Application of sedimentary-structure interpretation to geoarchaeological investigations in the Colorado River Corridor, Grand Canyon, Arizona, USA

Geomorphology 101 (2008) 497–509

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Application of sedimentary-structure interpretation to geoarchaeological investigations in the Colorado River Corridor, Grand Canyon, Arizona, USA Amy E. Draut a,⁎, David M. Rubin a, Jennifer L. Dierker b, Helen C. Fairley c, Ronald E. Griffiths c, Joseph E. Hazel Jr. d, Ralph E. Hunter a, Keith Kohl c, Lisa M. Leap b, Fred L. Nials e, David J. Topping c, Michael Yeatts f a

U.S. Geological Survey, 400 Natural Bridges Drive, Santa Cruz, CA 95060, United States National Park Service, 823 San Francisco St., Suite B., Flagstaff, AZ 86001, United States c U.S. Geological Survey, Grand Canyon Monitoring and Research Center, 2255 N. Gemini Drive, Flagstaff, AZ 86001, United States d Department of Geology, Northern Arizona University, Flagstaff, AZ 86011, United States e GeoArch, 10450 W. 8th Place, Lakewood, CO 80215, United States f Hopi Cultural Preservation Office, Northern Arizona University Department of Anthropology, Flagstaff, AZ 86011, United States b

A R T I C L E

I N F O

Article history: Accepted 1 April 2007 Available online 14 March 2008 Keywords: Sedimentary structures Flood deposits Fluvial processes Geoarchaeology Grand Canyon Glen Canyon Dam

A B S T R A C T We present a detailed geoarchaeological study of landscape processes that affected prehistoric formation and modern preservation of archaeological sites in three areas of the Colorado River corridor in Grand Canyon, Arizona, USA. The methods used in this case study can be applied to any locality containing unaltered, nonpedogenic sediments and, thus, are particularly relevant to geoarchaeology in arid regions. Resolving the interaction of fluvial, aeolian, and local runoff processes in an arid-land river corridor is important because the archaeological record in arid lands tends to be concentrated along river corridors. This study uses sedimentary structures and particle-size distributions to interpret landscape processes; these methods are commonplace in sedimentology but prove also to be valuable, though less utilized, in geoarchaeology and geomorphology. In this bedrock canyon, the proportion of fluvial sediment generally decreases with distance away from the river as aeolian, slope-wash, colluvial, and debris-flow sediments become more dominant. We describe a new facies consisting of ‘flood couplets’ that include a lower, fine-grained fluvial component and an upper, coarser, unit that reflects subaerial reworking at the land surface between flood events. Grain-size distributions of strata that lack original sedimentary structures are useful within this river corridor to distinguish aeolian deposits from finer-grained fluvial deposits that pre-date the influence of the upstream Glen Canyon Dam on the Colorado River. Identification of past geomorphic settings is critical for understanding the history and preservation of archaeologically significant areas, and for determining the sensitivity of archaeological sites to dam operations. Most archaeological sites in the areas studied were formed on fluvial deposits, with aeolian deposition acting as an important preservation agent during the past millennium. Therefore, the absence of sediment-rich floods in this regulated river, which formerly deposited large fluvial sandbars from which aeolian sediment was derived, has substantially altered processes by which the prehistoric, inhabited landscape formed, and has also reduced the preservation potential of many significant cultural sites. Published by Elsevier B.V.

1. Introduction 1.1. Effects of Glen Canyon Dam on the Colorado River Corridor, Grand Canyon The Colorado River corridor through Grand Canyon, Arizona, contains nearly 500 archaeological sites that collectively record several thousand years of prehistoric human occupation. Archaeological research and monitoring in Grand Canyon National Park focus in⁎ Corresponding author. Tel.: +1 831 427 4733; fax: +1 831 427 4748. E-mail address: [email protected] (A.E. Draut). 0169-555X/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.geomorph.2007.04.032

creasingly on the potential effects of Glen Canyon Dam operations on the landscape in which these cultural sites are preserved (e.g., Hereford et al., 1993; Yeatts, 1996; Thompson and Potochnik, 2000). To assess the degree to which selected archaeological sites and the geomorphic surroundings are sensitive to dam operations, we combined techniques of sedimentology, geomorphology, and archaeology to investigate erosional, transport, and depositional processes that have influenced the landscape from prehistoric times through today. Particularly valuable in this work is the use of sedimentary structures, sometimes combined with grain-size analyses, to identify depositional facies. Such methods, commonly used by sedimentologists to infer depositional setting and to characterize flow strength, direction, and depth, are also valuable in

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geoarchaeological studies as a means of identifying processes that formed prehistoric, inhabited landscapes and that affect modern preservation of cultural sites. Since the closure of Glen Canyon Dam in 1963, the natural hydrologic and sedimentary regimes along the Colorado River in the reach through Grand Canyon have changed significantly (e.g., Andrews, 1986; Webb et al., 1999; Topping et al., 2003; Hazel et al., 2006a). The dam has reduced the fluvial sediment supply at the upstream boundary of Grand Canyon National Park by ∼95%. Regulation of river discharge by dam operations has important implications for storage and redistribution of sediment in the river corridor. In the absence of floods, sediment cannot be deposited at the higher elevations that received sediment regularly before dam closure. Riparian vegetation has colonized areas at lower elevation than in pre-dam time when annual floods removed young vegetation (Turner and Karpiscak, 1980). These factors have caused a system-wide decrease in the size and number of subaerial sand deposits over the past four decades, punctuated by episodic aggradation during exceptional highflow intervals in 1983–1984, 1996, and 2004, and by sediment input from occasional tributary floods (Beus et al., 1985; Schmidt and Graf, 1987; Kearsley et al., 1994; Hazel et al., 1999; Schmidt et al., 2004). Post-dam alterations in the flow and sediment load of the Colorado River may affect the preservation potential of archaeological sites within the river corridor, even above the annual flood zone (Hereford et al., 1993; Yeatts, 1996; Thompson and Potochnik, 2000). The annual flood zone is defined here by the mean annual pre-dam flood, 2410 m3/s (85,000 ft3/s); the ‘pre-dam flood limit’, the highest elevation at which fluvial deposits are locally present, was roughly equivalent to a rare, major event of 8500 m3/s (300,000 ft3/s; Topping et al., 2003). Many cultural sites located in or on sediment deposits are actively eroding because of aeolian deflation and incision by gullies (Leap et al., 2000; Neal et al., 2000; Fairley, 2003). Hereford et al. (1993) suggested that gully incision of sediment deposits, and the base level to which small drainage systems respond, were linked to dam operations; they hypothesized that pronounced arroyo incision was caused by lowering of the effective base level at the mouths of ephemeral drainages to meet the new, post-dam elevation of high-flow sediment deposition, ∼3–4 m below the lowest pre-dam alluvial terraces. Thompson and Potochnik (2000) modified this hypothesis to include restorative effects of fluvial deposition in the mouths of gullies and arroyos, which raises effective base level, and new aeolian deposition on pre-dam alluvial deposits as wind reworks flood-deposited sand. Thompson and Potochnik (2000) concluded that sediment deprivation and lack of floods, caused by dam operations, reduce the potential for new deposition that could heal gullies formed by precipitation runoff. To understand how the presence and operation of Glen Canyon Dam may influence the stability of archaeological features downstream, sitespecific stratigraphic and geomorphic knowledge is essential. Establishing the local importance of fluvial, aeolian, and other processes in predam and post-dam time is an important prerequisite for accurate assessments of dam effects. Detailed investigations of the sedimentary record at three locations along the Colorado River corridor in Grand Canyon were initiated to determine the relative importance of various geomorphic processes in nearby archaeologically significant areas, information that can then be used to evaluate site sensitivity to dam operations. Management applications of this study were addressed in detail by Draut and Rubin (2007); here, we present this work as a case study in geoarchaeology within the river corridor of an arid-land bedrock canyon and discuss the applicability of the sedimentology methods used here to other systems. 1.2. Previous work: sedimentary structures and Grand Canyon geoarchaeology Fairley et al. (1994) completed the first comprehensive survey of archaeological sites along the Colorado River corridor in Grand Canyon,

