Research Article
Patterns in the Landscape and Erosion of Cultural Sites Along the
Colorado River Corridor in Grand Canyon, USA
Joel L. Pederson* and Gary R. O’Brien
Department of Geology, Utah State University, Logan, Utah, USA
Correspondence
* Corresponding author; E-mail:
[email protected]
Received
24 January 2014
Revised
23 May 2014
Accepted
27 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 in
the archaeological record, including sites where apparent increases in erosion
may be related to Glen Canyon Dam. To provide geoarchaeological context
for the Colorado River corridor and such issues, we explore first-order trends
in a database of field observations and topographic metrics from 227 cultural
sites. The patterns revealed may be expected in other river-canyon settings
of management concern. The spatial clustering of sites along the river follows
variations in width of the valley bottom and the occurrence of alluvial terraces
and debris fans, linking to bedrock controls. In contrast, the pattern of more
Formative (Ancestral Puebloan, 800–1250 A.D.) sites in eastern Grand Canyon
and Protohistoric (1250–1776 A.D.) sites in western Grand Canyon does not
follow any evident geomorphic trends. In terms of site stability, wider reaches
with more terrace and debris fan landforms host a disproportionate number
of sites with acute erosion. This links most directly to weak alluvial substrates,
and the primary erosion process is gullying with diffusive-creep processes also
pervasive. Although Glen Canyon Dam does not directly influence these erosion processes, overall sediment depletion and the loss of major flooding leaves
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erosion unhampered along the river corridor. INTRODUCTION
The Colorado River through Grand Canyon National Park
in the American Southwest occupies a canyon-bottom
landscape that is diverse in both topography and landforms, varying from gorges where only the river channel itself intercedes between polished cliffs to valley bottoms with a variety of interwoven geomorphic elements.
The river corridor also preserves a human record ranging
in age from Middle Archaic (5000–3000 B.C.) to historic
Anglo use (Table I), and the changing character of this
record through time and over space is surely tied to the
dynamics of the canyon landscape. Indeed, human settlement and utilization of the river corridor may be largely
dictated by geomorphology because, in Grand Canyon,
geology and landscape are amplified. Furthermore, understanding the geomorphology of cultural sites (term
used to represent the diversity of archaeological sites and
historic features) along the river corridor is critical for understanding their condition and predicting their stability.
Following the closure of Glen Canyon Dam in 1963, concerns have mounted over myriad ecological and physical
Geoarchaeology: An International Journal 29 (2014) 431–447
transformations that have occurred. Despite these concerns, there have been no systematic, canyon-wide studies relating geomorphic patterns to the archaeological and
historical record.
The modern riverine environment of Grand Canyon is
no longer subject to seasonal flooding and replenishment
of sand. Consequently, there is a reduced supply of sediment along the river corridor, as has been abundantly
documented (e.g., Schmidt & Graf, 1990; Webb et al.,
1999; Hazel et al., 2006; Wright et al., 2008). This sediment depletion along the flanks of the river should hypothetically exacerbate erosion in this setting, and processoriented studies have been conducted to document and
understand the increased erosion of cultural sites along
the river corridor. The most pronounced erosion at select 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 eolian
processes of inflation and deflation (Draut, 2012), trailing
and visitation, diffusive processes such as creep and rainsplash, and even mass wasting.
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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
Middle Archaic
Late Archaic
8000–5000 B.C.
5000–3000 B.C.
3000–1000 B.C.
0
Late Archaic/Early
Agricultural
Basketmaker III
Pueblo I
Pueblo II
Pueblo III
Protohistoric
Historical
Archaic
22
Preformative
1000 B.C. to A.D. 500
A.D. 500–800
6
Formative
A.D. 800–1000
A.D. 1000–1150
A.D. 1150–1250
133
A.D. 1250–1776
A.D. 1776–1950
120
135
PEDERSON AND O’BRIEN
sites. These data were then combined with calculations of
topographic metrics as well as an existing National Park
Service (NPS) database of site attributes.
