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<br />Forest-Fire Devegetation and Basin Adjustments <br /> <br />1978. During the summer, vertical accretion on stable channel bars caused <br />net deposition in the higher-order channels (fig. 9). Filling of these chan- <br />nels occurred in response to revegetation along the margins of these streams <br />(fig. 11). Tall_ grasses which grew on stable bars trapped fine sediments <br />during summer runoff events. After summer channel fillin.g, channel scouring <br />dominated through the fall, 1978. Dormancy of channel margin vegetation <br />during the fall resulted in increased erodibility of the stream banks. Further- <br />more, because stream transport power is typically'lower during fall and spring <br />runoff events, commonly only suspended sediments are transported. <br /> <br />Process-Response Model in Devegetated Watersheds <br /> <br />The ephemeral fluvial systems in mountainous terrain show complex respon- <br />ses to forest-fire devegetation. These complexities are related to variations <br />in amounts and source areas of sediments. Sediment delivered from mesa-top <br />fluvial systems to the trunk drainage, Rita de 10s Frijoles. caused channel <br />geometry modifications. Alterations in the Rita de los Frijoles geomorphology <br />cannot be transmitted back to the mesa-top tributaries because of the bed- <br />rock nickp6ints at the canyon walls. Only tributary streams topographically <br />lower than the bedrock nickpoints (within the canyon walls) can react to <br />changes induced by Rita de los Frijoles. <br /> <br />I <br />I <br />, <br />) <br /> <br />A process-response model for fluvial systems in bedrock terrain and in <br />forest-fire devegetated areas is given in figure 12. The model shows the im- <br />portance of seasonal variations in sediment delivery from the mesa-top tribu- <br />taries. In figure 12 geomorphic responses and sediment transport are illus- <br />trated by the arrows. and the volume of sediment storage on hillslopes, in <br />channels, and in valleys is indicated by the r,e1ative size of the boxes. The <br />model (1) illustrates the continuous decrease in sediment supply from the <br />mesa-top systems to the Frijoles canyon, (2) illustrates the continued response <br />of fluvial systems in the canyon to devegetation after geomorphic stability is <br />attained on the mesa tops, and (3) emphasizes that tributaries in a fluvial <br />systemn.may adjust dependently or independently of the master drainage line. <br />Complex responses (Schumm, 1977) are typical in streams with alluvial valleys. <br />wherea~' in mountainous terrain the bedrock nickpoints act as barriers to <br />interactions between cODJponents of the fluvial systems and reduce adjustments <br />between tributaries and trunk streams. <br /> <br />l <br />, <br />i <br /> <br />~I <br /> <br />DRAINAGE BASIN ADJUSTMENTS <br /> <br />I <br />; <br />! <br /> <br />Altered runoff and sediment yield illustrate drainage basin adjustments to <br />varying degrees of devegetatio.n. These two geomorphic parameters,_ have been <br />. measured in numerous southwestern drainage basins and have been 'compared <br />to the drainage basin geomorphology (Hadley and Sch\Unm, 1961: Strand, 1975: <br />Schumm. 1977). Such comparisons show high correlations between these <br />drainage basin processes and morphologic elements. However, these drainage <br />basins represent undisturbed conditions, and in devegetated watersheds fac- <br />tors other than basin morphology influence runoff and sediment yield. <br /> <br />Runoff <br /> <br />Water discharge in the instrumented watersheds of Burnt Mesa was mea- <br />sured for single, peak runoff events. Runoff during the summer of 1978 <br /> <br />,,, <br />