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 Relationship Northwest 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 Response Variable After Bull, 1991

Figure 5.3, p. 158 in Knighton, D Fluvial Forms and Processes: A New Perspective. Arnold, London.

From Yubanet.com

From Gilbert, 1917

After Graves and Eliab, 1977

From Miller, 1997

Reservoir 0 1 1B 2 2B B 7C 7D Gaging station Gaging Station Mineral Canyon Dayton Carson City Virginia City Table Mtn. Canyon (Brunswick) Fort Churchill Miles Km 95 Six Mile Canyon Fan Gold Canyon S i x M i l e C a n y o n F o r k Carson City Carson River Watershed Boundary Carson Lake Carson Playa Stillwater Wildlife Refuge Lahontan Reservoir Lake Pyramid SIERRA NEVADA Nevada California F o r k E a s t T r u c k e e R. C a r s o n R. T r u c k e e R. W e s t F o r k C a r s o n R.. Lake Tahoe Lahontan

Light colored materials are Hg contaminated mine tailings

Temperature - Precipitation GeologyVegetation 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 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: