Title: Our Dynamic Desert
Headings: 
Pediments and Alluvial Fans
Content: In many areas throughout the Mojave region it is nearly impossible to see where a pediment ends and alluvial fans begin, however, geophysical data and water-well drilling shows that in many places sediment filled basins do occur adjacent to pediment areas. The impact of climate change on alluvial fans has been the focus of much research. Studies show that a period of elevated alluvial fan deposition occurred between the time of the Last Glacial Maximum (about 15,000 years ago) and the beginning of arid conditions in the early Holocene (about 9,400 years ago). McDonald et al, (2003) suggest that the climatic transition from seasonable wet conditions to arid conditions, punctuated by extreme storm event (possibly associated with tropical cyclones) may be responsible for this change. Today, heavy rainfalls rarely provide enough precipitation to allow enough surface runoff to occur on highly porous soils and colluvium. Only during major stream event will water discharge in volume and intensity to move material from mountain source areas to lower fan areas. In addition to extreme storm events,the buildup of alluvial fan deposits at this Pleistocene/Holocene time transition may be linked with the transition from widespread plant cover to the more barren character of the modern Mojave landscape. Die-back of plants would decrease rooting, making more mountain-side material available for erosion and transport to alluvial fans.