<br />00G24
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<br />Modelling of sand deposition in Colorado River 359
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<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
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