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003168 . CHAPTER II DESCRIPTION OF STUDY AREA boundary. The Navajo Mountain, at an elevation of 10,388 feet and stand- ing about 5,000 feet above the surrounding plateau, is of special interest because it is so young that its igneous rocks are still entirely covered with sedimentary material. Elevations within the study'area range from 13,089 feet on top of Mt. Peale in the La Sal Mountains to 3,160 feet near the point now inun- dated by Lake Powell, where the Colorado River crosses into Arizona. The Colorado River, forming the northwest boundary of most of the study area, flo~s in a deep, rugged canyon with many beautiful entrenched meanders receiving tributaries in similar canyons. The San Juan, Dolores, and Green Rivers flowing on or near other boundaries of the area are also deeply entrenched. The headwaters of these rivers are in mountains lo- cated great distances north and east of the study area. Streams originat- ing within the area are SITsll and most of them flow only intermittently. Soil Erosion . Soil erosion caused by both wind and water is a serious problem in many parts of the study area. Both causes of erosion have had a pronounced effect over long periods of time in shaping the topography. The aeolian soils of the Monticello Area extending many miles eastward from the base of the Abajo Mountains have been carried there from wind-cal~ed locations such as Monument Valley. While the rapid erosion rate that characterizes this region is troublesome to modern residents and planners of resource developments, it is largely the erosive forces operating over the cen- turies that have carved out the unique and scenic landscape. Wind erosion occurs particularly in areas of sparse vegetation and low precipitation. It is most active in the south and west parts of San Juan County in (1) the vicinity of Bluff and Mexican Hat and extending south of the San Juan River, (2) an area extending northeast from the junction of the San Juan and Colorado Rivers, (3) an area immediately east of the Colorado River and adjacent to Utah Highway 95, and (4) Lockhart Basin east of Canyonlands National Park. . Water erosion is prevalent throughout the study area. It is gener- ally caused by spring runoff and by torrential but brief rainstorms that commonly occur during the late summer and early fall. Great volumes of soil are carried a~ay by water. Illustrative of this is the fact that the sediment load carried by the San Juan River at Bluff averaged about 46 million tons annually for the 1930-48 period. This is equivalent to an average of 1.46 acre-feet of sediment annually from each square mile of the river's drainage area above Bluff. Of course, only a small part of the drainage area above Bluff is in the study area. About the same ero- sion rate per square mile, however, is estimated for White Canyon, an in- termittent stream entirely within the study area that enters the Colorado 13