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

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 hypothesisi.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 extremestorm 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.

The influence of Holocene vegetation changes on topography and erosion rates: a case study at Walnut Gulch Experimental Watershed, Arizona

“…Such an expansion of the fluvial drainage network could have mobilized hillslope deposits stored as colluvium during the last glacial epoch, mobilizing large volumes of sediment into the fluvial system during the transition to the present interglacial (Bull, 1991;Pelletier, 2014). This hypothesis is consistent with measured ages of the onset of aggradation in valley floor and alluvial fan depositional zones in the central Mojave Desert, in which aggradation occurs earliest (ca.…”

“…In especially arid areas of the southwestern USA such as the central Mojave Desert, the range of elevations affected by late Quaternary conversions of grasslands/woodlands to shrublands extends to elevations as high as 1800 m a.s.l. (Pelletier, 2014).…”

“…15 ka) in depositional zones fed by source regions with relatively low elevations in the 800-1800 m a.s.l. range and progressively later in areas fed by eroding regions at higher elevations (Pelletier, 2014). Alternative explanations for the punctuated nature of late Quaternary aggradation in the southwestern USA invoke changes in the frequency and/or magnitude of floods (Antinao and McDonald, 2013b) and argue that hillslope vegetation changes played a limited role (Antinao and McDonald, 2013a).…”

A gridded global data set of soil, intact regolith, and sedimentary deposit thicknesses for regional and global land surface modeling

“…On uplands, the distinction between hillslopes and valley bottoms is important for the purposes of estimating soil, intact regolith, and sedimentary deposit thicknesses because upland valley bottoms, while erosional over long time scales, can be areas of significant deposition over short time scales, which can result in locally thick sedimentary deposits. For example, variations in erosion rates caused by Quaternary climatic changes have led to recent (i.e., 10 3 to 10 4 years) deposition in many valley bottoms, even those within mountain ranges that are undergoing net erosion over longer time scales [Bull, 1991;Pelletier, 2014]. The distinction between hillslopes and valley bottoms is usually not significant for estimating sedimentary deposit thicknesses in lowlands because sedimentary deposit thicknesses are most strongly controlled by structural geologic factors (e.g., fault geometries that define the boundaries between mountain ranges and adjacent depositional basins), with surface topography acting as a relatively minor controlling factor.…”

“…Upland valley bottoms are areas that have undergone net erosion over geologic time scales but are often infilled with sedimentary deposits as a result of Quaternary climatic changes that have episodically delivered large volumes of sediment from hillslopes to valley bottoms that are not adjusted to transport such high sediment loads [e.g., Bull, 1991;Pelletier, 2014]. As a result, many of these valley bottoms are filled with meters to tens of meters of sediment above bedrock.…”