providing baseline data for defining the depositional context of many archaeological sites. Subsequent monitoring summaries by the National Park Service (NPS) document geomorphic observations related to archaeological-site location, condition, and preservation (e.g., Leap et al., 2003). Geomorphic mapping by Hereford (1993) and by Hereford et al. (1993, 1996) in an area known as the Palisades generated detailed interpretations of the surficial geology and radiocarbon dates that complement this study; the Palisades was one location used by Hereford et al. (1993) and Thompson and Potochnik (2000) to formulate the base-level hypotheses discussed above. Grams and Schmidt (1999) used historical photographs of the Palisades area to document reduction in the extent of surficial sand deposits since 1890. High-resolution mapping by Yeatts (1996) and Hazel et al. (2000) demonstrated net aggradation of sand deposits at Palisades as a result of a 1996 experimental flood released from Glen Canyon Dam, inferred aeolian migration of sediment to higher elevation over the following year, and identified those consequences of the 1996 flood as potentially beneficial for archaeological-site preservation. Many studies have demonstrated the utility of sedimentary structures for characterizing depositional environments and paleo-flow conditions, notably Walker (1963), Stokes (1968), Harms et al. (1975), Hunter (1977a,b), McKee (1979), Rubin and Hunter (1982, 1987), Rubin (1987), and Southard and Boguchwal (1990). Various sedimentary environments associated with archaeological sites have been discussed in an overview by Stein and Farrand (1985), within which Gladfelter (1985) addressed sediment storage and chronostratigraphy of cultural sites in alluvial settings and Hassan (1985) reviewed aridland fluvial geomorphology in a geoarchaeological context. Within Grand Canyon, McKee (1938) first presented facies descriptions of Colorado River flood strata. Rubin et al. (1990) and Schmidt (1990) used stratigraphic exposures in river-level sand bars to describe the evolution of separation and reattachment bars in zones of flow recirculation in eddies. Rubin et al. (1994) used sedimentary structures in flood deposits from the early 1980s to estimate rates of deposition and to evaluate the potential effect of various dam-controlled flow regimes on erosion and accumulation of sediment on sandbars, concepts later modeled by Wiele and Franseen (2001). Grain-size trends, in particular upward coarsening, within Grand Canyon flood deposits were shown to indicate a limitation of sediment supply in pre-dam and post-dam floods by Rubin et al. (1998), Topping et al. (2000a,b), and Rubin and Topping (2001). To complement the present study, Draut and Rubin (2005, 2006) measured wind, aeolian sedimenttransport, and precipitation patterns in the river corridor over more than two years. 1.3. Study sites This study focuses on the Palisades, Lower Comanche, and Arroyo Grande areas of Grand Canyon (Fig. 1); by law, specific details of archaeological-site locations cannot be disclosed. These reaches of the river corridor are characterized by alluvial terraces that represent multiple episodes of floodplain aggradation within the pool-and-drop bedrock canyon of the Colorado River. The ‘pools’ are reaches of the channel up to several km long, bounded at each end by constrictions formed by rockfalls and debris fans at the mouths of side canyons. This environment is broadly similar to the Class A1 (high-energy stream, non-cohesive sediment) floodplain classification described by Nanson and Croke (1992), in which isolated deposits of alluvial sand, silt, and clay overlie poorly sorted gravel and boulders derived from local bedrock. The cross-channel distance between exposed bedrock walls at each study location is on the order of hundreds of meters. The highest alluvial terraces at each site contain deposits left by pre-dam flood events of over 5660 m3/s (200,000 ft3/s; Topping et al., 2003), much higher than any post-dam floods have been. The terraces at all three sites contain arroyo networks (sensu Patton and Schumm, 1981) up to several meters deep and wide resulting from incision by local

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Fig. 1. Location map of Grand Canyon with the three study areas and National Park boundary indicated.

precipitation runoff. Coppice dunes partially cover the terrace surfaces at all three sites. Talus piles are commonly present at the base of bedrock walls, and debris fans occur at the mouths of side-canyon tributaries (Fig. 2). Aerial photographs show that the Palisades, Lower Comanche, and Arroyo Grande sites have all experienced reduction in open (unvegetated) sand area since the closure of Glen Canyon Dam in 1963. At Palisades, the area investigated measured ∼600 m long (with respect to the orientation of the river) by 150–300 m wide, and included the confluence of a tributary with the mainstem Colorado River (Fig. 2a). The Palisades site has a series of pre-dam alluvial terraces dated by Hereford et al. (1996) on the basis of archaeological artifacts and 14C charcoal dates; the oldest well defined terrace dates from before A.D. 950 to A.D. 1075–1200. Aeolian dunes (now deflated and relatively inactive, covered with cryptogamic crust) are present atop the fluvial terraces. The dune field is bordered along its landward margin by ponded (playa-like) deposits that show evidence of local sediment derivation (with lithic fragments and dark red coloration of the adjacent Dox Formation sandstone and shale) and recent desiccation. The ponded area and alluvial terraces are incised by an arroyo that began to form after 1890 and that has deepened since 1965 (Hereford et al., 1993). Several cultural sites are affected by this arroyo incision, which contributes to artifact loss and deterioration of structural features. Eight prehistoric archaeological sites and one historic site are recorded at Palisades; some contain multiple habitation and artifact features (NPS, 2004). Common artifact assemblages include pottery, roasting features with fire-altered rock, and lithic flakes associated with the shaping of stone for tools. Sites have been dated by radiocarbon methods and artifact identification largely to the Pueblo I and Pueblo II periods (see Table 1 for age ranges; Fairley et al., 1994) with some evidence for earlier occupation (Dierker and Downum, 2004). The study area at Lower Comanche spanned ∼400 m by 60–150 m and was bounded at its downstream end by a tributary channel (Fig. 2b). This area includes, at elevations above the pre-dam flood limit, an aeolian dune field with dunes N10 m high. Active sand transport and dune migration occur there, although sparse vegetation and cryptogamic crust are present. Several interdune ponded areas with desiccation cracks indicate the occasional presence of standing water. At lower elevation than the dune field, pre-dam alluvial deposits are incised by an arroyo network up to 2 m deep. Cultural features at Lower Comanche date to the late Pueblo I–early Pueblo II Formative period (Fairley et al.,

