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<br />Grams and Schmidt 14 <br /> <br />canyon reaches have an average debris-fan frequency of 3.1 fans/km compared to 0.4 <br />fans/km in non-canyon reaches. Fan frequency is similar in all the canyon reaches, but is <br />highest in Whirlpool Canyon. However, the fans in Whirlpool Canyon are on average <br />about one-half the size of fans in Lodore Canyon and Split Mountain Canyon. <br />There are a total of 10 1 mapped deposits in Lodore Canyon that are gravel or a <br />combination of fine sediment and gravel (Table 3). The rest of the canyon reaches contain <br />an additional 91 gravel deposits. The proportion of the total area of all alluvium in the <br />canyons that is gravel varies from 38 percent in Lodore Canyon to 73 percent in Split <br />Mountain Canyon. In the non-canyon reaches, about 35 percent of the area of all <br />alluvium consists either completely or partially of gravel. Most of the exposed gravel in <br />all reaches occurs as mid-channel and expansion gravel bars. <br />The downstream variation of bed material sizes within the study area was <br />compared with estimates of reach average boundary shear stress. Shear stress was <br />estimated from the product of the specific weight of water 'Y , the hydraulic radius R and <br />the water-surface slope S, <br /> <br />'t = 'Y RS. <br />Even in the relatively deep and narrow channel in the canyon reaches R is well <br />approximated by mean depth (cross-sectional area / channel top width). The shear stress <br />was calculated for the stage that inundates the most prominent geomorphic surface in the <br />canyon and meandering reaches. This is a broad terrace with mature cottonwood trees in <br />the meandering reaches and a narrow but distinct terrace with boxelder trees in the <br />canyons. Figure 7 is a typical cross section in Lodore Canyon that shows this prominent <br />terrace as well as the post-dam bankfull stage and corresponding floodplain. This <br />prominent terrace was chosen because the post-dam bankfull-flow surface is much lower <br />and not representative of the unregulated flow regime to which the gravel bars are <br />presumably adjusted. Photographs taken during extreme events indicate that the <br />prominent terrace is inundate.4~by flows of about a 10-yr recurrence interval in the pre- <br />