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<br />draft 3120/98 ' <br /> <br />allowed observation of sediment transport and deposition during peak-flow conditions. For example, <br /> <br /> <br />750-mm diameter boulders were observed at peak stage being deposited, primarily as mid-channel <br /> <br /> <br />bars. These clasts came to rest with their tops just protruding above the projected HWMs; nearby <br /> <br /> <br />sand deposited in channel-margin areas equalled HWMs. <br /> <br />An example demonstrating where the top of boulder bars protrudes above HWMs (about 2.7 m) is <br /> <br /> <br />for the 1982 dam-failure flood in the Roaring River (3 sites), Rocky Mountain National Park, <br /> <br /> <br />Colorado (Jarrett and Costa, 1986). In reaches having channel slopes from about 0.07 to 0.35 m1m, <br /> <br /> <br />sediments up to 8 m in diameter were mobilized and subsequently deposited where channel slope was <br /> <br /> <br />less than about 0.07 m1m (Jarrett and Costa, 1986). It is likely that such large boulders are mobilized <br /> <br /> <br />by flow around the base of boulders, the force of flowing water, and momentum transfer when struck <br /> <br /> <br />by boulders already in transport. Boulder transport continues until flow depth or discharge decreases <br /> <br /> <br />and sediments are deposited. Thus, for large bouldery, flood deposits located mid-channel and <br /> <br /> <br />downstream from reaches with large gradient (- 0.04 m/m) reaches, then the paleoflood depth (PSI) <br /> <br /> <br />needs to be lower than the tops of the boulders for discharge reconstruction. <br /> <br />At four sites, the top of the flood deposits (PSIs) were as much as 0.5 m (-15 percent of flood depth) <br /> <br /> <br />lower than HWMs (fig. 4); also noted at two sites in House et al. (1995) and Springer and Kite <br /> <br /> <br />(1997). These sites were located in relatively straight reaches confined by bedrock channels with <br /> <br /> <br />very steep granitic valley walls in streams in the Sierra Nevada Mountains, California. Sediment <br /> <br /> <br />supply generally is relatively small from decomposing granites (compared to softer rocks) or alluvial <br /> <br /> <br />channels. Perhaps the relatively small sediment supply combined with very large rainfall amounts <br /> <br /> <br />and frequent flooding in these basins, which tend to flush most sediments through these uniform <br /> <br /> <br />reaches, may explain the lower PSIs. Rainfall also removed thinner fine-grained deposits very soon <br /> <br /> <br />after the floods. <br /> <br />The important conclusion for the sites we visited is that there were sites nearby where new flood <br /> <br />deposits (top of PSIs) from floods that were within +/-0.06 m of adjacent HWMs, but locating these <br /> <br />8 <br />