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.