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<br /> Particle Size (mm) <br /> 2 0.5 0.25 0.125 0.053 <br /> 100 <br /> A <br /> 80 <br /> 60 <br /> 40 <br /> 20 - ~ - - - Streamflow deposits <br />c (n = 89) <br />.. <br />~ - Minimum/maximum <br />~ 0 <br />u: <br />E 100 <br />I'l B <br />;r X <br /> 80 x <br /> x <br /> 60 <br /> x <br /> 40 -ReachA(n=6) <br /> - - -Reach B (n = 71) <br /> -- - -- Reach C (n = 6) <br /> 20 _Reach D (n = 3) <br /> . x. . Reach F (n = 3) <br /> 0 <br /> -1 0 2 3 4 <br /> Particle Size (op) <br />Figure 10. Particle-size distribution of sand delivered by streamflow from <br />ungaged tributaries in Grand Canyon. A. All streamflow deposit samples. <br />B. Reach-averaged distributions. <br /> <br /> <br />delivery in Glen and Marble Canyons (Reaches A <br />and B) are 0.032'106 and 0.305'106 Mg/yr <br />respectively. <br /> <br />The outcome of interaction between mainstem <br />flows and tributary sediment input depends in par1 <br />on the par1icle-size distribution of the sediment, <br />with finer sand being more readily mobilized by a <br />given discharge than coarser sand. Although <br />tributary streamflow deposits may overrepresent <br />total sand content, they can give an accurate picture <br />of the composition of the sand fraction itself. <br />Topping (1997) has found that the distribution of <br />the sand fraction from this type of deposit is very <br />similar to that of sand measured in suspension <br />during flood peaks on the Paria River. <br /> <br />When examined by reach, sand contributed by <br />tributaries in Glen Canyon is notably coarser (050 = <br />0.24 mm) than sand from other reaches (050 = 0.11 <br />- 0.20 mm), including the Marble Canyon reach <br /> <br /> <br /> <br />(050 = 0.20 mm; table 13; fig. 10). The coarse sand <br />in Glen Canyon derives from the Navajo Sandstone <br />which strongly dominates the bedrock geology of <br />the reach. <br /> <br />FREQUENCY OF HISTORICAL <br />DEBRIS FLOWS IN GRAND CANYON <br /> <br />Evidence of Historical Debris Flows <br /> <br />Dffectobservaffons <br /> <br />We compiled notes from river runners on when <br />debris flows, rockfalls, or significant streamflow <br />floods occurred in Grand Canyon from 1984 <br />through 1998. In addition, we examined all diaries <br />of historical river trips for any reports of floods or <br /> <br />FREQUENCY OF HISTORICAL DEBRIS FLOWS IN GRAND CANYON 21 <br />