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I I I I I I I I I I I I I I I I I I i, 8 07/16/99 3D Boundaries of an Alluvial Fan Define the toe and lateral boundaries of the alluvial fan. 3D (I) Toe The distal terminus, or toe, of an alluvial fan commonly is defined by: . A stream that intersects the fan and transports deposits away from the fan . A playa lake . An alluvial plain . Smoother, gentler slopes of the piedmont plain Such boundaries can often be identified on topographic maps by changes in contour lines or identified on aerial photographs or by field inspection as changes in vegetation due to sediment changes or increased water table depth. 3D (2) Lateral Boundaries Lateral boundaries of alluvial fans are the cdges of deposited and reworked alluvial materials. The lateral boundlll)' of a single alluvial fan typically is a trough, channel, or swale formed at the lateral limits of deposition. The lateral boundary may also be a confining mountainside. Lateral boundaries of single alluvial fans can often be identified as a contact of distinct differences between light-colored, fresWy abraded, alluvial deposits and darker-colored, weathered deposits with wcll developcd soils on piedmont plains. Care should be taken to ensure that the contact is not simply a divide betwccn oldcr and more reccnt deposits of the alluvial fan. The lateral boundarics of alluvial fans that coalesce with adjacent alluvial fans are generally less distinct than thosc of single alluvial fans. These lateral boundaries may be marked by a topographic trough or ridge. It is sometimes possible to distinguish between surfaces of adjacent alluvial fans based on different source-basin rock types. Defining the lateral boundaries of coalescing fans will likely require additional fieldwork, use of surficial geologic and soils maps, and consultation with a geomorphologist or soil scientist. 4 STAGE 2: DEFlNING ACTIVE AND INACTIVE AREAS Stage I identified whether the landform in question is an alluvial fan. Stage 2 seeks to delineate areas of the alluvial fan that are active or are inactive in the deposition, erosion and unstable flow path flooding that builds alluvial fans. Stage 2 attempts to narrow the area of concern for Stage 3, which is thc specific identification of the extent of the I DO-year flood. Although active alluvial fan flooding has occurred on all parts of an alluvial fan at some time in the geologic past in order to construct the landform itself, this does not mean that all parts are equally