The linkages among hillslope-vegetation changes, elevation, and the timing of late-Quaternary fluvial-system aggradation in the Mojave Desert revisited

J D Pelletier

doi:10.5194/esurf-2-455-2014

Abstract

Abstract. Valley-floor-channel and alluvial-fan deposits and terraces in the southwestern US record multiple episodes of late-Quaternary fluvial-system aggradation and incision. Perhaps the most well-constrained of these episodes took place from the latest Pleistocene to the present in the Mojave Desert. One hypothesis for this episode – i.e., the paleovegetation-change hypothesis (PVCH) – posits that a reduction in hillslope vegetation cover associated with the transition from Pleistocene woodlands to Holocene desert scrub generated a pulse of sediment that triggered a primary phase of aggradation downstream, followed by channel incision, terrace abandonment, and initiation of a secondary phase of aggradation further downstream. A second hypothesis – i.e., the extreme-storm hypothesis – attributes episodes of aggradation and incision to changes in the frequency and/or intensity of extreme storms. In the past decade a growing number of studies has advocated the extreme-storm hypothesis and challenged the PVCH on the basis of inconsistencies in both timing and process. Here I show that in eight out of nine sites where the timing of fluvial-system aggradation in the Mojave Desert is reasonably well constrained, measured ages of primary aggradation are consistent with the predictions of the PVCH if the time-transgressive nature of paleovegetation changes with elevation is fully taken into account. I also present an alternative process model for PVCH that is more consistent with available data and produces sediment pulses primarily via an increase in drainage density (i.e., a transformation of hillslopes into low-order channels) rather than solely via an increase in sediment yield from hillslopes. This paper further documents the likely important role of changes in upland vegetation cover and drainage density in driving fluvial-system response during semiarid-to-arid climatic changes.

Highlights

Introduction and motivationQuaternary deposits of the southwestern US are dominated by valley-floor-channel and alluvial-fan deposits and their associated terraces that record multiple regionally correlative episodes of aggradation, channel incision, and terrace abandonment (Christensen and Purcell, 1985; Bull, 1991; Harvey et al, 1999; Menges et al, 2001; McDonald et al, 2003; Anders et al, 2005)
In his work on the paleovegetation-change hypothesis (PVCH) for fluvial-system response to climatic changes in the southwestern US, Bull (1991) argued that a reduction in vegetation cover during late-Quaternary semiarid-to-arid transitions led to sediment pulses characterized by an initial increase in sediment yield followed by a decrease in sediment yield as the reservoir of colluvium stored on hillslopes was depleted
Rather than grouping fan-aggradation and paleovegetation sites according to spatial proximity and elevation ranges that are similar but are associated with differences in the timing of assumed versus actual paleovegetation changes of up to 6 ka, my methodology honors the dominant role of elevation in controlling plant distributions in the Mojave Desert by first quantifying the relationship between the elevational lower limit of woodland plants versus time in the Mojave Desert and applying that relationship to a GIS analysis that predicts the timing of both primary aggradation and channelincision/secondary-aggradation based on the timing of the woodland-to-desert-scrub transition in the source catchments upstream from every point on the landscape

Summary

Introduction and motivation

Quaternary deposits of the southwestern US are dominated by valley-floor-channel and alluvial-fan deposits and their associated terraces that record multiple regionally correlative episodes of aggradation, channel incision, and terrace abandonment (Christensen and Purcell, 1985; Bull, 1991; Harvey et al, 1999; Menges et al, 2001; McDonald et al, 2003; Anders et al, 2005). In his work on the paleovegetation-change hypothesis (PVCH) for fluvial-system response to climatic changes in the southwestern US, Bull (1991) argued that a reduction in vegetation cover during late-Quaternary semiarid-to-arid transitions led to sediment pulses characterized by an initial increase in sediment yield (resulting in aggradation in valley-floor channels and/or alluvial fans downstream) followed by a decrease in sediment yield as the reservoir of colluvium stored on hillslopes was depleted (resulting in channel incision and terrace abandonment). In the central Mojave subregion of their study, Antinao and McDonald (2013a) tested the PVCH by comparing the timing of fan aggradation at a site (southern Death Valley – DV) sourced by an eroding catchment with lowest elevations of ≈ 400 m a.s.l. I present an improved process model of how the PVCH works, which includes transient changes in drainage density associated with semiarid-to-arid climate transitions

A test of the PVCH for the Mojave Desert

An improved process model for PVCH

Findings

Discussion

Conclusions