Jose Luis Antinao

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

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.

Occurrence of landslides during the approach of tropical cyclone Juliette (2001) to Baja California Sur, Mexico

The approach of tropical cyclone (TC) Juliette, in 2001, to the Baja California Peninsula triggered at least 419 landslides. Most of the landslides were shallow slips and debris slides of limited areal extent, which were converted rapidly into debris flows to be exported quickly out of the mountain areas towards the lowlands. Main factors affecting landslide occurrence were total storm rainfall and intensity, aspect, geology and vegetation association. Two processes can be distinguished as initiating slope failure. Accumulation of rainfall from exposed bedrock slopes, generating excess overland flow, was the main process linked to failures in concave topography. A combination of wind and excess overland flow in the more convex or planar upper slopes was a secondary process related to heterogeneity of vegetation associations in the oak-dry tropical forests ecotone, as uprooted trees dislodged large regolith and bedrock blocks, priming hillslopes for further runoff concentration. An estimative threshold curve for triggering landslides in this region is sketched. From the analysis of historical information, storms like Juliette approach the southern peninsula on average once every 100 years. Denudation estimates are in the higher end of the spectrum for a tectonically passive margin. These estimates should be considered when taking decisions regarding management of water resources in this area through damming of streams. The results emphasize the need for a more detailed representation of the spatial distribution of the rainfall and winds for this mountainous region affected by TCs.