patterns in the landscape and erosion of cultural sites ... and obrien 2014... · patterns in the...

Research Article

Patterns in the Landscape and Erosion of Cultural Sites Along theColorado River Corridor in Grand Canyon, USAJoel L. Pederson* and Gary R. O’Brien

Department of Geology, Utah State University, Logan, Utah, USA

Correspondence*Corresponding author; E-mail:

[email protected]

Received24 January 2014

Revised23 May 2014

Accepted27 May 2014

Scientific editing by Gary Huckleberry

Published online in Wiley Online Library

(wileyonlinelibrary.com).

doi 10.1002/gea.21490

The geologic and geomorphic template of Grand Canyon influences patterns inthe archaeological record, including sites where apparent increases in erosionmay be related to Glen Canyon Dam. To provide geoarchaeological contextfor the Colorado River corridor and such issues, we explore first-order trendsin a database of field observations and topographic metrics from 227 culturalsites. The patterns revealed may be expected in other river-canyon settingsof management concern. The spatial clustering of sites along the river followsvariations in width of the valley bottom and the occurrence of alluvial terracesand debris fans, linking to bedrock controls. In contrast, the pattern of moreFormative (Ancestral Puebloan, 800–1250 A.D.) sites in eastern Grand Canyonand Protohistoric (1250–1776 A.D.) sites in western Grand Canyon does notfollow any evident geomorphic trends. In terms of site stability, wider reacheswith more terrace and debris fan landforms host a disproportionate numberof sites with acute erosion. This links most directly to weak alluvial substrates,and the primary erosion process is gullying with diffusive-creep processes alsopervasive. Although Glen Canyon Dam does not directly influence these ero-sion processes, overall sediment depletion and the loss of major flooding leaveserosion unhampered along the river corridor. C© 2014 Wiley Periodicals, Inc.

INTRODUCTION

The Colorado River through Grand Canyon National Parkin the American Southwest occupies a canyon-bottomlandscape that is diverse in both topography and land-forms, varying from gorges where only the river chan-nel itself intercedes between polished cliffs to valley bot-toms with a variety of interwoven geomorphic elements.The river corridor also preserves a human record rangingin age from Middle Archaic (5000–3000 B.C.) to historicAnglo use (Table I), and the changing character of thisrecord through time and over space is surely tied to thedynamics of the canyon landscape. Indeed, human settle-ment and utilization of the river corridor may be largelydictated by geomorphology because, in Grand Canyon,geology and landscape are amplified. Furthermore, un-derstanding the geomorphology of cultural sites (termused to represent the diversity of archaeological sites andhistoric features) along the river corridor is critical for un-derstanding their condition and predicting their stability.Following the closure of Glen Canyon Dam in 1963, con-cerns have mounted over myriad ecological and physical

transformations that have occurred. Despite these con-cerns, there have been no systematic, canyon-wide stud-ies relating geomorphic patterns to the archaeological andhistorical record.

The modern riverine environment of Grand Canyon isno longer subject to seasonal flooding and replenishmentof sand. Consequently, there is a reduced supply of sed-iment along the river corridor, as has been abundantlydocumented (e.g., Schmidt & Graf, 1990; Webb et al.,1999; Hazel et al., 2006; Wright et al., 2008). This sedi-ment depletion along the flanks of the river should hypo-thetically exacerbate erosion in this setting, and process-oriented studies have been conducted to document andunderstand the increased erosion of cultural sites alongthe river corridor. The most pronounced erosion at se-lect cultural sites is due to overland flow and gullying(Hereford et al., 1993; Pederson, Petersen, & Dierker,2006), but degradation of sites is also influenced by eolianprocesses of inflation and deflation (Draut, 2012), trailingand visitation, diffusive processes such as creep and rain-splash, and even mass wasting.

Geoarchaeology: An International Journal 29 (2014) 431–447 Copyright C© 2014 Wiley Periodicals, Inc. 431

PATTERNS OF LANDSCAPE AND CULTURAL SITES IN GRAND CANYON PEDERSON AND O’BRIEN

Table I Temporal classification of Grand Canyon culture history.

Archaeological identificationa Temporal range

Number of

sites in

datasetb

Paleo-Indian 12000–8000 B.C. 0

Early Archaic 8000–5000 B.C. 0

Middle Archaic 5000–3000 B.C.Archaic 22

Late Archaic 3000–1000 B.C.

Late Archaic/Early

Agricultural 1000 B.C. to A.D. 500Preformative 6

Basketmaker III A.D. 500–800

Pueblo I A.D. 800–1000

Pueblo II Formative A.D. 1000–1150 133

Pueblo III A.D. 1150–1250

Protohistoric A.D. 1250–1776 120

Historical A.D. 1776–1950 135

aNames based on a modified Pecos classification (Fairley, 2003).bA total of 162 sites have multiple cultural components, so are counted

here more than once, whereas 19 have undetermined affiliation.

Although Grand Canyon is of course an erosionallandscape in general, over the climate changes of theHolocene, a shifting balance has been struck between ero-sion and episodic deposition along the river corridor. Theevidence for this is a complex Holocene stratigraphy thatpreserves cultural sites, at least for some epochs. Rela-tively wet and dry intervals, both in Grand Canyon itselfand in the river’s Rocky Mountain headwaters, modu-lated the sediment supply and the flooding of the river aswell as other geomorphic processes at corridor archaeo-logical sites (O’Connor et al., 1994; Hereford et al., 1996;Davis et al., 2000; Draut et al., 2008; Tainer, 2010). Un-derstanding this deeper time context of site-formationprocesses is supremely challenging, but first-order spatial-geoarcheaological patterns can provide the groundwork,and both a spatial and temporal context is essential foruntangling the milieu of human and natural processespreserving and destroying cultural sites.

To provide part of the larger context for theseland-management issues, this study explores the first-order trends in a large observational and topographicdataset constructed through collaboration with the GrandCanyon Monitoring and Research Center of the U.S. Ge-ological Survey and the National Park Service (O’Brien& Pederson, 2009). Here, we present systematic geomor-phic data recorded at 227 of the cultural sites (somewith multiple loci) distributed along the length of theColorado River through Grand Canyon, visited by theauthors during several river trips. This diverse and largesample was chosen by land managers as being of inter-est for long-term monitoring and mitigation from a to-tal population of about twice as many recorded corridor

sites. These data were then combined with calculations oftopographic metrics as well as an existing National ParkService (NPS) database of site attributes.

We query this geoarchaeological database with a dualfocus on patterns linking the river-corridor landscape tothe archaeological/historic record and the geomorphicprocesses that preserve and destroy that record. Our find-ings confirm that the broadest patterns in the culturallandscape are tied to basic geologic controls on topog-raphy, though more specific correspondences betweensite archaeological identification and geomorphology arelacking along the Colorado River corridor. There are alsoclear relations between erosion and preservation of cul-tural sites and particular geomorphic processes and set-tings, and such patterns may be expected in other rivercanyons of management concern.

BACKGROUND

Geomorphic Setting

Topography has two modes in the Grand Canyon re-gion, with surrounding plateaus lying in sharp contrastwith the threshold hillslopes that define the canyon itself(Figure 1). Within the canyon, the steepness of both hill-slopes and drainages is strongly influenced by varyingbedrock properties. Erosionally resistant bedrock causestributary streams and the mainstem river to be confinedto narrow, steep-walled canyons, whereas reaches of thecanyon underlain by mechanically weaker bedrock andaffected by fault zones have wider valley floors (Howard& Dolan, 1981; Mackley, 2005; Pederson & Tressler,2012). Along the mainstem corridor, this latter condi-tion provides the accommodation space for larger de-bris fans at tributary junctions, a wider channel for theColorado River, and better preservation of Holocene de-posits. The notable examples of such reaches are FurnaceFlats in eastern Grand Canyon and the greater westernGrand Canyon reach, including the Granite Park area(Figure 1). These lower relief reaches in turn correspondto a greater number of recorded archaeological sites, andperhaps with more intense utilization throughout humanhistory (Fairley et al., 1994).

Another broad topographic pattern is the directionaltrend, or aspect, of the Colorado River corridor. Both thestretch of Marble Canyon and Furnace Flats to the eastand the western-central portion of the canyon that en-counters the Hurricane and Toroweap fault zones trendfrom north-northeast to south-southwest (Figure 1).Contrasting with this are the intervening stretches thattrend southeast to northwest, through the upper andmiddle Granite gorges as well as the far-western reachof the canyon through the Shivwits Plateau. These latter

432 Geoarchaeology: An International Journal 29 (2014) 431–447 Copyright C© 2014 Wiley Periodicals, Inc.

PEDERSON AND O’BRIEN PATTERNS OF LANDSCAPE AND CULTURAL SITES IN GRAND CANYON

Figure 1 Grand Canyon region of northern Arizona and its major physiographic features. Study sites (n = 227) along the Colorado River corridor are

marked by circles and are representatively clustered, especially in the Furnace Flats and western Grand Canyon reaches.

reaches generally correspond with Proterozoic basementrock at river level and relatively steep, narrow, and inac-cessible inner gorges, with fewer recorded cultural sitesconsidering the dearth of canyon-bottom real estate toutilize (Fairley et al., 1994). Because aspect has a strongcontrol on local climate conditions such as effective mois-ture, there also may be a correspondence of aspect to bothsettlement patterns and historic erosion problems.

Setting the stage more specifically, the major land-forms and deposits occupying the river corridor—fromadjacent slopes to the channel margin—include bedrockslopes/cliffs, talus, tributary debris fans, finer grainedalluvial and colluvial fans derived from smaller hills-lope catchments, alluvial terraces, and localized eoliandunes (Figure 2). The mainstem alluvial terraces includeboth relatively common, finer grained Holocene fill ter-races and higher Pleistocene gravelly fill terraces thatare preserved in the very widest reaches of the corridor.The surficial geology in key-wide reaches of the corri-dor has been mapped, described, numerically dated, andinterpreted in detail in other studies (Hereford, 1996;Hereford, Burke, & Thompson, 1998, 2000; Pederson

et al., 2011). Based upon that work, the Holocene ter-races of concern here are generally composed of silty veryfine to fine sand with well-preserved sedimentary struc-tures indicating flood deposition in eddies or backwa-ter settings. In many deposits, this alluvium interfingerswith pebbly colluvium or boulder-gravel debris fan sedi-ment toward the valley margin (Hereford et al., 1996). Fi-nally, vegetation-stabilized coppice dunes or active dunesof eolian sand mantle other landforms, or they are com-monly cored by alluvium (Hereford et al., 1993; Draut,2012).

The arid river corridor at the bottom of Grand Canyonreceives 213 mm of mean annual precipitation and has amean annual temperature of 20.4oC (at Phantom Ranch).Like topography, Grand Canyon’s precipitation regimehas two modes, with about half of the annual precip-itation in the form of high-intensity summer and earlyfall monsoonal storms and half in longer duration frontalsystems in the late fall and winter. Surficial processes inthe canyon link to this precipitation, and debris flows arearguably the most important way that sediment is de-livered from hillslopes to tributary drainages and to the



Figure 2 Schematic illustration of the major landforms and their relations along the river corridor. Alluvial terraces, debris fans, and eolian dunes are

grouped as “valley-bottom” landforms flanking the river axis, whereas bedrock, talus, and colluvial/alluvial fans are grouped as “canyon-slope” landforms

at the edges of the corridor.

Colorado River (e.g., Hereford et al., 1996; Melis, 1997;Griffiths, Webb, & Melis, 2004). The resultant boulderand cobble-rich debris fans deposited at tributary junc-tions largely define the Colorado River’s channel geome-try as well as the settings where historic and prehistoricflooding has deposited alluvium flanking and betweendebris fans (Schmidt, 1990; Figure 3). Eolian sedimenttransport is highly variable across the river corridor, withthe dunes mantling debris fans and alluvial terraces de-rived largely from the reworking of unstabilized flood de-posits along the channel margin (Draut, 2012; Figure 3).

Previous Geoarchaeological Studies in GrandCanyon

Geoarchaeological work along the Colorado River cor-ridor over the past few decades has been motivated byerosion problems. The first monitoring of the erosion ofcultural sites along the river corridor came immediatelyafter the unexpected July 1983 flood release from GlenCanyon Dam, which significantly reworked predam flooddeposits and negatively impacted some cultural sites. Anarchaeological inventory was completed in May 1991 bythe NPS along 410 km of the Colorado River corridor(Fairley et al., 1994). This led to the compilation of adatabase of corridor sites, including the surface observa-tions of archaeological identifications used in this study(Table I) as well as the continued monitoring of sites withdegrading integrity.

Many of the cultural sites of interest lie in the con-text of a suite of Holocene stream terraces, adjacent debrisfans, and capping eolian deposits. Although the chronos-tratigraphy of these Holocene deposits is not within thepurpose or scope of this particular study, a review of pre-vious work related to both deposition and erosion is inorder. Studies of the Holocene stratigraphy by Herefordand others (1993, 1996) found that archaeological sitesare frequently located in deposits they called the “allu-vium of Pueblo II age” and “striped alluvium,” datingfrom A.D. 700 to 1200 and 2500 B.C. to A.D. 300, re-spectively. These late Holocene deposits generally havenot been inundated by historic flows of the ColoradoRiver. Yet, flooding did create two other inset deposits,the protohistoric “upper mesquite” and historic “lowermesquite” alluvial terraces. Hereford and others notedthe possibility that apparent time gaps in the archaeo-logical record actually result from the episodic erosionevident in the corridor stratigraphy. Likewise, the Daviset al. (2000) study at two sites in eastern Grand Canyonfound buried soils/cultural horizons overtopped and re-worked by flooding and episodic erosion that may havecaused breaks in occupation due to unsuitable farmingconditions. Finally, Anderson and Neff (2011), in theirstudy of other Ancestral Puebloan sites in eastern GrandCanyon, relate the changing position of cultural featuresthrough time to modeled flood lines and interpret thatthe Colorado River’s flood dynamics directly influencedsettlement patterns.



Figure 3 This overview is to the west at the downstream end of the wider Furnace Flats reach as it transitions into the narrower Upper Granite Gorge.

Examples of the alluvial terrace, debris fan, eolian, talus and bedrock landforms, substrates, and processes are evident, and valley-bottom width and the

gradient and aspect measurements taken at study sites are illustrated.

In terms of the surface processes associated with the re-cent erosion of cultural sites, Hereford et al. (1993) andThompson and Potochnik (2000) documented that gullyincision increased dramatically between 1973 and 1984,based on analysis of historic aerial images and repeat pho-tographs of sites. Hereford et al. (1993) also studied pre-cipitation records and proposed that a period of more in-tense precipitation from the late 1970s through the 1990sdrove accelerated erosion. These and subsequent empir-ical studies indicate that gullying is the most acute ero-sion process at cultural sites, driven by infiltration-excessoverland flow in this semiarid to arid landscape withhigh-intensity precipitation events (Pederson, Petersen, &Dierker, 2006).

An erosion process of secondary importance, but whichis ubiquitous across the canyon, is creep, especiallythrough rainsplash and bioturbation. Although creepprocesses are incremental and therefore subtle, they havea strong cumulative effect on site integrity and preserva-tion over the centuries. An empirical study at one site ineastern Grand Canyon indicates that particles can creepdownslope at rates of 5–10 cm/yr, rapidly taking out ofcontext any artifacts that have emerged onto the landsurface (Tressler & Pederson, 2010). Recent studies have

also focused on quantifying eolian sediment transport asa process affecting cultural sites. Draut’s (2012) researchat selected sites highlights the very strong spatial variabil-ity of wind, moisture, and eolian transport along the rivercorridor, making linkages between increased erosion, re-duced eolian deposition, and reduced sediment supplyfrom sandbars in the postdam era.

METHODS

Observational Field Data

Data from 227 cultural sites are presented here; 16 ofthem have two spatially distinct loci and so n = 243.This large sample of sites was assessed in the field dur-ing five Grand Canyon river trips in 2006 and 2007 aswell as one trip upstream into the Lower Granite Gorgefrom Lake Mead. Systematic observations were recordedusing a standardized form designed to capture specific ge-omorphic attributes (Table II). With the exception of re-stricted information on site location and archaeologicalidentification, the full data illustrated and discussed hereare available in Supplementary Table 1. Sites frequentlyextend across more than one landform and have a variety



Table II Components of the field database explored in this study.

Category −−−−−−−→ Subcategories

Landforms Bedrock Talus Debris fan Alluvial terrace Colluvial/alluvial fan Eolian dunes

Substrate Debris-fan

diamicton

Eolian sand Alluvial sand Slopewash Bedrock Talus

Area covered by

soil crust

0% 1–25% 26–50% 51–75% 76–90% >90%

Characteristics of

soil crust

Consistent, mature Consistent,

immature

Intermittent,

mature

Intermittent,

immature

Geomorphic

processes

Overland flow Diffusive/creep Eolian Visitation effects Depositional

Erosion rankinga 1 = stable (erosion

absent or barely

discernable)

2 = mild (subtle

erosion within an

overall stable

site)

3 = intermediate

(active impacts

to site, but

treatable)

4 = serious

(active erosion

posing threat to

features)

5 = severe (active

degradation of most of

the site)

Archaeological

identificationbArchaic Preformative Formative Protohistoric Historic

aSee O’Brien and Pederson (1994) for detailed erosion-ranking criteria.bPre-existing data provided by NPS (see Fairley et al. 1994, for example).

of substrate and surface characteristics. Our approach wasto rank landforms, substrates/deposits, and surface pro-cesses according to their predominance within the officialsite areas. In the case of subequal landforms that transi-tion across a study site, they were ranked from bottom-up stratigraphically. Each site was also assigned an overallranking expressing stability or the degree of erosion evi-dent (Table II). Details of the criteria used for each subcat-egory of field observations can be found in O’Brien andPederson (2009), beyond the basic explanation providedhere.