1994; NPS, 2004). Additional sites contain artifacts related to Late Prehistoric and Early Historic habitation (Fairley et al., 1994). Many of the artifacts found in this area are roasting features that were used to cook food. The Arroyo Grande site is located on land managed by the Hualapai Nation and Grand Canyon National Park. The area studied there spanned ∼ 250 m by ∼ 70 m; the nearest side-canyon tributaries enter the Colorado River N1 km upstream and 300 m downstream of the study area (Fig. 2c). A large eddy is present on river left (the left side of the river when facing downstream) in the Arroyo Grande area even at non-flood stage. An arroyo system up to 5 m deep has incised two levels of pre-dam alluvial terraces in this area. Aeolian coppice dunes, which are now relatively stable with cryptogamic crust and sparse vegetation, are present on the terraces. Terrace surfaces are deflated, indicated by development of pedestals holding pebbles and cultural artifacts at heights up to 5 cm above the surrounding land surface. Four archaeological-site complexes are present in alluvial-terrace and tributary-delta regions of the Arroyo Grande area. The largest of these, Site G:03:064, is affected by the arroyo. This site contains 15 roasting pits on the land surface with additional hearths exposed within the arroyo walls. Surface features date from the Protohistoric and Early Historic era (Pai and Paiute occupation), whereas Preformative and Pueblo I–III Formative dates were identified for features at lower stratigraphic levels (as much as 3 m below the surface) within the arroyo by Fairley et al. (1994). Numerous artifacts documented on the land surface reflect the cultural importance of this area; the artifact assemblage indicates that occupants participated in an extensive trade network. The Hualapai and Southern Paiute Tribes view this area as one of great cultural significance. 2. Methods 2.1. General approach General geomorphology of the study areas was examined during reconnaissance work in 2003, and again before detailed work began in 2004. Detailed accounts of these investigations, and of weather monitoring undertaken as part of the same project, are described by Draut et al. (2005), and by Draut and Rubin (2005, 2006). After viewing natural sediment exposures, the research group chose to focus on those that were most complete vertically and that offered good spatial coverage throughout various terrain. Data were obtained

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Fig. 2. Aerial photographs of study areas with geomorphology shown in colored overlays. Circles with numbers are sedimentary profiles measured in detail; circles labeled ‘T’ are test pits studied in less detail and to shallower depth. (a) Palisades area, with geomorphology from this study and Hereford (1993). The northern area in pale green is a debris fan. The darker green strip along the shoreline is a cobble bar, likely a reworked part of the debris fan. In light blue is a relict (pre-dam) terrace surface. In darker blue are modern fluvial sandbars at an elevation within the usual dam-controlled fluctuations (below 566 m3/s [20,000 ft3/s]; in all three photographs the flow is 226 m3/s [8000 ft3/s]). Areas in yellow are coppice dunes. In orange is a playa-like surface where slope-wash and colluvial sediments accumulate. An arroyo is outlined in white. (b) Lower Comanche. The green area is a boulder bar; the blue shows fluvial deposits forming a narrow terrace. The large yellow area contains coppice dunes. In orange is a talus and colluvial slope drained by an arroyo outlined in white. (c) Arroyo Grande: two terrace surfaces are apparent, one at higher elevation (light blue) and one at lower elevation (darker blue). Aeolian coppice dunes, in yellow, occur on the upper terrace and to its east. The area in orange is talus and colluvium with bedrock exposed. The arroyo is outlined in white.

through detailed examination of sedimentary profiles and numerous shallow test pits in alluvial terraces, coppice-dune fields, and, at Palisades, near the distal margin of a debris fan from a tributary canyon (Fig. 2). Sediment exposures studied during this work were located in the general area of archaeological features but no cultural artifacts were exposed or collected. The presence of arroyo networks facilitated the exposure of vertical faces; at Palisades and Lower Comanche, three of the sections studied in detail were exposed by digging pits into areas without cultural artifacts. Thicknesses of sedimentary units were measured and sedimentary structures, clast lithology, and color of the sediment were described. We have not

applied the Munsell soil-color classification to these data because the materials are not pedogenic soils but unaltered regolithic sediments; however, these sediments all belong generally within the 7.5YR (6/2 to 6/4) Munsell category. At selected locations, sediment samples were collected for grain-size analysis using a Coulter laser particle-size analyzer (Coulter LS-100Q) at the Grand Canyon Monitoring and Research Center laboratory in Flagstaff, Arizona. The Coulter LS-100Q processes smaller samples, and more rapidly, than conventional sieving. This instrument is calibrated using dry-sieving techniques to produce grain-size distributions that agree to within 5% for grain sizes 63–900 μ.

A.E. Draut et al. / Geomorphology 101 (2008) 497–509 Table 1 Chronology of human occupation in the Grand Canyon area, Arizona (after Fairley, 2003) Period

Age

Paleoindian Archaic Early Archaic Middle Archaic Late Archaic Preformative (Basketmaker II) Formative Early Formative (Basketmaker III) (Pueblo I) Late Formative (Pueblo II) (Pueblo III) Late Prehistoric Protohistoric Early Historic Late Historic

ca. 12,000–8000 B.C. ca. 8000–1000 B.C. ca. 8000–5000 B.C. ca. 5000–3000 B.C. ca. 3000–1000 B.C. ca. 1000 B.C.–A.D. 400 ca. A.D. 400–1250 A.D. 400–1000 ca. A.D. 400–800 A.D. 800–1000 A.D. 1000–1300 A.D. 1000–1150 A.D. 1150–1300 A.D. 1300–1540 A.D. 1540–1775 A.D. 1776–1850 ca. A.D. 1850–1950

Names of periods listed in parentheses refer specifically to stages of development within ancestral Puebloan cultures living in this part of North America. Because cultural affiliations of some archaeological sites in Grand Canyon are uncertain, more general terms (e.g., Formative) are often applied in lieu of period names that have cultural connotations (e.g., Pueblo II). No Basketmaker I period has been defined because artifact assemblages have not been found that would represent a transitional phase between cultures without basket-making technology and those with the skills reflected in Basketmaker II artifacts. In Grand Canyon, Puebloan habitation sites were not occupied after ca. A.D. 1220. Dates that define the Protohistoric and Early Historic periods are related to contact with Anglo-European cultures; the first contact of early Native American cultures with Spanish explorers occurred around A.D. 1540, Spanish missionaries reentered the Grand Canyon region from both the north and south in 1776, and tribes came into frequent contact with United States governmental entities after 1850.

2.2. Identification of depositional environments using sedimentary structures Water-lain deposits in the Colorado River corridor, and in other aridland bedrock canyons that contain archaeological remains, include alluvium left by floods of the mainstem river, sediment deposited at the mouths of tributaries whose flow is commonly ephemeral, and slopewash sediment reworked by precipitation runoff. Distinguishing these deposits from each other and from aeolian sediment is an essential task in identifying landscape processes associated with cultural-site formation and preservation. Within a vertical sediment exposure, sedimentary structures often provide the best diagnostic indicator of depositional environment. These textures form as a result of fluid flow or sediment gravity flow and can be modified by soft-sediment deformation or biogenic activity. Structures such as ripples and cross-bedding can be used to identify the sediment-transport mechanism and flow conditions that resulted in the formation of a particular sedimentary unit. Structures characteristic of fluvial and aeolian environments can often be distinguished by the dimensions, scale, grain-size sorting, and spatial orientation of bedding apparent in outcrop exposure. Although some of the same structures (such as ripple marks or cross-bedding) are common to aeolian and subaqueous deposits, differences in the appearance of each often allow the depositional settings to be distinguished (Fig. 3). Sediment will assume different bedform structures depending on the flow depth, flow velocity, and grain size of the sediment (e.g., Allen, 1982; Rubin, 1987; Southard and Boguchwal, 1990). Subaqueous ripples may be symmetrical or asymmetric depending on whether they form under oscillating or unidirectional flow, respectively. Fluvial ripples are, therefore, most commonly asymmetric, although symmetric ripples can form under wave swash or from eddy pulsations at the margin of a river channel (Rubin and McDonald,1995). Asymmetric ripples migrate along the river bed as sediment is eroded from the stoss side of each ripple and deposited on the lee side; each ripple form is thus gradually translated downstream. The migration direction preserved in the resulting ripple cross-lamination indicates the orienta-

