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<br />3.4 <br /> <br />Cascade Creek drains the area sometimes referred to as the "American <br /> <br />Flats". In contrast to the unconsolidated glacial sediments filling the lower <br /> <br /> <br />part of the Amphitheater, Cascade Creek cuts through the near vertical cliffs <br /> <br /> <br />of older sandstone and limestones. For example, Cascade Falls is formed on <br /> <br /> <br />the Hermosa Formation, a thick, gritty red sandstone with intercalated conglo- <br /> <br />merate and limestone. Rockfall material in this catchment is produced from <br /> <br /> <br />freeze thaw action of water in jointed and bedded rocks and seepage of ground- <br /> <br /> <br />water along bedding planes. <br /> <br />Both creeks deposit their water and sediment loads onto overlapping <br /> <br /> <br />debris fans, with the Portland Creek fan being somewhat larger in area. <br /> <br /> <br />Mudflows carrying debris follow the local fall-line but can be easily plugged <br /> <br /> <br />and deflected to any part of the fan (discussion below). <br /> <br /> <br />Differences in the sediment supply and morphology of the upper watersheds <br /> <br /> <br />are reflected in differences in channel characteristics of the two creeks. <br /> <br /> <br />These characteristics, in turn, are reflected in the movement and frequency of <br /> <br /> <br />reoccurring debris flow events. Portland Creek is confined within vertical <br /> <br /> <br />walls )0 to 30 feet high until it diverges at the head of the debris fan at an <br /> <br /> <br />elevation of about 7,930 feet. The creek carries well graded bedload ranging <br /> <br /> <br />from boulders several feet in diameter to cobbles, gravel, sand, (particularly <br /> <br />very fine sand), silt and clay derived from weathered volcanics and weathered <br /> <br /> <br />glacial material mantling the watershed. Likewise, the large volume of uncon- <br /> <br /> <br />solidated glacial and volcanic debris is essentially accumulating in the upper <br /> <br /> <br />watershed until a significant precipitation event flushes the material down- <br /> <br /> <br />slope. The avalanche chutes and small gullies and tributaries of Portland <br /> <br /> <br />Creek collect debris, including logs and boulders, and may eventually empty it <br /> <br /> <br />into Portland Creek and deposit it onto the associated debris fan. <br /> <br /> <br />The material in Cascade Creek is predominantly angular blocks of <br /> <br /> <br />sandstone, limestone and some intrusives. A smaller fraction of fine <br /> <br /> <br />material, volcanics, and rounded glacial cobbles was observed. The angular <br /> <br /> <br />cobbles and boulders were observed to accumulate in a broad channel at the <br /> <br /> <br />base of Cascade Falls (about 8,075 feet) compared to the narrow channel of <br /> <br /> <br />Portland Creek. Debris flows carrying this angular material do not flow as <br /> <br /> <br />easily as flows carrying more fines, and tend to plug up within the channel <br /> <br /> <br />thereby diverting to other parts of the debris fan. This could possibly <br /> <br /> <br />explain the difference in the fan slopes for the two creeks. Portland Creek <br /> <br />r <br /> <br />, <br />