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64 ALLUVIAL FAN FLOODING of the latter, storms in southern Arizona have changed in this century from moderate-sized summer events with sources in the south to much larger fall and winter events coming from the west-southwest (pacific Ocean). Some of this change in storm conditions is attributed to the increasing frequency of El Niiio climatic events over the past few decades. In such cases, it probably is prudent to define active as more than just those parts of the fan that have been the sites of flooding and deposition in the past 100 or 1,000 years, Identifying Areas of Flooding and Deposition on the Active Part of an Alluvial Fan It is important to idel/tify both active and inactive pariS of the fan, because this provides a map of where flooding cal/ occur as well as where it probably wi/lnot occur. Preparing a Geomorphic Map of Different Age Fan Surfaces Once a time period is chosen to represent the active part of a fan for the purpose of flood hazard assessment, the flood evaluator must determine which deposits are less than the chosen age. A simple place to start is to examine the historical record of flooding and sedimentation. Aerial photographs from different years can be compared to identity sites of deposition that are less than about 60 years in age. If humans have lived in the area, historical deposits often contain relicts of human activity, such as pieces of machinery, bottle caps, lumber, and scraps of metal. These deposits can be examined and described to gain a good understanding of the nature of fresh deposits for that alluvial fan. The flood evaluator can map different deposits, placing them in relative chronological order from youngest to oldest, as mapping of the entire fan progresses. On a surface of essentially continuous deposition, gradational relations are the rule. The product of this part of the investigation should be a basic geomorphic map of the entire fan, with particular emphasis on the active parts of the fan. An example of such a geomorphic map is shown for the Cedar Creek fan (Figure 3-2b). This map divides alluvial fan surfaces into different age categories, from as old as middle Pleistocene to as young as late Holocene. Soil profiles were described at different sites, and weathering characteristics such as those described below were used to assess the relative age of each surface. In this example, only a small part of the total fan can be considered to be active. Morphologic and Weathering Criteria Used to Prepare a Geomorphic Map A variety of properties can be used to separate deposits of different ages. These include features such as fan surface morphology and sediment weathering characteristics. The surface of a recent deposit typically is irregular, whereas older deposits generally are smoother. Fresh stream-laid deposits commonly have bar and swale topography, whereas fresh debris flow deposits might have sharply defined levees along lateral margins. With time, loose, unconsolidated flood deposits weather, and some fine-grained material is added to the deposit from eolian influx. As deposits weather, clast size becomes smaller, and edges of deposits from individual flood episodes become more subdued. The net result is that the morphology of older surfaces becomes increasingly more subtle as micro- and macro-relief features are worn down. An