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<br /> <br />., <br /> <br /> <br /> <br />- 7 - <br /> <br />N <br />00 <br />--.J <br />CJ') <br /> <br />The Pacific Southwest is made up of land with all ,classes of ground cover. <br />The high mountain areas generally have the most vegetation, while many <br />areas', in the desert regions have practically none. The abundance of, vege- <br />tation is related in a large degree to precipitation. If vegetative ground <br />cover is destroyed in areas where precipitation is high, abnormally high <br />erosion rates may be experienced. <br /> <br />Differences in vegetative type have a variable effect on erosion and sedi- <br />ment yield, even though percentages of total ground cover may be th" same. <br />For instance, in areas of pinyon-juniper forest having the same percentage <br />of ground cover as an area' of grass, the absence of understory in some of <br />the pinyon-juniper stands would allow a higher erosion, rate than in the area <br />of grass. <br /> <br />Land tise <br /> <br />The use of land has a widely variable impact on sediment yield, depending <br />largely on the susceptibility of the soil and rock to erosion, the amount <br />of stress exerted by climatic factors and the type and intensity of ,use. <br />Factors other than the latter have been discussed in appropriate places in <br />this guide. <br /> <br />In almost all in$tances use either removes .or reduces the amount of natural <br />vegetative cover which reflects the varied relationships within the environ- <br />ment. Activities which remove all vegetation for parts of each year for <br />several years or permanently are cultivation, urban development and road <br />construction. Grazing, logging, mining, and fires artifically induce per- <br />manent or temporary reduction in cover density. <br /> <br />High erosion hazard sites, because of the geology, soils, climate, etc., are <br />also of high hazard from the standpoint, of type and intensity of llse. For <br />example, any use which reduces cover density on a steep slope with erodible <br />soils and severe climatic conditions will strongly affect sediment yield, <br />The extent of this effect will depend on the area and intensity of use <br />relative to the availability of sediment from other causes. Construction of <br />roads or urban development with numerous cut and fill"slopes through a large <br />area o'f widespread sheet or gully erosion will probably not CaUse a change <br />in sediment yield classification. Similar construction and continued dis- <br />turbance in an area of good vegetative response to a favorable climate can <br />raise yield by one or more classifications. <br /> <br />Use of the land has its greatest potential impact on sediment yield where a <br />delicate balance exists under natural conditions. Alluvial valleys of fine, <br />easily dispersed soils from shales and sandstones are highly vulnerable to <br />erosion where intensive grazing and trailing by livestock have occurred. <br />Valley trenching has developed in many of these valleys and provides a large <br />part of the sediment in high yield classes from these areas. <br /> <br />