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tion of the current flow at the time of sediment deposition (Fig. 3); fluvial sediment deposited in an eddy typically shows an upstream ripple-migration direction, consistent with upstream flow in the eddy return current (cf. Kochel and Baker, 1982). Ripples are often seen to “climb” upward (as in Fig. 3), with the climb angle a function of the relative rates of sediment deposition and lateral migration of the bedform (Rubin and Hunter, 1982; Rubin, 1987). Structures that are characteristic of wind-deposited sediment have some similarities to subaqueous structures but differ in geometry and grading, reflecting fluid dynamics that result from the greater density contrast between the sediment and the fluid (air). Like fluvial ripples, aeolian climbing ripples are formed as individual bedforms migrate (translate) downcurrent, but typically at a much lower climb angle than is seen in subaqueous ripples. Aeolian subcritically climbing translatent strata, in contrast to fluvial counterparts, are, therefore, very thin (∼2 mm in Fig. 3) and tabular (Hunter, 1977a,b). As each ripple form migrates over the next, a pattern of ‘pinstriped’ strata commonly forms from climbing wind ripples (Hunter, 1977b; Fig. 3). Within each individual stratum, sediment particles are inversely graded (coarsening upward), a product of the grain-size sorting of sand by wind. In contrast, inverse grading in subaqueous climbing ripples is rare or absent. In addition to climbing wind ripples, aeolian sedimentary structures can result from avalanching of sand down the lee side of a dune (e.g., Rubin and Hunter, 1987) or form by grainfall deposition in areas of flow separation in the lee of dune crests (Hunter, 1977b). Some manifestations of fluvial and aeolian sedimentary structures appear similar and can be difficult to distinguish (such as aeolian ripples with low climb angles and fluvial upper-plane-bed structures; see Southard and Boguchwal, 1990). In such cases, the depositional environment can sometimes be inferred by observation of lateral gradation into other, more diagnostic structures (subaqueous upperplane-bed lamination may grade laterally into subaqueous climbing ripples, e.g.). In a bedrock canyon such as Grand Canyon, fluvial and aeolian deposits commonly interbed with sediment derived from local bedrock by slope-wash events, debris flows, or rockfalls. Deposits of such processes contain locally derived lithic clasts and are more poorly sorted and immature (with respect to mineralogy and weathering of sediment grains) than fluvial or aeolian deposits (e.g., Benito et al., 2003). Overland flow traversing an alluvial-terrace surface commonly incises the terrace to form channel features (cm-scale rills to meterscale arroyos; Patton and Schumm, 1981) that can fill later with locally derived sand and gravel.

Fig. 3. Sedimentary structures indicate a fluvial deposit in the upper, left part of this vertical sediment exposure above the dashed line (with fluvial climbing ripples that migrate from right to left, indicating right-to-left current). The fluvial deposit overlies an aeolian deposit with climbing wind ripples (Hunter, 1977b) visible below the dashed line. This example was photographed in the Colorado River corridor upstream of the Palisades study site (Draut et al., 2005).

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Table 2 Summary of sedimentary processes affecting archaeological areas at Palisades Site number

Age and affiliation of site

Sediment on which site was originally built

Sediment overlying site

Sedimentary and geomorphic characteristics

C:13:033

Debris-fan cobbles, gravel

Aeolian (minor)

Dominated by debris-flow deposition

C:13:098

Uncertain; probable Pueblo I–III Historic

Debris-fan cobbles, gravel

None

C:13:099

Pueblo I–II

Fluvial; slope-wash sand and gravel

Aeolian; distal debris-flow material

C:13:100 C:13:101 C:13:272

Pueblo I–II Pueblo I–II Pueblo I–II

Fluvial Rounded cobbles, likely fluvial Debris-fan cobbles, gravel

Fluvial/aeolian Aeolian (minor) Fluvial and aeolian

C:13:334 C:13:336

Pueblo I–II Pueblo I–II

Fluvial Fluvial/aeolian

C:13:355

Pueblo II

Fluvial; ponded slope-wash Likely fluvial (artifacts not in original position) Fluvial; distal debris-flow

Remains of 19th century cabin built on debris-fan sediment Interbedding of fluvial, aeolian, debris-flow and slope-wash sediments Fluvial sediment modified by wind Coppice dunes overlying rounded cobbles Fluvial sediment modified by wind into coppice dunes; bioturbated Eroding out of apparent fluvial deposit Fluvial sediment reworked by wind into coppice dunes Area dominated by debris fan, smaller volumes of fluvial and aeolian sediment

Fluvial and aeolian

Archaeological-site numbers are those assigned by the National Park Service (NPS). Detailed discussions of processes in these areas may be found in Draut et al. (2005). Prehistoric sites have been dated by radiocarbon methods and artifact identification to the Pueblo I and Pueblo II (PI and PII) periods (Fairley et al., 1994) although some evidence exists for Preformative occupation (Dierker and Downum, 2004). The historic site is associated with mining activity and was occupied from A.D. 1890–1910.

It is particularly advantageous to identify landscape processes using sedimentary structures in addition to surface morphology because landforms may be unrelated to depositional processes that emplaced the sediment in the sub-surface. Documented examples include yardangs, fluvial ripples scoured into beach swash (Fig. 11 of Rubin, 1987), and marine tidal ridges scoured into the surface of a fluvial deposit following sea-level transgression (Berné et al., 2002). In each case, the surface forms represent erosion into older sediment that was deposited by processes other than those that generated the surface form. Therefore, if sedimentary structures are well preserved, the most complete record of prehistoric and historic landscape processes will be obtained by investigating sub-surface sedimentology and surficial geomorphology. 3. Results and interpretation Very detailed descriptions of sedimentary profiles and grain-size analyses from the three study sites were discussed by Draut et al. (2005). Here, we summarize the local depositional environments in these regions and use the data to focus on the broader applicability of these techniques in addressing geoarchaeological problems. Geomorphic and sedimentary processes that affect each studied archaeological site are summarized for Palisades, Lower Comanche, and Arroyo Grande in Tables 2, 3, and 4, respectively. Stratigraphic sections at Palisades (Fig. 4a) indicate repeated inundation of this area by Colorado River floods in pre-dam time. Fluvial deposits containing climbing ripples were present in three of

the four profiles studied, and the fourth contained sediment for which fluvial deposition was suspected but could not be confirmed by sedimentary textures. The extent of fluvial deposits, defined by sedimentary structures, demonstrates that the entire terrace area in the Palisades region was submerged episodically during pre-dam high flows. No post-dam flows (since 1963) have inundated this terrace. The consistent upstream migration direction of fluvial ripples indicates that an eddy existed in this area during pre-dam floods. A stage–discharge relationship developed from driftwood deposits indicated that the terraced area was almost entirely inundated by a flow of ∼5940 m3/s (210,000 ft3/s; Hazel et al., 2006b), a flood level reached most recently in 1884. Reworking of fluvial sediment by wind was apparently common, evidenced by aeolian deposits between fluvial deposits; aeolian material was also found interbedded with playa-like sediment where slope-wash runoff had formed small, isolated ponds. The proportion of water-lain sediment derived from local sources (slope-wash and distal debris flows) was found to increase landward and toward a debris fan at the mouth of a tributary north of the study area (Fig. 2a). Six of the nine archaeological sites at Palisades were formed in or on fluvial sediment, some of which had been reworked by wind (Table 2). Five of the nine sites are preserved at least in part by a cover of aeolian sediment, with minor contributions from aeolian sedimentary cover at two additional sites. Erosion associated with arroyo incision affects two of the sites. Exposure of artifacts can be attributed at least in part to aeolian deflation at five sites. Influence of debris flows from a side canyon was apparent at five of nine sites. Four sites

Table 3 Summary of sedimentary processes affecting archaeological areas at Lower Comanche Site number

