Laserfiche WebLink
<br />62 <br /> <br />ALLUVIAL FAN FLOODING <br /> <br />A change in the rate of tectonic uplift along a mountain front also can result in <br />abandonment of parts of alluvial fans. For example, a decrease in the rate of uplift at a mountain <br />front relative to the fan could result in stream-channel downcutting at the mountain front/fan apex <br />over a period of time. As a consequence, the upper part of the fan would become entrenched and <br />the area of active alluvial fan deposition would shift downfan3 (Figure 3-4). The opposite also can <br />happen. In this case, an increase in the rate of uplift can result in rapid deposition at the fan head <br />and development of a young untrenched fan segment overlying the older fan surfaces. <br />It is clear from the examples of segmented fans that only certain parts of the fan, or <br />segments, are active at anyone time. In the entrenched fan, the di stal segment downstream of the <br />hydrographic apex is typically the active part of the fan. In the untrenched fan, the segment of the <br />fan proximal to the source area, at the topographic break, is typically the active part of the fan. <br />These examples, however, are simplistic in that few fans have only one active segment that is <br />clearly distinguished from an older, inactive segment. More typically, fans must be mapped to <br />identity surfaces of different ages, from youngest to oldest. <br />To determine what parts of a fan are active and inactive, the investigator must examine the <br />whole fan using indicators of activity as described here. <br /> <br />Defining Active <br /> <br />Because it is not possible to predict with zero uncertainty where the next flood will occur <br />on an alluvial fan, we resort to the traditional method of the geologist and rely on the dictum "the <br />past, as preserved in the geologic record, is key to understanding the present and to predicting the <br />future." Using this reasoning, the geologist concludes that the area of deposition on an alluvial fan <br />shifts with time, but the next episode of flooding is more likely to occur where the most recent <br />deposits have been laid down than where deposits of greatest antiquity occur. <br />Once the planner has incorporated this basic philosophy into efforts to identifY those parts <br />of the fan that are active, the next step is to decide what time period will be used to define active. <br />As a conservative standard, in some areas it is easy to separate the parts of a fan that formed more <br />than 10,000 years ago from those parts that formed during the past 10,000 years, the Holocene <br />epoch. In many places in the southwestern United States, it is less easy to see a clear <br />PleistocenelHolocene change, but one can subdivide Holocene deposits on a much finer timescale. <br />In areas with long records of flooding and aerial photographs that date back to about 60 years, it <br />is possible to identity areas of historic flooding within the past 100 years. <br />The term active refers to that portion of a fan where flooding, deposition, and erosion are <br />possible. If flooding and deposition have occurred on a part of a fan in the past 100 years, clearly <br />that part of the fan is active. If flooding and deposition have occurred in the past 1,000 years, that <br />part of the fan can be considered to be active. However, it becomes more difficult to determine <br />whether or not a part of the fan that has not experienced sedimentation for (say) more <br /> <br />3 Note thai the Cedar Creek fan is similar 10 the entrenched fan shown in Figure 3-4, except that its loe is bounded <br />by a river that transports sediment away from the distal part of the fan. As a result a young fan segmenl is not <br />forming or preserved at Ihe fan loe. <br />