The linkages among hillslope-vegetation changes, elevation, and the timing of late-Quaternary fluvial-system aggradation in the Mojave Desert revisited
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...
| Main Author: | |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2014-08-01
|
| Series: | Earth Surface Dynamics |
| Online Access: | http://www.earth-surf-dynam.net/2/455/2014/esurf-2-455-2014.pdf |
| Summary: | 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. |
|---|---|
| ISSN: | 2196-6311 2196-632X |