Age and affiliation of site

Sediment on which site was originally built

Sediment overlying site

Sedimentary and geomorphic characteristics

C:13:273

Fluvial, aeolian; colluvial

Aeolian (minor)

C:13:274

Early Formative (Basketmaker III) Pueblo I–II

Aeolian

None

C:13:333

Pueblo I–II

Aeolian

Aeolian (minor)

C:13:335 C:13:337 C:13:373

Pueblo I–II Pueblo I–II Prehistoric to Early Historic Hopi

Aeolian Aeolian Aeolian

Aeolian None Aeolian

Fluvial and aeolian deposits interbedded with slope-wash colluvium Aeolian substrate overlies distal debris-fan sediment Site destabilized by aeolian deflation and dune migration Exposed by aeolian deflation Aeolian dunes; site in interdune area Site destabilized by aeolian deflation and dune migration

Site numbers have been assigned by NPS. Most cultural features in this area date to the Late PI–Early PII Formative period (A.D. ∼900–1000; Fairley et al., 1994; NPS, 2004). At least one site contains artifacts related to late prehistoric and early historic use by the Hopi Tribe (Fairley et al., 1994). The Early Formative age given for Site C:13:273 is based on radiocarbon ages obtained from a hearth feature by Yeatts (1998).

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Table 4 Summary of processes affecting archaeological areas at Arroyo Grande Site number

Age and affiliation of site

Sediment on which site was originally built Sediment overlying site

G:03:064, G:03:064, G:03:064, G:03:064,

Protohistoric to Early Historic Pai Protohistoric to Early Historic Pai Protohistoric to Early Historic Pai Protohistoric to Early Historic Pai

Aeolian Aeolian Aeolian Aeolian

F1 F2 F3 F4

Aeolian (minor) Aeolian (minor) Aeolian (minor) Aeolian (minor)

G:03:064, F5

Protohistoric to Early Historic Pai Aeolian

G:03:064, F6

Protohistoric to Early Historic Pai Terrace surface (dominantly fluvial)

G:03:064, F7 G:03:064, F8 G:03:064, F9 G:03:064, F10 G:03:064, F11 G:03:064, F12

Protohistoric to Early Historic Pai Protohistoric to Early Historic Pai Protohistoric to Early Historic Pai Protohistoric to Early Historic Pai Protohistoric to Early Historic Pai Protohistoric to Early Historic Pai

Terrace surface Terrace surface Terrace surface Terrace surface Terrace surface Aeolian

(dominantly fluvial) (dominantly fluvial) (dominantly fluvial) (dominantly fluvial) (dominantly fluvial)

G:03:064, F13 Protohistoric to Early Historic Pai Aeolian G:03:064, F14 Protohistoric to Early Historic Pai Interbedded fluvial/aeolian/colluvial G:03:064, F15 Protohistoric to Early Historic Pai Colluvium Not assigned

Unknown

Sedimentary and geomorphic characteristics

Slope-wash colluvium and aeolian

Coppice dunes on upper-terrace surface Coppice dunes on upper-terrace surface Coppice dunes on upper-terrace surface Interdune area in coppice-dune field; arroyo incision affects feature stability Aeolian (minor) Interdune area in coppice-dune field; affected by aeolian deflation None Arroyo incision affects terrace surface in the vicinity of this feature None Surface of upper terrace None Surface of upper terrace None Surface of upper terrace None Surface of upper terrace None Surface of upper terrace Aeolian (partially covers feature) Small coppice dunes on upper-terrace surface. Feature affected by arroyo incision None Small coppice dunes on upper-terrace surface. Feature affected by arroyo incision Aeolian Exposed in small drainage within upper terrace None Near landward edge of upper terrace, in area dominated by slope-wash sedimentation Fluvial, aeolian, and colluvial Charcoal hearth feature exposed in arroyo wall within upper terrace, near measured Section 8

Site and feature numbers are those assigned by NPS. The site numbered G:03:064 includes 15 separate features (most of which are roasting pits) spread across several thousand square meters. Detailed discussions of these areas can be found in Draut et al. (2005). Most cultural features that can be examined without excavation in this area are exposed on the land surface. In general these are associated with the Protohistoric and Early Historic-era Pai culture related to the Paiute and Hualapai Tribes. Many of the exposed sites are on the broad upper terrace where aeolian coppice dunes occur locally. This aeolian sand apparently represents reworked fluvial sediment; coppice dunes overlie a terrace composed of interbedded fluvial, aeolian, and colluvial (slope-wash) materials. Thus these features were built on aeolian sediment directly, but this relatively thin aeolian substrate is underlain by volumetrically more substantial flood deposits.

were formed directly on debris-flow sediment (including cobble-sized clasts at the historic and one prehistoric site) and at least one site had been overlain by distal debris-flow material after initial human occupation (Table 2). Stratigraphy of the southern, arroyo-incised part of the Lower Comanche area indicated multiple episodes of fluvial sedimentation followed by reworking at the land surface by wind and local runoff that follows rainfall events (Fig. 4b). Aeolian climbing ripples were present in some of the deposits; much of the silt and fine sand did not contain preserved diagnostic sedimentary structures. Colluvium (recognized by sandstone and shale clasts derived from the local Dox Formation bedrock) was present in all six profiles studied at Lower Comanche. Fluvial sedimentary structures were observed only in the two profiles closest to the river. Numerous channel-fill structures were apparent in arroyo-wall exposures. Five of the six cultural sites at Lower Comanche are located within the large aeolian dune field (Table 3). Four of those were originally situated on aeolian sediment, with the fifth built on an interdune, playalike surface. Four sites were at least partially buried by aeolian sediment; two of those four have only minor sediment cover (b10 cm thick). Three of the five sites within the dune field are affected by modern aeolian deflation and dune migration. The one archaeological site not located in or among the large coppice dunes at Lower Comanche (Site C13:273; Table 3) was constructed on a terrace that contained slope-wash material interbedded with lighter-colored, better-sorted fine sediment with poorly preserved sedimentary structures that appeared to grade laterally into fluvial and aeolian deposits. Colorado River flood deposits dominate the stratigraphic record around the extensive archaeological site at Arroyo Grande. Profiles within the upper terrace (Fig. 2c) showed the proportion of fluvial sediment generally increasing toward the river. The record of flood deposition is best preserved in sedimentary profiles closest to the river, with 15 individual floods evident in one profile (Section 6; Fig. 4c). All profiles showed evidence of subaerial reworking and incorporation of colluvial and slope-wash sediments between floods. The repeated occurrence of fluvial deposits overlain by subaerial sediment led to the description of a

‘flood couplet’ facies consisting of a lower fluvial and upper subaerial member (Fig. 5). The fluvial portion of many of these flood couplets begins with a white to pale buff-colored silt-and-clay lower layer. Sedimentary structures in fluvial deposits most commonly indicate flow toward the north or west, evidence of a large eddy at high flow in this area similar to the one that exists there today even at non-flood flow. Bioturbation and the presence of biotite and amphibole lithic clasts in the upper parts of flood couplets (common minerals in the local gneiss bedrock) indicate reworking of the flood sediment at the land surface; pedogenic soil horizons are not developed. Poorly sorted, lithic-rich channel-fill deposits in two of the measured profiles, and in other exposures in the arroyo walls where strata were not described in detail, indicate multiple episodes of gully formation and filling during subaerial exposure. Aeolian sediment constitutes a relatively minor volume in the sections measured at Arroyo Grande; where present, aeolian dune deposits extended as much as 2 m beneath the land surface. The most likely sediment source for the small, deflated coppice dunes visible on the land surface today is reworking of the extensive fluvial deposits that created the terrace morphology in this area. Of the 16 cultural features studied at Arroyo Grande, 15 are exposed at the land surface (Table 4). Fourteen were formed on the surface of the upper terrace, of which seven are located within coppice dunes on the terrace surface. Seven of the 14 features on the terrace surface are partially covered by aeolian sediment. Of the two features not on the upper terrace surface, one was built on colluvium near the landward edge of the terrace and the other, for which no site name or affiliation has been assigned, was exposed within an arroyo wall. That hearth feature had been dug into slope-wash and aeolian sediment and was buried by fluvial, aeolian, and colluvial sediments after its use. 4. Discussion 4.1. Interpretation of sedimentary textures Analysis of sedimentary textures yielded new insights, at a level of detail not reached by previous studies, into the suite of landscape

