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Valley-floor-channel and alluvial-fan deposits and terraces in thesouthwestern US record multiple episodes of late-Quaternary fluvial-systemaggradation and incision. Perhaps the most well-constrained of theseepisodes took place from the latest Pleistocene to the present in the MojaveDesert. One hypothesis for this episode – i.e., the paleovegetation-changehypothesis (PVCH) – posits that a reduction in hillslope vegetation coverassociated with the transition from Pleistocene woodlands to Holocene desertscrub generated a pulse of sediment that triggered a primary phase ofaggradation downstream, followed by channel incision, terrace abandonment,and initiation of a secondary phase of aggradation further downstream. Asecond hypothesis – i.e., the extreme-storm hypothesis – attributes episodes ofaggradation and incision to changes in the frequency and/or intensity ofextreme storms. In the past decade a growing number of studies has advocatedthe extreme-storm hypothesis and challenged the PVCH on the basis ofinconsistencies in both timing and process. Here I show that in eight out ofnine sites where the timing of fluvial-system aggradation in the MojaveDesert is reasonably well constrained, measured ages of primary aggradationare consistent with the predictions of the PVCH if the time-transgressivenature of paleovegetation changes with elevation is fully taken intoaccount. I also present an alternative process model for PVCH that is moreconsistent with available data and produces sediment pulses primarily via anincrease in drainage density (i.e., a transformation of hillslopes intolow-order channels) rather than solely via an increase in sediment yieldfrom hillslopes. This paper further documents the likely important role ofchanges in upland vegetation cover and drainage density in drivingfluvial-system response during semiarid-to-arid climatic changes

earth surface dynamics

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