In terms of landforms, “bedrock” includes canyonwalls, cliffs, shelters below ledges, and caves or alcoves(Table II). We make a distinction between debris fans andcolluvial/alluvial fans. The former are steep and coarsefans of debris-flow diamicton that constrict the ColoradoRiver at tributary junctions, whereas the latter are un-derlain by finer sediment from overland flow off smallerhillslope catchments onto the valley bottom in widerreaches. Our observations include the condition and cov-erage of biotic-soil crust, which is ubiquitous throughoutthe corridor and influences both sediment cohesion andinfiltration (Pederson, Petersen, & Dierker, 2006). Theserange from insipient rainsplash crusts to dark and ruggedbiological-soil crusts.

Many distinct geomorphic processes were documentedin the field, which we have categorized into five groupshere: (1) “overland flow” includes slopewash, rilling, pip-ing, and gullying (Figure 3); (2) “diffusive” processesrecorded include soil and particle creep, rainsplash, bio-turbation, and in situ physical weathering; (3) “eolian”processes include both deposition and deflation; (4) “vis-itation effects” are mostly erosion caused by trailing, but

include rare instances of artifact relocation and graffiti;and finally, (5) “depositional” processes were noted inthe rare instances of significant alluviation within culturalsite areas (Table II).

One of our priorities in the field was to document ateach site how acute and prevalent erosion was, or con-versely, how stable the site was. To make this systematic,we utilized an erosion ranking of 1–5, ranging from “sta-ble,” where little or no erosion is documented across asite, to “mild,” “intermediate,” “serious,” and finally “se-vere” where acute erosion was destroying much of thecultural value of the site (Table II). It is important to rec-ognize that our erosion ranking reflects the condition ofthe site area and cultural features at the time the obser-vations were made, which may not accurately reflect thestability over longer timescales or in the archaeologicalrecord.

Archaeological Identifications

We combined into our dataset the site archaeologicalidentifications provided by the NPS, as determined byarchaeologists from surface features or artifacts apparentduring the original survey of sites (Fairley et al. 1994),as well as during subsequent monitoring visits. To enablefirst-order analysis of a greater number of each, we sum-marized the more detailed archaeological identificationsapplied by the NPS into general categories of Archaic,Preformative, Formative, Protohistoric, and Historical(Tables I and II). These associations are not mutuallyexclusive, such that about two-thirds of sites have multi-ple affiliations, and about 8% of sites are undetermined.These surficial archaeological determinations inevitably



underrepresent cultural features that are buried withindeeper stratigraphy, as well as generally older culturesthat are less preserved. Examples of this are illustratedin the relative dearth of Preformative (1000 B.C. toA.D. 800) associations, which have only been found inthe subsurface in this setting, as well as older Archaicassociations that may be buried or poorly preserved(Table I). Despite these caveats, we can still draw outbroad patterns of the overall frequency of recordedarchaeological identifications across the corridor.

Site Distribution and Terrain Metrics

The database is partly analyzed with respect to site loca-tion, specifically by river mile as measured downstreamfrom Lee’s Ferry (Figure 1), extending to river mile 260in the western end of Grand Canyon. The calculationand extraction of terrain metrics for each site was con-ducted in ESRI ArcMap software. Mean values of topo-graphic slope and aspect (azimuth direction of slope, orthe direction a landform faces, Figure 3) were extractedfor the area of each NPS site polygon. For slope, this uti-lized a 1-m terrain model of the river corridor developedthrough photogrammetry and provided by the GrandCanyon Monitoring and Research Center. In the case ofaspect, mean values were more accurately obtained us-ing less detailed 30-m digital elevation models (DEMs)from the U.S. Geological Survey, calculated across thosesite polygons that have an overall slope. Finally, calcu-lations of valley-bottom width are from Mackley (2005)and were made at 500-m intervals along the river, nor-mal to the channel, utilizing a 10-m DEM. Valley-bottomwidth in this study is the distance from hillslope/bedrockwall to the opposite hillslope/bedrock wall, 5 m above thechannel, just above the level of the flood plain and mostHolocene terrace deposits (Figure 3).

RESULTS AND DISCUSSION

Geoarchaeological Patterns Along the RiverCorridor

This study involves about half the total recorded ColoradoRiver corridor cultural sites, and the spatial distributionof our large sample matches the overall pattern of allsites (O’Brien & Pederson, 2009). Archaeological sites arenot evenly distributed through the corridor, with concen-trations found in the Furnace Flats and western GrandCanyon reaches, and relatively few in the interveninggorges (Figure 4).

It has been hypothesized that this uneven distribu-tion is linked to broader geomorphic controls; for exam-ple, the steepness of surrounding terrain as it dictates

routes of human access from the canyon rim to bot-tom (Euler & Chandler, 1978; Fairley et al., 1994). Set-tlement patterns may also reflect susceptibility to flood-ing and preservation potential, as well as the presenceof key terrace landforms and a broader riparian land-scape that could be more intensively utilized by people(Fairley et al., 1994; Fairley, 2003). A comparison of sitedensity and our measurements of canyon-bottom widthconfirm these trends (Figure 4). The narrow bedrockgorges, where the canyon bottom is hardly wider than theriver channel itself, are strongly associated with low cul-tural site density. Conversely, the abundant cultural sitesof the Furnace Flats and western Grand Canyon reachescorrespond with canyon bottoms that are twice the av-erage width of the narrow gorges, providing accommo-dation space for the distinct landforms and resources ofthose areas.

An instructive exception to this trend is the upper Mar-ble Canyon reach (river miles 0–35), which has moderateto high average canyon-bottom width, but relatively fewsurveyed cultural sites (Figure 4). This reach is marked bydeeply entrenched bedrock meanders, and although thecanyon bottom does have some accommodation space,the river corridor is also typically hemmed in by nearlyvertical cliffs of Paleozoic bedrock. Thus, in these upperreaches, the metric of canyon-bottom width fails to cap-ture some key controlling factor, such as accessibility ofthe canyon bottom by foot (Fairley et al., 1994). This up-per Marble Canyon exception also argues against theirbeing significant roles played by preservation potentialrelative to floods and space for resources. That is, in up-per Marble Canyon, the relatively wide canyon-bottomprevents wholesale flood scouring and it allows for morevalley-bottom resources, yet the density of sites is rela-tively low for some other reason not discernable with ourdata focused along the river corridor.