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Fig. 4. Strata mapped in detail at (a) Palisades, (b) Lower Comanche, and (c) Arroyo Grande. Horizontal axes indicate cross-shore distance from the river. Vertical axes indicate elevation (m) referenced to the NAD83(1999) datum for the profiles; numbers above each profile are the elevation of the land surface, in meters. Numbers beneath each profile are the stratigraphic section numbers by which these profiles are identified in Draut et al. (2005). Individual sedimentary units within each measured profile are shown; colors of strata correspond to depositional environment (assigned based on sedimentary structures, clast composition, and, for ∼ 10 strata, grain-size distribution). Blue units are fluvial deposits, yellow are aeolian, orange are slope-wash and colluvial sediments derived from local bedrock and talus, green are distal debris-flow deposits, and blank (white) units are those for which a depositional setting was not assigned, either because sedimentary textures were inconclusive or because those parts of the sedimentary profiles were inaccessible. Width of each sedimentary unit represents grain size (the wider the unit, the coarser the sediment); some units coarsen upward or fine upward. Grain-size properties were determined primarily by grain-size-comparator charts with ∼10% of strata sub-sampled for laser particle-size analysis.

processes that affected archaeologically significant areas before, during, and after prehistoric occupation at the Palisades, Lower Comanche, and Arroyo Grande areas of Grand Canyon. Sedimentary structures were used to identify the number and thickness of fluvial deposits, aeolian reworking of fluvial sediment, and interaction of those processes with local runoff sedimentation (Fig. 4). Valuable information about landscape modification can be gained from observing geomorphic features on the land surface (coppice dunes, playa-like desiccated ponds) but such features reflect only the most recent surficial modification and tell little about landscape evolution in the past. Accurate interpretation of sedimentary structures in strata beneath the land surface is a complementary and essential step in deciphering landscape processes that affected cultures in the past and that determine the preservation of cultural sites today. Therefore, the methods used in this case study are applicable to bedrock-canyon settings foremost and can also be applied to any cultural locality worldwide where the substrate and any overlying material consist of unaltered, non-pedogenic sediments. This approach to assessing depositional and erosional processes will be particularly relevant to geoarchaeology in arid regions, where chemical weathering is essentially absent and where the scarcity of vegetation promotes preservation of intact sedimentary structures. Resolving the interaction of fluvial, aeolian, and local runoff processes in an arid-land river corridor, as discussed here, is especially significant because the

Fig. 5. Couplets of fluvial and subaerially reworked strata within measured profile #6 at Arroyo Grande, defining a flood-deposit facies (Draut et al., 2005). The photograph shows five units each interpreted as a flood deposit that has been reworked at the land surface. Fluvial climbing ripples are present in the lower parts of each couplet. The subaerial portion of each couplet contains sediment derived from local runoff (identified by lithic clasts) as well as charcoal and ash staining. Common charcoal and ash in subaerial parts of couplets may result from grass fires started by the prehistoric occupants of this area. Bioturbation has mixed slope-wash sediment (lithic grains) downward into underlying fluvial deposits. Scale bar is 10 cm long.

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archaeological record in arid lands tends to be concentrated in river corridors, where human populations have regular access to water (e.g., Hassan, 1985). Even in arid regions, however, not all strata preserve diagnostic structures. In most cases, absence of sedimentary structures in finegrained sediment is attributable to biologic effects. Vegetation interferes with the development of aeolian sedimentary structures; wind ripples forming on a sparsely vegetated surface have shorter crests and less regular trough elevation than on unvegetated surfaces, while denser vegetation reduces or prevents aeolian transport of sand (Olson, 1958; Ash and Wasson, 1983; Buckley, 1987). Organic debris on the land surface can also interfere with the development of wind ripples, as can a variable wind direction during deposition. Bioturbation by trampling, animal burrowing, and the growth of plant roots, as seen at the Grand Canyon study sites, can disturb or destroy sedimentary structures. Bioturbation in subaerial and subaqueous environments can quickly obscure sedimentary structures, particularly if the rate of sediment deposition is slow (e.g., Dott and Bourgeous, 1982). If sedimentary structures are not apparent, reworking of a primary sedimentary deposit can sometimes be inferred from the morphology and composition of the deposit. In the absence of lithic grains or a distinctive color that implies incorporation of local runoff sediment (such as, in this study, clasts from gneiss bedrock), fine-grained deposits in the Colorado River corridor are generally interpreted as having been derived from Colorado River sediment. Flood deposits that have been reworked by wind commonly exhibit surface morphology that includes sand shadows or coppice dunes. If neither sedimentary structures nor aeolian landforms are evident, it is difficult if not impossible to determine whether light-colored finegrained sediment within terraces remains in its original position as fluvial units or has been reworked by wind. Grain-size analysis may in some cases distinguish fluvial from aeolian material if sedimentary structures are indistinct. Previous studies have shown that grain size and moment statistics are of little use for identifying fluvial and aeolian deposits when comparing samples from a wide variety of geographic regions (Ahlbrandt, 1979; Tucker and Vacher, 1980; Gladfelter, 1985). This is not surprising given the variations in clast composition and environmental conditions, both of which control grain weathering, introduced by studying a geographically broad sample population. This study analyzed particlesize distributions of fluvial and aeolian samples collected only within the Colorado River corridor of Grand Canyon, to assess whether enough consistency is present within this sedimentary system to use grain size to classify samples of unknown origin also collected within this same river corridor. Grain-size distributions of 100 samples collected from pre-dam fluvial deposits, post-dam fluvial deposits, and aeolian deposits are shown in Fig. 6. Although a range of grain sizes exists within Colorado River fluvial deposits (a function of sediment availability, shear stress of the flow, and proximity to flooding tributaries), this study found essentially no overlap between textural characteristics of pre-dam fluvial (Fig. 6a) and aeolian deposits (Fig. 6c) in Grand Canyon, such that the finest aeolian deposits are coarser than the coarsest pre-dam fluvial units sampled (Fig. 6e; see also Burke et al., 2003). Because aeolian deposits in the river corridor form as the wind winnows flood sediment, it is not surprising that aeolian deposits are coarser than the flood deposits from which they were derived (Rubin et al., 2007). Grain-size distributions of aeolian samples do overlap, however, with those from post-dam flood deposits (Fig. 6f), because the post-dam fluvial samples shown (which were deposited by an experimental high flow in November 2004) are coarser, as a population, than the pre-dam flood deposits (Fig. 6d). This is consistent with previous analyses of other post-dam flood deposits and reflects the coarsening of sediment transported and deposited by this river after closure of Glen Canyon Dam, in response to limitation of the upstream sediment

