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. . . 000159 CHAPTER II DESCRIPTION OF STUDY AREA River near Hite, Utah. Erosion caused by flood flows frequently cuts out diversion structures, deposits sediment in canals, ditches, and on devel- oped lands, or washes away from farm lands near the streams. Sediment deposition in reservoirs is a factor requiring special consideration in planning new water resource developments. Manis activ-Lties also influence erosion rates. Problems are created in places by excessive removal of vegetation and 'by wantoc, cutting of the land. On the other hand improved land management progr'ams and water stor- age projects reduce erosion. Activities such as these have reduced the sediment load of the San Juan River in recent yeaTS as discussed in con- nection ~li th the poter'.tial Mexican Hat Project in Chapt,er VIII. Climate The San Juan Area has a dry ~limate except in the high mountains. Summers are hot and dry with daily temperatures usually reaching highs in the 90' s and lows in the 50' sand 60' s. Maximum temperatures reach 100. almost every year and 105. in about half of the years. Winters are dry and cold but usually not severe. As a rule snowfall amounts to only a few inches during the winter, but freakish storms occasionally deposit much more. Heaviest rainfall occurs in the spring and fall. Storms from the Pacific Ocean move through the area in March and April while in August and September the area is occasionally subjected to thunderstorms asso- ciated with moist air masses moving in from the Gulf of Mexico. A map showing the location of climatological stations and isohyitals of annual precipitation appears on the following page. Water year precipitation (October through the following September) at Bluff, Utah, ranged from 3.17 inches to 11.73 inches in all years from 1912 through 1963 except in one year (::'941) when it shot up to 18.05 inches. The precipitation at Mexican Hat, Utah, for the 1946-63 period ranged from 2.72 to 9.62 inches. Evaporation rates are highest at the lower elevations which receive the least precipitation, Generally evaporation rates increase toward the southwestern part of the area. National forest lands in the La Sal and Abajo Mountains have greater precipitation and lower temperatures than other lands of the study area and provide correspondingly greater amounts of stream runoff. Native Plant and Animal Life The dominant plant growth includes Douglas fir, ponderosa pine, and mountain shrub on the Abajo and La Sal Mountains, pinon pine, juniper, 14