The distribution of different landforms and their asso-ciated deposits varies along the river corridor, as recordedat any ranking of prevalence at the study sites (Fig-ure 4C). For first-order trends, we group landform typesinto those lining the river banks along the “valley-bottom” axis (alluvial terraces, eolian dunes, debris fans)versus the “canyon-slope” landforms at the edges of thecorridor (talus, colluvial/alluvial fans, bedrock; Figures 2,3). The relative percentage of these landform groups illus-trates an expected and strong trend of axial valley-bottomlandforms being more predominant in wider reaches withmore accommodation space for such deposits, gener-ally matching Figure 4A. This includes the lower Mar-ble Canyon, Furnace Flats, and western Grand Canyonreaches, and the predominance of riparian valley-bottomlandforms is therefore associated with a greater numberof recorded cultural sites (Figure 4B).



Figure 4 Trends along the length of the Colorado River corridor through Grand Canyon, by river mile in 5- or 10-mile bins. (A) Mean valley-bottom width,

which reflects changing geologic controls along the corridor. There are no data below river mile 235 due to Lake Mead. (B) Uneven distribution of the 243

cultural sites and loci in our dataset, illustrating clusteringof sites in Furnace Flats andwesternGrandCanyonanda correspondence to valley-bottomwidth

except in upper Marble Canyon. (C) Normalized distribution of landforms recorded at cultural sites by river mile, grouped as canyon/slope landforms at

the corridor edge (talus, colluvial/alluvial fans, bedrock) versus riparian valley-bottom landforms (alluvial terraces, debris fans, eolian dunes). Note general

correspondence to valley-bottom width. (D) Mean erosion ranking within bins appears to correspond to the occurrence of valley-bottom landforms;

higher ranking indicates more severe erosion, mostly by gullying.



Looking more closely at the distribution of the individ-ual valley-bottom landforms that tend to contain sites,alluvial terraces and debris fans comprise a relatively pre-dictable component wherever valley-bottom landformsare significant, whereas eolian dunes at cultural sites aremore variable in occurrence, with no systematic trendapparent at this scale. At those cultural sites found onthe valley-bottom suite of terraces, alluvial terraces are aconsistent component at 20–40% of them in both east-ern and western Grand Canyon. Debris fans are a moredominant component (40% or more) along the narrowerinner gorge reaches, and they also dominate cultural sitesrecorded from river miles 20–40, including the reachknown as the “roaring 20s” by whitewater rafters for thefrequent rapids caused by debris-fan constrictions. Yet,important for our investigation into broad trends alongthe corridor, the distribution of landforms at cultural sitesdoes not show any particular distinction between easternand western Grand Canyon. In the main Furnace Flatsand western Grand Canyon reaches, both with a healthyproportion of valley-bottom landforms, the alluvial ter-races, debris fans, and eolian dunes all comprise a sube-qual proportion of site areas.

Finally, a somewhat unexpected spatial pattern existsregarding the current erosional condition of sites, seem-ingly matching the overall distribution of sites and valley-bottom landforms along the river corridor (Figure 4D).Of course, in reaches with a greater number of culturalsites to begin with, one will find more sites with erosionproblems. But even when our ranking of site erosion isaveraged over 10-mile bins, and we include only thosebins with more than two sites to reduce this frequencybias, more intense erosion of sites appears to mirror theoccurrence of valley-bottom landforms and therefore alsothe overall density of sites along the corridor (Figure 4).For example, there is a higher proportion of unstable siteswithin Furnace Flats and western Grand Canyon, witherosion rankings of serious (4) or severe (5) clustered inthose reaches (O’Brien & Pederson, 2009). This supportsthe idea that these wider areas with their distinctive land-forms are more active in terms of erosion processes, par-ticularly gullying, as discussed below.

Trends in Archaeological Identifications

The distribution along the corridor of the generalizedarchaeological affiliations recorded for the study sitesillustrates a long-recognized trend across the canyon(Fairley et al., 1994; Figure 4). In order to removethe bias from varying site density across the canyon,we illustrate archaeological identifications as a relativepercentage of those recorded within 10-river-mile bins(note that “Preformative” or Early Agricultural and

Basketmaker III affiliations are not included because oftheir low overall frequency in the dataset). The primarytrend is that “Formative” or Puebloan is an associationnotably more abundant in the eastern river corridor,whereas Protohistoric site associations are recordedwith increasing dominance in western Grand Canyon.Although Archaic sites appear to be more abundant ina short intermediate reach within the Upper GraniteGorge (Figure 5), this is uncertain because the river mile100–110 bins contain only three sites total.

There are many possible reasons for this distinct trendin the human record of the corridor, including those thatare purely cultural or territorial and others that relate tochanging climate and plant resources across the canyon.Our task here is to investigate any possible relations togeomorphology. It has been hypothesized that systematiceast to west trends in both the type and the age of depositscontrol what archaeologists record at the surface com-pared with what may be hidden in the subsurface (Fair-ley, 2003). Furthermore, beyond stratigraphy and preser-vation, cultural patterns in the landscape also may followunderlying geomorphic trends in landforms and deposittypes, as past cultures may have adapted to certain corri-dor settings.

Although landform distribution relates most broadly tovalley-bottom width (Figure 4), and the relative propor-tion of the specific terrace and debris-fan landforms mostassociated with sites changes little across the corridor,there still may be a geomorphic explanation for the east-to-west trends in archaeological identifications. A fruit-ful approach is to take each generalized archaeologicalidentification and inquire about the landform types it isassociated with at sites, regardless of location along thecorridor. When the dominant (first-ranked) landformsrecorded at sites are plotted according to associated ar-chaeological identifications (Figure 6), the relatively fewsites with Archaic affiliation have the strongest trend.Fully 43% of sites dominated by debris fans include anArchaic component, whereas this is true for only 5% ofsites dominated by alluvial/colluvial aprons and eoliandunes. This disproportionate appearance of Archaic sitesat debris fans may be due to greater stability of debris-fansubstrates and preservation of older archaeology there(e.g., Hereford et al., 1996), or it could possibly reflect areal preference for these features by mobile Archaic pop-ulations.

