Relative scales of time and effectiveness of climate in watershed geomorphology

Abstract

Peak rainfalls and peak runoff rates per unit area are comparable over a worldwide spectrum of climates. However, while the magnitude of the external contribution of energy or force in diverse regions is similar, the impact on the landscape varies markedly between regions. Absolute magnitudes of climatic events and absolute time intervals between such events d o not provide satisfactory measures of the geomorphic effectiveness of events of different magnitudes and recurrence intervals. Although geomorphic processes are driven by complex sets of interrelated climatic, topographic, lithologic, and biologic factors, the work done by individual extreme events can be scaled as a ratio to mean annual erosion and the effectiveness of such events in forming landscape features can be related to the rate of recovery of channel form or mass wasting scars following alteration by the extreme event. Thus, a time scale for effectiveness may relate the recurrence interval of an event to the time required for a landform to recover the form existing prior to the event.River channels in temperate regions widened by floods of recurrence intervals from 50 to more than 200 years may regain their original width in matters of months or years. In semi-arid regions, recovery of channel form depends not only upon flows but upon climatic determinants of the growth of bottomland vegetation resulting in variable rates of recovery, on the order of decades, depending upon coincidence of average flows and strengthened vegetation. In truly arid regions the absence of vegetation and flow precludes recovery and the width of channels increases in drainage areas up to 100 km2 but remains relatively constant at larger drainage areas.Area as well as time controls the effectiveness of specific events inasmuch as the likelihood of simultaneous peak discharges or rainfalls and large areas is less, particularly in arid regions where events spanning areas of more than several thousand km2 are extremely rare if experienced at all. To some extent a decrease in area in a humid region is comparable with a regional change from humid toward more arid climate reflected in the increase in importance of episodic as contrasted with more continuous processes. Exceedingly rare floods of extreme magnitudes, estimated recurrence intervals of 500 years or longer, may exceed thresholds of competence otherwise unattainable in the 'normal' record resulting in 'irreparable' transformations of valley landforms.Denudation of hillslopes by mass wasting during relatively rare events can also be related to mean rates of denudation and to recovery of hillslope surfaces after scarring by different kinds of landslides. Measured recovery times described in the literature vary from less than a decade for some tropical regions to decades or more in temperate regions. Recurrence intervals of high magnitude storms which trigger mass wasting range from 1 to 2 years in some tropical areas, to 3 or 4 per hundred years in some areas of seasonal rainfall and to 100 or more years in some temper...

Closing the Gap Between Watershed Modeling, Sediment Budgeting, and Stream Restoration

“…The net exchange of sediment between stream channels and floodplains depends, in part, on the space accommodation within alluvial valleys, as well as local base level controls provided by structures such as dams and culverts [Schenk and Hupp, 2009]. Flood magnitude and the elapsed time between major runoff events also influence sediment deposition and storage within valley networks [Wolman and Gerson, 1978].…”

“…Even with the known existence of those processes, the net contribution calculated for the UPRW alluvial valley network was one of sediment storage over decadal time scales. The timing and rate of valley sediment evacuation is governed by the occurrence of relatively large runoff events, complicating predictions over time scales of less than a decade [Wolman and Gerson, 1978].…”

Erosional Consequence of Saltcedar Control

“…Because of a high variability in flow and relatively sparse riparian vegetation, stream ecosystems in the arid and semi-arid western United States often carry high sediment loads and can fluctuate greatly in channel width and location (Wolman and Gerson 1978). Many native riverine species require such physical disturbance for long-term persistence (Friedman and Lee 2002).…”

A stochastic approach to modeling the role of rainfall variability in drainage basin evolution

“…The importance of the frequency spectra of g½omorphic events is a fundamental problem in g½omorphic research. Much of the previous research on this problem has focused on defining the recurrence interval of the most effective g½omo•hic event, with "effectiveness" defined either on the basis of denudation rate [e.g., Wolman and Miller, 1960 Wolman and Gerson, 1978]. Scant attention has been paid, however, to the question of how intrinsic variability in g½omo•hic forces impacts the morpholo• and ra•½ of evolution of landforms.…”