Title: Pediments and Alluvial Fans - Mojave Desert
Headings: Pediments and Alluvial Fans
Pediments and Alluvial Fans
Content: In the foreground, a relatively stable alluvial fan surface consists of desert pavement broken by braided stream channels and a patchwork of vegetation (mostly white bursage [gray] and creosote bush [green]). Total surface relief in this lower portion of the fan is in the range of one meter. The development of pediments and alluvial fans is progressive with the uplift of mountains and subsidence of adjacent basins. Pediments reflect a relative "static equilibrium" between erosion of materials from upland areas and deposition within an adjacent basin. The slope of the landscape is gentle enough that weathering and transport of sediments from upland areas and the pediment that no significant stream incision occurs. 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.