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<br />in the arroyos depend both on local <br />&ischarges from upstream alluvial fans. <br /> <br />drainage and <br /> <br />On alluvial fans, the location of the stream channel is <br />often erratic, due to the rapid expansion of the width of <br />the fan and the highly v~tiable sediment load and flow rate <br />of the discharges leaving the mountain watershed. Cutting <br />of a channel near the apeH may occur during one flood event. <br />to be followed by channel backfilling in subsequent events. <br />Conversely, a flood pathway may remain stable over several <br /> <br />events. causing aggradation of that part of the fan. <br />Eventually, the establ ished channel will be filled by <br />sediment and the flow will move to a new, lower elevation <br />area in a process called an nvulsion. Through multiple <br /> <br />avulsions over geologic time, the fan aggrades uniformly so <br />that it tends to exhibit the concentric, semi-circular <br />contour 1 ines shown in figure 3.1. <br /> <br />.' <br /> <br />If the sediment supply from the upstream watershed to a fan <br />is reduced due to changes in rainfall patterns or increased <br />vegetation, incision of a channel wi,ll begin at the apex. <br />If the change in sediment supply is permanent, a stable <br />channel or entrenchm~nt of the fan surface ~esults. Normal <br />depositional patterns on the fan are altered by the <br />entrenchment such that little deposition occurs near the <br />apex and fan building commences at the downslope point where <br />the entrenchment ends. A new. secondary fan is established <br />with its apex at the end of the entrenchment and grows in <br />the same pattern as the original fan. <br /> <br />Extensive variation in alluvial fan morphology can be <br />observed both between fans in different geographic and <br />climatic areas and between adjacent fans in one valley. <br />These variations make generalizations about fan structure as <br />well as hydraulic behavior difficult. Geomorphologists <br />often compare fan morphologies based on the relative age of. <br /> <br />27 <br /> <br />on <br /> <br />