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<br />1 <br /> <br />1 <br /> <br />11-2 <br /> <br />I <br /> <br />LEYDEN CREEK <br /> <br />GEOLOGIC SETTING <br /> <br />I <br /> <br />The upper Leyden Creek subbasin begins at the foothills just south of Rocky <br />Flats and extends west of State Highway 93. Elevations range from 7,500 feet <br />to 5,400 feet, The ground slope ranges from 18 percent in the upper portion <br />of the Leyden Creek subbasin to 5 percent in the lower areas, The lower <br />Leyden Creek subbasin is characterized by an increased density of farm build- <br />ings, as well as more intense urban development at the lower end. <br /> <br />Included in the Ralston-Leyden basin are geologic formations of widely <br />varying ages and I ithologies. The area is divided into two sections for <br />the purpose of describing the overall geology. <br /> <br />I <br /> <br />VAN BIBBER CREEK <br /> <br />The portion of the Ralston-Leyden basin to the east of the Rocky Mountains <br />is located on the western flank of the Denver Basin. This basin is a <br />large asymetrical structural downwarp just east of the front range, <br /> <br />1 <br /> <br />The upper Van Bibber Creek subbasins exhibit many of the mountainous character- <br />istics of subbasins of the upper Ralston Creek. Elevations range from 9,700 <br />feet to 5,300 feet and ground slopes vary from 40 percent down to 3 percent, <br />There is an extensive forest canopy, with a cover density of 60 percent in some <br />areas and rocky transitions from plateau areas to the valley. The lower <br />reaches of Van Bibber Creek range from rural-urbanized to highly urbanized. <br /> <br />The western area is that of the Rocky Mountains and associated rock <br />types. The area differs from the eastern area in structure and lithology. <br /> <br />I <br /> <br />1 <br /> <br />Individual subbasin characteristics for the lower (rural) Ralston Creek basin <br />are tabulated in Table 11-3 below. <br /> <br />I <br /> <br />TABLE 11-3 <br />LOWER (RURAL) BASIN PARAMETERS <br /> <br />I <br /> <br />r':: <br /> <br /> Over 1 and Overl and Streambed <br />Sub- Area Percent Flow Length Slope Slope <br />basin (Sq. Mi.) Impervious (Feet) Percent Percent <br />7 3.8 10 850 40 9 <br />8 4,3 10 1,000 35 5 <br />10 4.2 5 1,200" 18 7 <br />11 4.8 5 1,100 12 2 <br />12 2.4 10 1,100 18 15 <br />13 4,5 5 1,2001, 15 3 <br />14 4.5 8 900 5 <br />15 6,9 7 1,100 6 14 <br />16 2,8 5 1,000 5 <br />*Maximum overland flow length used is 1,200 feet; defined channel generally <br />occurs at greater lengths, <br /> <br />1 <br /> <br />I <br /> <br />1 <br /> <br />I <br /> <br />I <br /> <br />1 <br /> <br />1 <br />I <br /> <br />" <br /> <br />I <br />