Schumm and Licthy (1963) Figure from Ritter et al., 2002.
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Transcript of Schumm and Licthy (1963) Figure from Ritter et al., 2002.
Schumm and Licthy (1963)
Figure from Ritter et al., 2002
Thresholds(after Schumm, 1973)
• Extrinsic: limits of equilibrium exceeded due to an external factor (e.g., climatic change or tectonism).
• Intrinsic: limits are exceeded due to an internal change in the system.
• Geomorphic Threshold: originally special kind of intrinsic, but later suggested it could also include extrinsic; change in landform morphology results in a period of disequilibrium.
– Suggests that “normal” landscape development may lead to instability and changes do not necessarily require a change in the external environmental controls
Threshold RelationshipNorthwest Colorado
From Patton and Schumm, 1975
Characteristics of Threshold Crossing Events
• Lasting, non-reversible changes in process and form; – If recurrence interval is longer than the response time, then
a threshold has not been crossed; if next disturbance occurs before the system has recovered, then a threshold has been crossed
• System must tend towards a new equilibrium condition adjusted to the characteristics of the altered controlling factors;
• Thresholds are time-dependent phenomena; the graded time interval is most conducive to the threshold crossing concept;
• Thresholds are identified by parameters that characterized processes and landforms.
Time
Climate Change
Response variable
Reaction time
Relaxation time
Adjusted to new condition
Res
po
nse
Var
iab
le
After Bull, 1991
Figure 5.3, p. 158 in Knighton, D. 1998. Fluvial Forms and Processes: A New Perspective. Arnold, London.
105
104
103
102
101
100
10-1
10-1 100 104103102101
Increasing Time Scale, years
Incr
eas
ing
Len
gth
Sca
le, m
Bed ConfigurationSand-Bed Streams
ChannelWidth
ChannelDepth
Bed ConfiguratoinGravel-Bed Streams
MeanderWavelenght
ReachGradient
ProfileGradient
ProfileConcavity
PlanForm
Cross-sectionalform
Profileform
Time 0
Time 1
Time 2
Stable Channel
Aggradation
Re-incision & Terrace Formation
Incision
DepositionalZone
Time 2
Basin Mouth
A
A'
Deposition
Stable Channel
Time 1
Basin Mouth
A
A'
A'A
Plan View Cross-Section
From Yubanet.com
From Gilbert, 1917
After Graves and Eliab, 1977
From Miller, 1997
Reservoir
0
1
1B2
2B
3 4
5
67
7B
7C 7D9 10
11
1213 14
15
16 1718
Gagingstation
GagingStation
MineralCanyon
Dayton
CarsonCity
VirginiaCity
TableMtn.
Canyon
(Brunswick)
FortChurchill
395
0
0 1 2 3 4
1 2 3 4 5 Miles
Km
95
Six Mile
Canyon Fan
Gold Canyon
Six M
ile Canyon
Fork
Reno Fallon
CarsonCity
Carson River
Watershed Boundary
Carson Lake
Carson Playa Stillwater
WildlifeRefuge
Lahontan Reservoir
LakePyramid
SIERRA NEVADA
NevadaCalifornia
ForkE
ast
Truckee R
.
Car
son
R.
Tru
ckee
R
.Wes
tF
ork
C
arso
n R. .
Lake Tahoe
Lahontan
Light colored materials are Hg contaminated mine tailings
Temperature - Precipitation
Geology Vegetation
Hydrology/Discharge
Sediment Discharge
Sediment Size
Load Type
Profile Change
•Slope Adjustment
•Scour and Fill
•Terrace Formation
Channel Form Adjustment
•Width
•Depth
•Width/Depth Ratio
•Roughness
Systems
•Pattern Change
•Sinuosity
•Meander Wavelength
Modified from Ritter et al., 1995
Take Home Messages
• Threshold crossings are likely to result in rapid and dramatic changes in the system;
• Responses may occur as a series of reactions with erosion and deposition occurring out of phase with one another in the watershed;
• Initial response may not be the final response;• Response to a perturbation may take years to
complete;• Disrupted systems may impact other systems
(process linkage)
From Ritter 1978; Process Geomorphollogy
Figure 1.8, page 35, Brown 1997. Alluvial geoarchaeology: floodplain archaeology and environmental change. Cambridge Manuals in Archaeology, Cambridge University Press. Only half of figure used. Modify what is used so permission is not required.
Figure B is Fig. 3, p. 377, in Miller JR, Rowland J, Lechler PJ, Desilets M (1996) Dispersal of mercury-contaminated sediments by geomorphic processes, Sixmile Canyon, Nevada, USA: implications to site characterization and remediation of fluvial environments. Water, Air and Soil Pollution 86:373-388.