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<br />00G24 <br /> <br />Modelling of sand deposition in Colorado River 359 <br /> <br />. <br /> <br />eroded archaeological sites. They also describe a more pervasive process related to <br />the lowering of base level for side channels. One type of stream channel they <br />describe, their Type I, consists of short (300-400-m long), steep, ephemeral streams <br />that drain small near main-stem catchments during rainstorms. They concluded that <br />the erosion of sand deposi ts at the base of these streams since dam closure has <br />lowered the base level of the streams and led to the upstream migration of nick- <br />points. As side streams have deepened and widened, they have encroached upon <br />archaeological sites, and in some cases, the erosion has exposed and damaged the <br />sites. Hereford eta[. (1991, 1993) proposed that the periodic restoration of sand <br />deposits near river level would raise the base level for these side channels, promoting <br />refil1ing of the channels that would in turn help preserve the archaeological sites. <br />Thompson and Potochnik (2000), in their extensive study of reaches with abundant <br />artefacts and active gullies, concluded that renewed sand deposition could help preserve <br />some, but not all, sites. Observations soon after the 1996 test flow showed that under <br />the conditions at that time, the lerraces containing resources gained sand in some cases, <br />and no hanD to these sensitive terrace deposits was reported (Y callS, 1996). <br />Hereford eral. (\991, 1993) focused on water transport of sediment in gullies in <br />their suggestion that erosion of sand bars near the main-stem shorelines is linked to <br />erosion of archaeological sites. Evidence of local reworking of sand by wind, how- <br />ever, has been documented by several workers and suggests an alternative link <br />between the erosion of sand bar deposits along the channel sides and the formation <br />of artefact-damaging gullies. Aeolian deposits are widespread in many areas within <br />Grand Canyon (Schmidt and Graf, 199Q; Hereford eral., 1991, 1993,1998; Schmidt <br />and Leschin, 1995; Hereford, 1996). Thompson and Potochnik (2000) found that <br />half of the 199 catchments they studied in Marble Canyon, Furnace Flats, the Aisles <br />and Western Grand Canyon had some kind of aeolian deposition, and 42% had <br />active aeolian deposition. The possible significance of aeolian processes in the ero- <br />sion of streamside sand deposits has been noted in studies of the deposition and <br />longevity of streamside deposits (Howard and Dolan, 1981; Beus elat., 1985; <br />Hereford elat., 1993, 1996; US Department ofInterior, 1995; Yeatts, 1998; Schmidt, <br />1999). Thompson and Potocbnik (2000) suggested that wind may be a major mechan- <br />ism in restoring sand to gullies owing to the evidence of aeolian deposition at many of <br />their sites. They further suggested that aeolian reworking of newly deposited sand onto <br />higher lerraces would be significant as long as the supply of sand deposited by the river <br />is available and is not cut offfrom upper slopes by vegetation. Powell (1897), although <br />concerned more about survival than sediment transport on his pioneeringjoumeys in <br />Grand Canyon, noted in his diaries that fierce flames erupted from campfires as a <br />result ofbigh winds near river level. An implication of the observations of these studies <br />is that streamside sand deposits are an imponant source for windblown sand. <br />Although some of this windblown sand would be immediately lost to the river, some <br />would be redistri buted over the nearby slopes. <br />The effects of windblown sand would bave been made more significant in the pre- <br />dam era as a result of the lower winter river stages, which would expose larger <br />subaerial ponions of deposits, coincide with high winter winds and follow tbe prime <br />season for tributary contributions of bar.forming sand (Cluer, 1995; David J. <br />Topping, US Geological Survey, oral communication, 1999). The infrequent, <br />local. intense rainstorms associated with the initiation and development of gullying <br /> <br />. <br /> <br />. <br /> <br />.'.- <br />.-.: <br />~..j <br />~,' <br />':~~ <br />",. <br />....... <br />