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supply and winnowing of the fine sediment already in the river channel (Rubin et al., 1998; Topping et al., 2000a,b). Based on the results shown in Fig. 6, it appears possible to use grain size to distinguish pre-dam flood deposits (which underlie and overlie archaeological sites in Grand Canyon, unlike the lower-elevation postdam deposits) from mature aeolian deposits within this river corridor if sedimentary structures are absent. Particle size as a diagnostic tool should still be used with caution even within one sedimentary system unless the range of grain sizes for known aeolian and fluvial deposits in that system is well defined. (We refer here only to fine-grained fluvial deposits at flood-stage elevation that are relevant to geoarchaeology; deeper in the river channel, fluvial deposits include gravel through boulder sizes for which the sedimentology techniques discussed are obviously not applicable). The difference in grain-size distribution between a fluvial deposit and its winnowed, aeolian counterpart depends on the initial composition of the flood sediment and on the wind conditions affecting the sediment. A continuum, therefore, exists between the grain-size distribution of the initial flood sand and the eventual distribution within a mature aeolian deposit; Colorado River fluvial sediment that has undergone some wind reworking might not yet plot within the mature aeolian field of Fig. 6c, limiting these diagnostic applications. We infer from the patterns in Fig. 6 that sand in the mature aeolian deposits sampled was derived almost entirely from extensive, pre-dam flood deposits with substantially less contribution from the smaller, lower-elevation deposits left by rare post-dam floods. 4.2. Geomorphic processes affecting site formation and preservation Landscape processes, such as flooding, aeolian dune development and migration, and slope-wash and debris-flow deposition, affected the formation (by anthropogenic and natural processes) and preservation of the studied archaeological sites to different degrees, as indicated in Tables 2–4. Site-specific study is very important because the relative roles of various sedimentation processes differ widely between sites. Fluvial deposits are the most common substrate underlying archaeological sites at Palisades and Arroyo Grande. At Palisades, six of nine sites (including several habitation structures) were formed on flood deposits, indicating the potential for episodic flooding of the primary habitation area and perhaps a preference for site location on fluvial terraces. The remaining three sites were built primarily on distal debris-flow sediment. At one Palisades site, potsherds and hearth charcoal (dated to A.D. 1270–1460) have been found overlying debris-flow sediment that in turn overlies an older cultural feature dated to A.D. 900–1270, indicating reoccupation following a debris flow that partially buried the former living area (Hereford, 1993). At Arroyo Grande, the most extensive archaeological site was formed in and on a terrace that consists primarily of interbedded fluvial and locally derived sediment. Fourteen of the 15 documented surface features at Arroyo Grande are on that terrace, with the remaining one on colluvium at the landward margin of the terrace. Fluvial deposition was found generally to decrease with increased elevation and distance from the river as locally derived slope-wash and colluvial sediments became more prominent (Fig. 4). The proportion of fluvial sediment does not decrease regularly away from the river, however, in part because aeolian modification can occur across much of an alluvial terrace almost irrespective of distance from the river (although wind strengths are lowest at the river margin, especially in the post-dam era, where vegetation is commonly thick; Fig. 2c). In the Colorado River corridor through Grand Canyon, wind strength (and aeolian modification of fluvial sediment) is greatest during the spring windy season, which can last from April through early June. During this time, rates of aeolian sediment transport are 5–15 times greater than in other seasons, and the dominant wind direction transports sediment toward upstream (Draut and Rubin, 2005, 2006). Although aeolian dunes are present on terrace surfaces at Palisades and

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Fig. 6. Comparison of grain-size distributions, obtained by laser particle-size analysis, in fluvial and aeolian deposits. Values on vertical axes indicate the cumulative percent of the sediment sample that is finer than the corresponding grain diameter shown on the horizontal axes, ranging from 0.037 mm (coarse silt) to 1.000 mm (the boundary between coarse sand and very coarse sand; Boggs, 1995). (a) Grain-size distributions of 27 sediment samples from 10 different fluvial strata, all deposited by Colorado River floods that pre-dated the closure of Glen Canyon Dam. Samples were identified as fluvial deposits by the presence of unequivocal sedimentary structures. Multiple samples were collected from some flood deposits because grain size commonly varies vertically within each deposit (Rubin et al., 1998). (b) Grain-size distributions of 63 samples from 10 different deposits of a November 2004 experimental flood in Grand Canyon. (c) Grain-size distributions of 10 aeolian deposits in Grand Canyon. (d) Comparison of the pre-dam fluvial deposits from (a), the light-gray field outlined in a solid line, with the post-dam (November 2004) deposits of (b), the darker gray field and dashed line. Deposits of the 2004 flood are coarser, and show a tighter distribution, than the pre-dam deposits. (e) Comparison of the pre-dam fluvial deposits of (a), the light-gray field, with the aeolian deposits of (c), the dark gray field. (f) Comparison of the post-dam, November 2004 flood deposits of (b), the medium-gray field with dashed border, with the aeolian deposits of (c), the dark gray field.

Arroyo Grande, aeolian sediment was found to be more important as a protective cover of sites than as the substrate on which cultural features originated. At Lower Comanche, in contrast, five of the six archaeological sites were formed on and are partially buried by aeolian sediment; however, the largest and most extensive site at Lower Comanche is not in the aeolian dune field but is on a lower-elevation terrace constructed of fluvial, aeolian, and slope-wash deposits. In all three study areas, geomorphic processes that affect the condition of cultural sites today include arroyo incision, wind action, and episodic local deposition of slope-wash and debris-flow material. Arroyo incision and aeolian processes contribute to exposure and erosion of artifacts in all three areas. Aeolian deflation exposes artifacts that are then subjected to erosion by rainfall runoff, and dune migration (particularly at Lower Comanche) causes down-slope movement of artifacts. Episodic deposition of flood sediment, which formerly affected archaeologically significant areas at Palisades and Arroyo Grande, no longer occurs at those elevations under the current damcontrolled flow regime of the Colorado River. Abundant charcoal and ash material in many strata (commonly seen at Arroyo Grande; Fig. 5) is most probably attributable to grass fires during times of subaerial exposure. It is likely that these fires were deliberately set by the inhabitants of the area, although lightning strikes could also have caused occasional fires. The Hualapai and

Southern Paiute Tribes, both with ancestral ties to the Arroyo Grande region, have cultural traditions that include the deliberate setting of grass fires to initiate seed germination and to prevent other, larger fires from starting (L. Jackson and I. Bullets, oral commun. with L. M. Leap, 2004). The setting of grass fires was practiced at certain prescribed times of year, and continues today. Other possible reasons for deliberately set fires include hunting and warfare (Powell, 1878; Kelly and Fowler, 1986; Boyd, 1999). 4.3. Implications for site protection and sediment restoration Many archaeological sites, including most of those at Palisades, Lower Comanche, and Arroyo Grande, were formed on terraces that consist largely of alluvium. Many of those sites are preserved today by a combination of burial (both by flood deposits and aeolian dunes) and limited arroyo erosion. From prehistoric times until the closure of Glen Canyon Dam in 1963, the greatest fluvial deposition in this river occurred during spring snowmelt floods that regularly exceeded even the highest post-dam discharge; the highest post-dam flow of 2750 m3/s (97,000 ft3/s) occurred in 1983, while the pre-dam eight-year flood level was 3540 m3/s (125,000 ft3/s; Topping et al., 2003). Annual sedimentrich floods occurred concurrently with or just before the spring windy season, during which wind would have remobilized sand from the new