Regarding the primary pattern of more eastern GrandCanyon Formative sites and more western Grand CanyonProtohistoric sites along the river corridor, the two ar-chaeological identifications have very similar and equabledistributions among dominant landforms, with the ex-ceptions of alluvial/colluvial aprons and eolian dunes(Figure 6). Formative sites occupy alluvial/colluvial



Figure 5 Relative proportion of recorded archaeological identifications along the river corridor within 10-river-mile bins; gaps are bins with no sites. Note

the trend of Formative affiliations dominating in the upper reaches of the corridor versus Protohistoric associations downstream.

aprons somewhat more often than other archaeologi-cal associations. In fact, the majority of alluvial/colluvialaprons in our dataset lie in the wider Furnace Flatsreach where Formative sites are clustered. Similarly, Pro-tohistoric sites occupy eolian dunes in relatively higherproportions. To some degree, this is stratigraphically in-evitable because both Protohistoric sites and eolian dunesgenerally occupy the tops of other landforms along thecorridor.

A final possible link between culture and landscapeacross the length of the river corridor lies in topographicaspect—the direction a landform or surface dips or faces.

A rose diagram of all study sites where aspect could bemeasured (not including rock art sites and those with-out significant slope) illustrates strong variability and aslight tendency for site landforms to face either east-southeast or west-northwest (Figure 7A). This reflectsthe trend of the river and canyon itself in the Fur-nace Flats and western Grand Canyon reaches wheremost cultural sites lie (Figure 1), considering that land-forms along the canyon bottom tend to slope toward(normal to) the river channel (Figure 3). A compari-son of the aspect of sites with a Formative component(predominantly in eastern Grand Canyon) and sites with

Figure 6 The distribution of generalized archaeological identifications according to dominant (first-ranked) landform type at study sites. Archaic sites

appear disproportionately on debris fans, Formative sites occur more often than others on colluvial/alluvial fans, and Protohistoric sites are associated

more frequently with eolian dunes.



Figure 7 Topographic aspect of cultural-site areas. (A) The full sample of corridor sites where aspect could be calculated. (B) Such sites with Formative

listed as an identification at any ranking. (C) Such sites with Protohistoric as a component at any ranking, which are disproportionately southwest-facing.

Rose diagrams show relative number of sites (infilled gradations) that face each direction in 10-degree azimuth increments, with north being up (0

degrees).

a Protohistoric component (predominantly in westernGrand Canyon) reveals a contrast. Formative sites in oursample are set on landforms that preferentially face eithernorthwest or southeast, while Protohistoric sites tend toface a wider range of aspects, but with a disproportionatenumber in a southwesterly direction (Figure 7B and C).The aspects of Formative sites are consistent with theirdominance in the Furnace Flats reach where the rivertrends northeast to southwest (only 18% face south-west). Yet, the river in western Grand Canyon has thesame trend, and therefore such an explanation cannotaccount for the fact that 29% of Protohistoric sites facesouthwest and only 15% northeast. Although not a ma-jor pattern, this observation suggests Protohistoric peoplemay have preferentially utilized the southwest-facing as-pects of the landscape for their distinct, perhaps seasonal,purposes.

Analysis of Erosion and Site Stability

Principal landforms, substrates, and processes

Along the valley-bottom axis of the river corridor, de-bris fans and alluvial terraces are the most common first-ranked landforms, each dominating about 20% of sites.Eolian dunes are less common as the first-ranked land-form (12% of sites), yet they appear as subsidiary, lowerranked landforms at nearly half of the study sites. Ofthe canyon-slope landforms more distal from the river,bedrock is the most common, being first-ranked at 21%of study sites. These are mostly features under rock ledgesor shelters as well as rock art localities.

Related substrate materials should have a control onerosion processes through physical resistance measuredby the caliber and cohesion of sediment. At our studysites, sandy alluvium, eolian coppice, slopewash fines and

gravels, and coarser debris-fan sediments all exist as thedominant substrates in subequal proportions. In contrast,sites underlain by bare bedrock or talus, or that havebeen stabilized by vegetation and soil crusts are all rel-atively rare as primary substrate or surface cover. Yet,resistant biological soil crusts appear very frequently asa subsidiary surface cover, recorded at some ranking atnearly half of sites. In fact, soil crust is different from theother substrate categories of the database, in that it de-velops on any of the other fine-grained substrates, givensome surface stability.

More directly responsible for erosion than landforms orsubstrate are surface processes. Our ranking of processesactive at sites indicates that throughout the corridor,overland flow (including gullying, rilling, slope wash, andpiping) is indeed the dominant class of process at nearlyhalf of cultural sites, and it is also the most pervasiveacross all sites when tallied at any ranking (Figure 8). Dif-fusive processes of creep, rainsplash, bioturbation, and in

situ weathering are first-ranked less often (28%), but theyare similarly pervasive, with creep specifically being thesingle most pervasive individual surface process, recordedat more than half of all sites at some ranking. Theincremental but persistent nature of creep results in greatcumulative and detrimental effects, causing nearly all ex-posed site features and artifacts to move out of context.Finally, eolian processes and visitation impacts—typicallyhuman trailing, which breaks soil crusts and promoteschannelized overland flow—are present to a lesser yetstill significant degree (Figure 8).

