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<br />002105 <br /> <br />STUDY-AREA DESCRIPTION <br /> <br />Geography <br /> <br />Physiography and Drainage <br /> <br />The lower Gunnison River basin is the drainage area of the Gunnison River <br />downstream from the junction with the North Fork Gunnison River as shown in <br />figure 1, This basin is mainly within the Canyon Lands section; a small <br />section is in the Uinta Basin section of the Colorado Plateaus physiographic <br />province (Fenneman, 1946). <br /> <br />The highest point in the basin is 11,159 ft, in sec. 24, T. 11 S., <br />R. 94 W. The lowest point in the basin is about 4,550 ft, at the confluence <br />of the Colorado and the Gunnison Rivers at Grand Junction. Tributary streams <br />in the southwestern part of the basin generally flow perpendicular to the axis <br />of the Uncompahgre Plateau uplift (see pl. 1), until reaching more level <br />terrain below. Streams draining southern Grand Mesa have a more dendritic <br />pattern. Total drainage of the lower Gunnison River basin is 1,630 mi2. <br /> <br />Climate <br /> <br />Climate and vegetation vary within the basin and are very dependent on <br />elevation. Average annual precipitation ranges from less than 8 in. in the <br />lower valley west of Delta to 40 in. along the Grand Mesa divide. The <br />Uncompahgre Plateau (labeled Uncompahgre uplift on plate 2) receives as much <br />as 25 in. annually. Temperature extremes range from more than 950F near Delta <br />in July to less than -300F in the mountains in January. Climatological data <br />from four stations in or near the basin are summarized in table 1. <br /> <br />Geology <br /> <br />Two major structural features are within the lower Gunnison River basin-- <br />the Uncompahgre uplift west of the Gunnison River and the north-dipping <br />southwestern flank of the Piceance structural basin both shown on plate 2. <br />The Uncompahgre uplift is an asymmetrical anticline plunging northwest and <br />southeast with the axis nearly coinciding with the western boundary of the <br />study area. The average dip for the Uncompahgre anticline slope is 2.50 <br />northeast, ranging from 20 to 40 (see fig. 3, a general geologic section). <br />Nearly parallel drainage from the Uncompahgre Plateau trends northeast and <br />reflects the anticlinal dip and other displacements, such as large-scale <br />joints and minor faults. The southwestern flank of the Piceance structural <br />basin plunges north with an average dip of 20. The axis of the Montrose syn- <br />cline, within the Piceance structural basin, plunges north about 8 mi west of <br />Cedaredge (Williams, 1964). <br /> <br />5 <br />