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<br /> <br />- 8 - <br /> <br />A decline in vegetative density is not the only effect of livestock on <br />erosion and sediment yield. Studies at Badger Wash, Colorado, which is <br />underlain by Mancos shale, have indicated that sediment yield from un- <br />grazed watersheds is appreciably less than from those that are grazed. <br />This difference is attributed to absence of soil trampling in the ungrazed <br />areas since the density of vegetation has not noticeably changed since ex- <br />clusion began. <br /> <br />Areas in the arid and semi-arid portions of the Southwest that are surfaced <br />by desert pavement are much less sensitive to grazing and other use, since <br />the pavement affords a substitute for vegetative cover. <br /> <br />In certain instances the loss or deterioration of vegetative cover may have <br />little noticeable on-site impact but may increase off-site erosion by accel- <br />eration of runoff. This could be particularly evident below urbanized <br />areas where accelerated runoff from pavement and rooftops has increased the <br />stress on dot.nstream channels. Widespread destruction of cover by poor log- <br />ging practices or by brush and timber fires frequently increases channel <br />erosion as well as that on the directly affected watershed slopes. On the <br />other hand; cover disturbances under favorable conditions, such as a cool, <br />moist climate, frequently result in a healing of erosion sources within a <br />few years. <br /> <br />Upland Slope Erosion <br /> <br />This erosion form occurs on sloping watershed lands beyond the confines of <br />valleys. Sheet erosion which involves the removal of a thin layer of soil <br />over an extensive area is usually not visible to the eye. This erosion form <br />is evidenced by the formation of rills. Experience indicates that soil loss <br />from rill erosion can be seen if it amounts to about 5 tons or more per acre. <br />This is equivalent in volume per square mile to approximately 2 acre-feet. <br /> <br />Wind erosion from upland slopes and the deposition of the eroded material in <br />stream channels may be a significant factor. The material so deposited in <br />channels is readily moved by subsequent runoff. <br /> <br />Downslope soil movement due to creep can be an important factor in sediment <br />yield on steep slopes underlain by unstable geologic formations. <br /> <br />Significent gully erosion as a sediment contributor is evidenced by the pre- <br />sence of numerous raw cuts along the hill slopes. Deep soils on moderately <br />steep to steep slopes usually provide an environment for gully development. <br /> <br />Processes of slope erosion must <br />contribute to its development. <br />tions. <br /> <br />be considered in the light of factors which <br />These have been discussed in previous sec- <br /> <br />