Erosion end members

Does the problem of erosion of cultural sites alongthe Grand Canyon corridor exhibit patterns relative tothese principal landforms, substrates, and geomorphic



Figure 8 Relative frequency of first-ranked surficial process classes and surficial processes reported at any ranking at study sites. Overland flow is the

predominant erosion process and both overland flow and diffusive processes, including creep, are pervasive across sites to some degree. Active alluvial

deposition processes are rare at sites.

processes? Our visual assessment of erosion severity (Ta-ble II), when tallied for all study sites, reveals patternsthat provide important insight into the causes and poten-tial mitigation of the erosion. First, half of the sites in thisdataset are documented as stable or only mildly affectedby erosion (Figure 9). More than a quarter of sites areranked intermediate, and only 14% and 7% of sites areranked as having serious or severe erosion, respectively.On the other hand, these last two categories reflect acuteerosion at nearly 50 sites of critical concern along the cor-

ridor just in our sampled dataset. These are cultural siteswhere resources and information are actively being lost.

A first-order expectation regarding the stability of sitesunder geomorphic processes is that the gentlest, low-est gradient settings will be more stable and those inthe steepest places will be the most acutely eroded. In-terestingly, this trend does not occur in Grand Canyon(Figure 9); sites with serious or severe erosion actuallyhave a lower mean gradient than other sites. Those thatare stable have the highest mean gradient, even when

Figure 9 Percentage of study siteswith each erosion ranking (see Table II for ranking criteria), and themean slope gradientwithin each ranking, excluding

rock-art sites. Serious or severe erosion is the exception, not the norm, and note that steepness of site area does not correspond to increasing severity

of erosion.



excluding the 12 rock-art sites in the dataset, which lieupon anomalously steep ledges. The fact is that the ero-sion of cultural sites in Grand Canyon is not a simple storythat can be encapsulated by a basic metric such as gradi-ent. Instead, an end-member analysis is useful in under-standing the more complex relation of cultural-site stabil-ity and trends in the landforms, substrates, and processesat each site. Sites are grouped into those that are stableor mildly eroded and those that are seriously or severelyeroded, while ignoring those with intermediate erosion.

In terms of landforms, it is intuitive that sites on highlyresistant bedrock walls and within bedrock shelters aremostly stable (Figure 10). Other landforms with largelystable sites are coarse talus and debris fans, but as oneapproaches the valley-bottom axis with sites on alluvialterraces, stability is much less frequent. Nearly half ofsites with alluvial terraces as the primary landform ex-hibit acute erosion, and colluvial/alluvial fans are simi-larly unstable (Figure 10). Thus, a first-order pattern isthat relatively stable cultural sites are found on land-forms of resistant substrate farther from the river, whilethe least stable sites are on landforms with fine-grainedsediment nearer the river. Sites in fine-grained eoliandunes, which are nearer the river axis but neither par-ticularly stable nor acutely eroded, are an exception. Wenote that the very high infiltration rate of eolian sandserves as a buffer to overland flow in this setting (Ped-erson, Petersen, & Dierker, 2006). Indeed, the stabilitytrend is linked most directly to parallel trends in substratecaliber and cohesion, not landform position (O’Brien &Pederson, 2009). Bedrock and coarse, poorly sorted talusand debris-fan sediment is mechanically more resistant toerosion than silty-sand alluvial deposits. These relationsexplain the inverted trend with gradient noted above(Figure 9); the relative stability of steeper talus anddebris-fan landforms is largely due to them being under-lain by much coarser and more resistant sediment.

Regarding the five categories of geomorphic processes,there are very strong trends with stable and acutelyeroded sites (Figure 11). Because there are about twiceas many stable sites as acutely eroded sites along thecorridor (Figure 9), a relatively high proportion of sitesare also stable for most process categories. Yet, sites dom-inated by overland-flow processes are acutely eroded inby far the highest proportion, nearly half of them. Acuteerosion is also somewhat common at sites where humanvisitation is the dominant process, partly because trailsmay become channels for overland flow. Not surpris-ingly, stable sites are found in the highest percentageseither where incremental diffusive/creep processes aredominant or where there is actual deposition rather thanerosion.

A final interesting, but less intuitive result of the end-member analysis relates to the topographic aspect ofsites. Within the overall population of sites shown inFigure 7A, a disproportionately high number of acutelyeroded sites face (or slope) either northwest or east-southeast, whereas almost no acutely eroded sites facenortheast or southwest (Figure 12). First, we have es-tablished that the many sites lying along the FurnaceFlats and western Grand Canyon reaches, where the rivertrends northeast to southwest (normal to the trend of Fig-ure 12A), more frequently lie within the context of al-luvial terraces and weak substrates subject to erosion byoverland flow. A possible secondary influence could bea meteorological phenomenon of more intense moisturebeing focused where canyon topography lies parallel tothe prevailing storm tracks from southwest to northeast.This was suggested by Griffiths, Webb, and Melis (2004)to account for modeled tributary debris-flow frequencybeing notably higher in reaches of Grand Canyon withthis same aspect trend.

CONCLUSIONS

Our goal has been to explore the most basic patterns be-tween the diverse and dynamic river corridor landscapeand the archaeological and historic record, including itserosion and preservation. The unique geology and en-vironment of Grand Canyon makes such inquiry espe-cially pertinent, and this represents the first systematic,canyon-wide exploration of such patterns and linkages.Yet, this dataset warrants further development, updat-ing, and certainly statistical analysis, as a tool for land-management and broader inquiry.

Patterns of Landscape Context

The clustering of cultural sites into eastern and westernGrand Canyon reaches tracks the width of the valley-bottom landscape. Our metric of valley-bottom width isintended to capture the Colorado River’s lateral erosionand widening over geologic timescales, and is inverselycorrelated with bedrock strength (Mackley, 2005). Thisbackground geologic control ultimately results in spe-cific reach properties pertinent to geoarchaeology. Thegreater accommodation space of wider reaches results ina greater proportion of valley-bottom alluvial terrace anddebris-fan landforms. It also provides the potential forgreater access by foot, resources and habitability, and bet-ter preservation in the face of flooding, when comparedto the narrow gorges (Fairley et al., 1994). This pattern inGrand Canyon parallels that identified by Nials, Gregory,



Figure 10 Percentage of sites with a given first-ranked landform, which are associated with stability (light gray) and with acute erosion (dark gray) end

members. Sites on alluvial terraces are more subject to active erosion while those associated with resistant bedrock or talus are mostly stable.

and Hill (2011) in their study of geoarchaeological pat-terns across the broader alluvial valleys of the basin andrange of southern Arizona and New Mexico. There, cul-tural site density is greater near fluvial-reach boundarieswhere geologic and geomorphic conditions create largerfloodplain areas and more available surface and ground-water for irrigation agriculture.

