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. I I I I I I I I I I I I I I I I I I 10 07/16/99 4B Identification of Inactive Areas For a given area of the alluvial fan, if the situations described in Section 4A do not exist, then the area is considered inactive and not subject to the deposition, erosion and unstable flow path flooding that builds alluvial fans. Inactive areas may be subject to flooding though, most notably within entrenched channels. Evidence of inactive areas may include annoring along the margin of the area bordering active areas, older vegetation, and the lack of change in flow paths viewed over the aerial photographic record. This evidence, though, does not preclude the area from possibly being classified as an active area due to changes in or conditions within adjacent active areas. Older alluvial fan surfaces are considered active if any of the following are true: . The recently active sedimentation zone is presently migrating into the older surface . The elevation difference between the recently active sedimentation zone and the older surface is small relative to flood, deposition and debris depths conceivable in the current regime of climate, hydrology, or land use in the source area . Upstream of the site there is an opportunity of avulsions that could lead channels or sheetf100ds across the older surface 4C Identification Process Once a relative time period is chosen (i.e., <1000 years) to help in evaluating the active areas of an alluvial fan, the analyst must determine relative ages for the morphologic features on the alluvial fan. Indicators of land surface age for Stage 2 are based on relative age indicators. Absolute (numerical) dating techniques, such as radiocarbon dating, are generally beyond the scope of many studies. Detailed soils and surficial geological maps, when available, provide useful delineation of soil types and surface ages. An examination of the historical record of flooding and deposition can enhance the information gained from the soils map. Aerial photographs from different years can be used to identitY sites of deposition. Field examination of morphologic features on the alluvial fan surface, particularly noting relicts of human activity (recent or archaeological) or weathering characteristics such as desert pavement, rock varnish, B-horizon development in the soil profile, calcic-horizon development, and pitting and rilling of clasts may also provide relative age information. Density and type of vegetation can provide useful clues to the age of an alluvial fan surface area. Texture and composition of the sediment, in addition to the water holding capacity, relate to the surface vegetation. Fresh alluvial deposits contain little organic carbon or clay and as a result do not promote vegetation growth. Vegetation is limited on older surfaces because they receive only direct rain, are often erosional, and can be less fertile (carbonate soil cropping out at the surface, for example). Intermediate-age surfaces (middle to late Holocene) contain the most dense and diverse vegetation.