A reduced relevance of vegetation change for alluvial aggradation in arid zones

Abstract

A basic tenet of arid region geomorphology is that a decrease in hillslope vegetation density coupled with increases in monsoonal precipitation after a change to a warmer and drier climate is an essential factor for an increase in sediment yield and fan aggradation. Over the last two decades vegetative cover change is increasingly cited as a prominent factor in triggering the onset of fan aggradation in deserts of southwestern North America, especially in analysis of fan deposition during the late Pleistocene–Holocene (LPH) climatic transition. To assess this assertion, we compiled paleobotanical and alluvial fan stratigraphy histories from 18 published studies broadly representing four different areas in the Mojave and northern Sonoran Deserts. Instead of focusing on chronology aggregation and comparison with global or regional climate change proxies forced by orbital parameters, we discriminated by altitudinal regions, looking for linkages between local vegetation change and alluvial deposition. Results indicate that the onset of extensive alluvial fan deposition (1) began well before changes in catchment vegetative cover, and (2) can occur during several possible combinations of vegetation change. The ambiguous relation between vegetation change and alluvial fan aggradation indicates that vegetation had a reduced role in LPH aggradation. Other factors, such as local storm intensity and water and sediment redistribution pathways on hillslopes, need to be considered in this analysis given the important role that the combined hillslope/alluvial system has on arid region ecosystem functions.