In contrast, clear trends between broad geomorphol-ogy and recorded archaeological identifications along theColorado River corridor are lacking in our data, and thismay be partly due to the reconnaissance-survey nature

of the cultural data. For example, the more frequent Pro-tohistoric affiliation of sites in western Grand Canyonand Formative in eastern Grand Canyon has no clearlink to geomorphic differences from eastern to westernGrand Canyon in landforms, substrates, or active geo-morphic process. Also, the chronostratigraphic record ofthe river corridor indicates there is no discernable trendin deposit ages or preservation across the canyon thatwould account for this trend in archaeological identi-fications (Hereford et al., 1996; Pederson et al., 2011).Yet, a few trends are intriguing. Sites that include a

Figure 11 Percentage of sites with a given first-ranked geomorphic process, which are associated with stability and acute erosion. Nearly half of sites

dominated by overland flow are acutely eroded.



Figure 12 Study site aspect plotted for (A) sites ranked as serious or severe in terms of erosion; and (B) sites ranked stable or mild in terms of erosion.

Acutely eroded sites are disproportionately facing northwest and east-southeast and clustered in the Furnace Flats and western Grand Canyon reaches,

related to weak alluvial substrates and potentially storm tracks.

Protohistoric component preferentially have southwest-erly aspects, and Archaic sites tend to be set upon de-bris fans. Such patterns are worthy of further investiga-tion, and it is possible they may relate to past people’schoices regarding resources, seasonal activity, and sunexposure. Regardless, our results generally suggest that,although geology and geomorphology set the broad pat-terns of where sites occur along the Grand Canyon corri-dor, the specific trends within that cultural landscape areinstead mostly controlled by cultural, territorial, or bio-logical drivers.

Erosion and Stability of Cultural Sites

Preserved cultural sites are more prevalent in reacheswhere there are greater proportions of axial valley-bottom landforms, such as in the Furnace Flats and west-ern Grand Canyon reaches. Unfortunately, our data in-dicate that those valley-bottom landforms and weak sub-strates also host a disproportionate number of sites withacute erosion problems. This is not due to steep slopesalong the valley bottom; instead, the steeper landformsthat are generally farther from the river and underlain bycoarser and more cohesive sediment are associated withrelatively stable sites.

A linkage of site stability specifically to substrate resis-tance makes sense, considering that the primary erosionprocess at sites with acute problems is gullying, whichhinges upon the entrainment of grains by the flow of wa-ter. Although there is the secondary possibility of mete-orological controls, the preferential northwest-southeastaspect of acutely eroded cultural sites likewise must be a

result linked to the dominant process of overland flow.The finer, weaker alluvial substrates are more preva-lent in the wider, densely populated areas of easternand western Grand Canyon, and the river trends normalto those aspects in those reaches. Although gullying ac-counts for most acute erosion issues, human visitation isthe next most significant process, and it is one that per-haps can be more easily managed. Finally, diffusive-creepprocesses are pervasive across sites, and in the long run,they play an insidious role in the degradation of sites inGrand Canyon.

The driving land-management question behind thisstudy and other geoarchaeological research in GrandCanyon is how these erosion issues may relate to theoperation or presence of Glen Canyon Dam, which haschanged the balance between flooding and sediment sup-ply downstream. The goal of this study is to providespatial-geomorphic context, not to address process link-ages to the presence or operation of the dam. Yet, thislarge dataset quantitatively confirms that the dominanterosion process at sites is overland flow. In this, the Col-orado River corridor shares the irony of most drylandsettings—despite water deficit defining the landscape, it isflowing water that dominates surface processes. Gullyingrelates most directly to local topography and runoff, ascontrolled by weather patterns and climate shifts (Here-ford et al., 1993; Pederson, Petersen, & Dierker, 2006).Although the dam does not play into either of these di-rect controls, overall sediment depletion in the river cor-ridor may still have a role. Our data indicate erosion ismost acute in weak, fine-grained substrates proximal tothe river. With the loss of fine-grained sediment and lack



of replenishing floods along the river margin, the ero-sion of such deposits is unhampered, even if the processlinkage is indirect, as through lower local baselevel forgully systems (Hereford et al., 1993) or a decrease in eo-lian reworking of flood deposits (Draut, 2012). Althoughour field-survey data indicate eolian landforms and pro-cesses, with some exceptions, are not predominant fac-tors at most sites, the current state we have recorded maybe altered from previous conditions.

Grand Canyon serves to highlight these issues of geoar-chaeology and human impacts, but there are manydammed rivers with downstream landscapes where cul-tural sites are a management concern. We submit that themost robust patterns evident along the Colorado Rivercorridor between geology, river-corridor topography, andsite distribution, and between alluvial landforms, weaksubstrates, and site stability, are trends that should beexpected elsewhere. Though these are rather intuitive,quantifying and confirming such patterns provides thegroundwork for making predictions, including for man-agement purposes. Thus, inquiry with spatial datasets canprovide part of the context for the human record, as wehave done here, but we hope it may also help assure itspreservation.

This project was supported by the Cultural Program of the GrandCanyon Monitoring and Research Center, and we thank direc-tor Helen Fairley. National Park Service archaeologists JenniferDierker, Lisa Leap, and Amy Horn were essential for directionand interpretation of cultural sites. Two anonymous reviewsand the comments of the editors made this a clearer and bet-ter contribution. Thanks go to many student field assistants fromUtah State University, including Chris Tressler, Susannah Erwin,Mike Keller, Ben DeJong, Alex Steely, and Erin Tainer, MichelleSumma, and Jonathan Harvey for both field help and data re-duction. Finally, thanks to the boatmen and women who con-veyed us downriver: Brian Dierker, Chris Mengel, Bruno, AnnieAnderson, and Brian Hansen.

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