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<br />. <br /> <br />Forest-Fire Devegetation and Basin Adjustments <br /> <br />I <br /> <br />Table 5 <br /> <br />Comparison o~ Sediment Yield and Runoff from Instrumented <br />Watersheds and Selected Geomorphic Parameters of <br /> <br /> the Drainage Basins in Burnt Mesa Area <br /> Relief- Runoff Sediment <br /> Area Relief Length Drainage (10-5m3/ Yield <br />Drainage Basin (m2) (m) Ratio Density see/m2) (m3/yr) <br />Intense-burn 4116.5 11. 86 0.09 high 6.2 19.4 <br />Moderate-burn 813.8 8.65 0.13 medium 3.8 0.8 <br />Light-burn 1427.0 5.57 0.08 low 0.1 4.5 <br /> <br />erosional processes and the supply of sediment to fluvial systems. For ex- <br />ample, lower erosion rates of the moderate-burn basins correlate with the ex'" <br />tensive needlecast and pine canopy which reduce raindrop impact and hi11s1ope <br />runoff. Also, higher-than-expected erosion rates occur in the light-burn <br />watershed in response to sediment production by burrowing animals. <br /> <br />Another difference between fluvial adjustments in undisturbed drainage <br />basins and devegetated watersheds is illustrated by the relationship between <br />sediment yield and drainage basin size. Sediment yield per unit area de- <br />creases with increasing drainage basin size for undisturbed basins in the <br />southwestern United States !Strand, 1975). This relationship is quantitatively <br />express~d as Q = 2.4 A-a. 29 where Qs is sediment load in AFlmi2jyr, and <br />A is drainage b-ea in mi2. Larger drainage basins are capable of storing <br />more sediment than smaller drainage basins (Schumm, 1977). In larger drai- <br />nage basins, sediment is stored on hillslopes and in valleys for longer time <br />periods.. For example, given a single event, a higher magnitude of event <br />would be necessary to carry the material out of a large basin, relative to a <br />small basin. However, for the devegetated watersheds of Burnt Mesa sedi- <br />ment yield increases with increasing drainage basin size (table 5). In these <br />disturbed watersheds. vegetation reduction decreases the sediment-storage <br />potential of larger drainage basins and ulcreases the sediment removal effi- <br />cie;ncy. Net channel indsion, w:hich is common to the streams of the intense- <br />. burn watersheds (fig. 9) is another indication of decreased sediment-storage <br />potential. Sediment sto~age provided by hillslope and channel vegetation is <br />important "in these small, mountainous watersheds, as demonstrated by the <br />reappearance of summer vegetation in 1978 and the subsequent aggradation <br />of higher-order channels. <br /> <br />CONCLUSIONS <br /> <br />Devegetated mountainous watersheds have relationships between drainage <br />processes and form which differ from those for undisturbed drainage basins <br />