complexity
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Can lead to unintended
consequences
• E.g., geo-engineering
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“if you can’t think of three things
that can go wrong then you don’t
understand the system”
(Weinburg and Weinburg, 1979)
Solutions
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What happened?
• Assemble historical information and develop a
history of past events that can help lead to
prediction
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What controls it?
• Develop an understanding of the processes
that operate and determine the applicable
physical and chemical relationships
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Where does it fit into the spectrum of this
phenomenon?
• Compare the results in space and determine
the characteristics that exist at different
locations
“Always ask yourself
important questions”
- Luna Leopold