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flood deposits to form aeolian dune fields. The modern absence of sediment-rich floods eliminates the mechanism for depositing sediment at the elevations of prehistoric alluvium and also eliminates the source of new sediment to aeolian deposits. Dam operations have, therefore, substantially altered both the fluvial and aeolian processes that acted on the prehistoric, inhabited landscape. This alteration potentially hinders the preservation of tens to hundreds of cultural sites, including most of those in this study, by limiting the fluvial and aeolian sediments available to bury and cover those sites (the National Park Service lists more than 250 sites as potentially affected by dam operations but only the ones discussed here have been studied at this level of sedimentary detail; see Draut et al., 2005). Enhanced site protection could be achieved by restoring fluvial and aeolian deposition processes to resemble more closely those that occurred before the river was dammed: large, sediment-rich floods that left fluvial sand deposits from which sand was remobilized by wind. Fluvial deposits form the most extensive base on which archaeological sites at Palisades and Arroyo Grande are located, spanning hundreds of square meters in area (see also Hereford et al., 1996). Moderate-scale (∼1270 m3/s; 45,000 ft3/s) controlled Colorado River floods, as were conducted in 1996 and 2004, can build sandbars that supply sand to aeolian deposits covering archaeological sites at higher elevation. To effect large-scale restoration of the fluvial terraces in these and other areas, however, a flow of N4810 m3/s (170,000 ft3/s) would likely be needed, based on stage–discharge relationships developed for Palisades from historical flood debris (Draut et al., 2005; Hazel et al., 2006b). To cause deposition instead of erosion, flood waters would also require very high concentrations of suspended sediment (Hazel et al., 2006a; Topping et al., 2006). Concentrations of suspended sediment measured for pre-dam flows N1000 m3/s (35,000 ft3/s) were within an order of magnitude of 0.1 g/l (Topping et al., 2000a); although it is not known what the concentration of sediment would be in a post-dam flood of 4810 m3/s (170,000 ft3/s), in general the annually averaged postdam sediment load is around 5% of average pre-dam values. An experimental flood of the magnitude needed to inundate pre-dam fluvial terraces at Palisades, ∼4810 m3/s (170,000 ft3/s), would be nearly four times greater than the highest experimental flows currently under consideration by the Glen Canyon Dam Adaptive Management Program (http://www.usbr.gov/uc/rm/amp/index.html). It is likely, also, that the concentration of sediment would be too low in such a high flow to have a substantial restorative effect unless the flood were to follow an exceptionally large input of tributary sediment or unless sediment is added artificially downstream from the dam. Were a sediment-rich, highdischarge dam release to occur, a large volume of new high-elevation fluvial deposits could then act as a source for aeolian sediment, which could be remobilized by wind and transported to dune fields. Increased transport of aeolian sand to dune fields is expected to inhibit erosion by filling small gullies, which act as natural traps for wind-blown sand (Draut and Rubin, 2006, 2007), and by providing additional protective cover to archaeological sites (such as Lower Comanche; Table 3). Generating a major new fluvial deposit as a source for aeolian sand is anticipated to increase the volume of aeolian deposits much more than creating smaller sources of aeolian sand by simpler, local methods such as removing vegetation from selected channel-margin sand bars. Controlled dam releases of ∼ 1270 m3/s (45,000 ft3/s) in 1996 and 2004 deposited sand in arroyo mouths at Palisades and elsewhere (Yeatts, 1996; Hazel, 2004, unpublished data; Topping et al., 2006), which may have increased the preservation potential of some sites by temporarily raising the effective base level to which arroyos incise (Hereford et al., 1993). This effect is likely short-lived, however, because runoff from local rainstorms can quickly remove the new deposits in arroyo mouths and lower the effective base level once again. Relatively few data are available concerning the effects of sediment deposition during controlled floods, and its ability to control arroyo base level, that would definitively support or refute the stilldebated hypotheses of Hereford et al. (1993) and Thompson and

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Potochnik (2000). Local climate, drainage area, substrate composition, and river-related sediment and base-level effects all contribute to arroyo incision and healing. Arroyo-incision processes could be clarified by repeated high-resolution mapping of gullies and affected archaeological sites to quantify local erosion and deposition rates, and by quantifying precipitation and other climate parameters that vary widely within the complex canyon topography (e.g., Draut and Rubin, 2005, 2006). Palisades and Lower Comanche, where large arroyos erode multiple archaeological sites, are the two regions with the least rainfall out of six sites monitored by Draut and Rubin (2006); if rainfall measured there from 2003–2006 is representative of long-term spatial variations, enhanced local precipitation could be eliminated as a possible cause of preferential arroyo development at Palisades and Lower Comanche. In general, slope-wash and debris-flow sedimentations have less potential to be affected by dam operations than are fluvial and aeolian deposits. Deposits resulting from local runoff, however, will vary in extent and location from the pre-dam condition if the base level onto which local sediment is delivered is lowered. For example, if the configuration of aeolian dunes changes, or if a new gully breaches the dune field (as at Palisades), slope-wash events may cause additional gully incision that drains to the river instead of allowing slope-wash and colluvial sediments to accumulate in ponded areas. Loss of slopewash sediment would have the greatest effect farthest from the river, where the proportion of such sediment is highest. At our study sites, slope-wash and colluvial sediments were less volumetrically significant in site preservation than the protective cover provided by thicker fluvial and aeolian deposits. Exposures in other areas of the river corridor, however, include interbedded aeolian and local sediments; a thin stratum of poorly sorted debris-flow sediment forms a resistant cap that protects the more erodible aeolian sediment beneath. Therefore, reduced accumulation of the cap-forming slopewash sediment (caused by reconfiguration of local slope-wash drainage) could lead to increased erosion of aeolian sediment. 5. Conclusions Interpretation of sedimentary textures constitutes an important tool needed to understand geomorphic processes that affected the landscape on which prehistoric people lived, and those that affect the condition of cultural sites in Grand Canyon today. Similar methods are applicable to any geoarchaeological studies in arid regions where unaltered, nonpedogenic sediments are present. In the Colorado River corridor, the influence of fluvial sedimentation generally decreases away from the river as slope-wash and colluvial sediments become more significant. Flood deposits are commonly reworked by wind into aeolian coppice dunes. A facies consisting of ‘flood couplets’ was observed that would likely occur in other arid-land fluvial settings: a lower, fine-grained fluvial component and an upper, coarser, unit that reflects subaerial reworking at the land surface between floods. Grain-size distributions of strata can be used within this river corridor to distinguish mature aeolian deposits from finer-grained fluvial deposits that pre-date the influence of Glen Canyon Dam. Most archaeological sites in the three areas studied were formed primarily on interbedded fluvial and slope-wash sediments, with aeolian sand acting as an important agent of site burial and preservation. Sediment-rich floods are now absent in this regulated river, which, before dam construction, deposited fluvial sandbars annually that were sources of aeolian sediment. Archaeological sites that were formed on or buried by fluvial or aeolian sediment can, therefore, be considered sensitive, to varying degrees, to the effects of Glen Canyon Dam operations. Although preservation of artifacts on geologic time scales is unrealistic because of continual downcutting and backwasting of this bedrock canyon, cultural-site protection could likely be enhanced on decadal to century time scales by restoring fluvial and aeolian sedimentations to resemble more closely those

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processes that occurred before the river was regulated by upstream dams, including large, sediment-rich floods that left fluvial deposits from which sediment was remobilized by wind. Acknowledgements This project was sponsored by the U.S. Geological Survey and Bureau of Reclamation through the Grand Canyon Monitoring and Research Center. Permission to work in the study areas was granted by the Grand Canyon National Park and by the Hualapai Tribal Historic Preservation Office. N. Andrews, K. Burnett, M. Dai, B. Dierker, C. Fritzinger, S. Jones, T. Porter, M. Rubin, and E. Todd provided logistical support in the field and laboratory. This work has benefited from insightful discussion and comments by T. Melis, J. Balsom, M. Barger, L. Jackson, I. Bullets, and R. Hereford. We thank J. Warrick, C. Storlazzi, and D. Fenn of the USGS for their reviews of this manuscript, and three anonymous external reviewers for their comments and suggestions. References Ahlbrandt, T.S., 1979. 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