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<br />Sheet erosion. Areas of sheet erosion are common on gentle to moder- <br /> <br />ate slopes throughout the Big Thompson Canyon. As indicated by behavior <br /> <br />during the July 31 - August 1 rainstorms and flooding, sheet erosion occurs <br /> <br />most commonly where rainfall is so intense that the infiltration capability <br /> <br />of surficial materials is exceeded. On granitic terrain subjected to great <br /> <br />rainfall intensity during the flood (6 - 7 cm/hr), where protection from <br /> <br />erosion by soil cover was minimal, fluvial transport of grus resulted in <br /> <br />removal of as much as 25 cm of regolith. Due to better developed soils and <br /> <br />despite lower permeability, surficial materials derived from metamorphic <br /> <br />rocks appear to be somewhat less susceptible to sheet erosion than those de- <br /> <br />rived from granitic rocks. On steeper slopes sheet erosion is replaced by <br /> <br />rillwash and gullying. <br />Erosional downcutting of drainage ways. With increasing slope of well- <br /> <br />defined channels and side slopes, water velocity and volume in these channels <br /> <br />increases greatly, resulting in downcutting and removal of smaller caliber <br /> <br />lag material that accumulates in the channel during times of normal seasonal <br /> <br />runoff. In the higher parts of most tributary drainages to the Big Thompson <br /> <br />River, headward downcutting of channels through a riffle-and-pool sequence <br /> <br />appears to be the dominant mode of channel erosion. In these areas, eroded <br /> <br />material is fine (pebble size and smaller) and the microrelief of riffles and <br /> <br />pools is typically no more than 2 m. In the intermediate and lower parts of <br /> <br />these same drainage basins, stream gradients typically increase to 200 - 400 <br /> <br />m/km and stream competence is sufficient to move all but the largest boulders. <br /> <br />Near the confluence with the Big Thompson River or other trunk streams, <br /> <br />channel gradients decrease somewhat, but in many cases sufficient velocity <br /> <br />and volume of floodwaters were present during the recent heavy rainstorms to <br /> <br />carry and remove nearly all accumulated bedload, leaving a scoured bedrock <br /> <br />surface. <br /> <br />30 <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />