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<br />M. Grimm et at. / Geomorphology 00 (1995) ()()()...O()() <br /> <br />3 <br /> <br />LJ <br /> <br />COLORADO <br /> <br />N <br />1 <br /> <br />Denver <br /> <br /> <br />105030' W <br />39.30' :; ..J <br /> <br />mountainrroat <br /> <br />. sediment. sample site <br />o 10 laD + cnst-stage gage <br />I I .. study site <br />Fig. 1. Location map for the Bear Creek basin, Colorado. Drainage area of Bear Creek is shaded. The towns of Morrison and Evergreen are <br />indicated by circles. <br /> <br />size distributions upstream and downstream from the <br />tributary junction because snowmelt occurs more uni- <br />fonnIy than intense rainfall-runoff floods and is <br />unlikely to produce tributary unit peak discharges sub. <br />stantially larger than those in the main channel. <br /> <br />2. Field area <br /> <br />Bear Creek originates at the Continental Divide west <br />of Denver, Colorado and flows 72 Ian to its confluence <br />with the South Platte River (Fig. I). The majority of <br />the 680 kIn' basin lies in the Southern Rocky Moun- <br />tains physiographic province, but the lower basin is in <br />the Colorado Piedmont (Diebold, 1939). Elevation <br />ranges from 4,348 m to 1,615 m. The mountainous <br />portion of the basin is underlain by a complex assort- <br />ment of Precambrian granites, schist, and gneiss <br />(Smith, 1964; Scott, 1972; Sheridan et al., 1972; Sher- <br />idan and Marsh, 1976), The mountain-piedmont bor. <br />der is marked by N-S trending hogbacks formed of <br />eastward-dipping sandstones, shales, and limestones of <br />Carboniferous to Tertiary age. <br /> <br />Many faults and shear zones complicate the bedrock <br />geology of the mountainous area, and stream courses <br />often coincide with mapped faults. The canyon section <br />of Bear Creek varies from a wide valley localized by <br />faulting to a steep-sided, narrow canyon cut into gran- <br />ites and pegmatite, Channel gradient, with ranges from <br />0.095 to 0.019. is related to bedrock resistance and <br />valley width. The creek has built longitudinally discon- <br />tinuous floodplains up to 150 m across in the wider <br />reaches, and these reaches are characterized by fine <br />sediment deposition overlying strath terraces. The <br />underlying lithology also affects coarse sediment avail- <br />ability. Granitic terrain generally yields larger and more <br />abundant boulders to erosive processes than does met- <br />amorphic terrain partly because the granite is more <br />massive and the metamorphic rocks tend to be strongly <br />foliated and closely jointed. Consequently, river <br />reaches in granitic terrain may contain more boulders <br />and be hydraulically rougher than reaches flowing <br />across metamorphic rocks. <br />The soils of the upper Bear Creek basin are shallow <br />and patchy, with low infiltration rates, rapid runoff <br /> <br />Journal: GEOMOR Article: 368 <br /> <br />--"'lIiIiIIo.._~_ <br />