We query this geoarchaeological database with a dual
focus on patterns linking the river-corridor landscape to
the archaeological/historic record and the geomorphic
processes that preserve and destroy that record. Our findings confirm that the broadest patterns in the cultural
landscape are tied to basic geologic controls on topography, though more specific correspondences between
site archaeological identification and geomorphology are
lacking along the Colorado River corridor. There are also
clear relations between erosion and preservation of cultural sites and particular geomorphic processes and settings, and such patterns may be expected in other river
canyons of management concern.
a
Names based on a modified Pecos classification (Fairley, 2003).
A total of 162 sites have multiple cultural components, so are counted
here more than once, whereas 19 have undetermined affiliation.
b
BACKGROUND
Geomorphic Setting
Although Grand Canyon is of course an erosional
landscape in general, over the climate changes of the
Holocene, a shifting balance has been struck between erosion and episodic deposition along the river corridor. The
evidence for this is a complex Holocene stratigraphy that
preserves cultural sites, at least for some epochs. Relatively wet and dry intervals, both in Grand Canyon itself
and in the river’s Rocky Mountain headwaters, modulated the sediment supply and the flooding of the river as
well as other geomorphic processes at corridor archaeological sites (O’Connor et al., 1994; Hereford et al., 1996;
Davis et al., 2000; Draut et al., 2008; Tainer, 2010). Understanding this deeper time context of site-formation
processes is supremely challenging, but first-order spatialgeoarcheaological patterns can provide the groundwork,
and both a spatial and temporal context is essential for
untangling the milieu of human and natural processes
preserving and destroying cultural sites.
To provide part of the larger context for these
land-management issues, this study explores the firstorder trends in a large observational and topographic
dataset constructed through collaboration with the Grand
Canyon Monitoring and Research Center of the U.S. Geological Survey and the National Park Service (O’Brien
& Pederson, 2009). Here, we present systematic geomorphic data recorded at 227 of the cultural sites (some
with multiple loci) distributed along the length of the
Colorado River through Grand Canyon, visited by the
authors during several river trips. This diverse and large
sample was chosen by land managers as being of interest for long-term monitoring and mitigation from a total population of about twice as many recorded corridor
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Topography has two modes in the Grand Canyon region, with surrounding plateaus lying in sharp contrast
with the threshold hillslopes that define the canyon itself
(Figure 1). Within the canyon, the steepness of both hillslopes and drainages is strongly influenced by varying
bedrock properties. Erosionally resistant bedrock causes
tributary streams and the mainstem river to be confined
to narrow, steep-walled canyons, whereas reaches of the
canyon underlain by mechanically weaker bedrock and
affected by fault zones have wider valley floors (Howard
& Dolan, 1981; Mackley, 2005; Pederson & Tressler,
2012). Along the mainstem corridor, this latter condition provides the accommodation space for larger debris fans at tributary junctions, a wider channel for the
Colorado River, and better preservation of Holocene deposits. The notable examples of such reaches are Furnace
Flats in eastern Grand Canyon and the greater western
Grand Canyon reach, including the Granite Park area
(Figure 1). These lower relief reaches in turn correspond
to a greater number of recorded archaeological sites, and
perhaps with more intense utilization throughout human
history (Fairley et al., 1994).
Another broad topographic pattern is the directional
trend, or aspect, of the Colorado River corridor. Both the
stretch of Marble Canyon and Furnace Flats to the east
and the western-central portion of the canyon that encounters the Hurricane and Toroweap fault zones trend
from north-northeast to south-southwest (Figure 1).
Contrasting with this are the intervening stretches that
trend southeast to northwest, through the upper and
middle Granite gorges as well as the far-western reach
of the canyon through the Shivwits Plateau. These latter
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PEDERSON AND O’BRIEN
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 basement
rock at river level and relatively steep, narrow, and inaccessible inner gorges, with fewer recorded cultural sites
considering the dearth of canyon-bottom real estate to
utilize (Fairley et al., 1994). Because aspect has a strong
control on local climate conditions such as effective moisture, there also may be a correspondence of aspect to both
settlement patterns and historic erosion problems.
Setting the stage more specifically, the major landforms and deposits occupying the river corridor—from
adjacent slopes to the channel margin—include bedrock
slopes/cliffs, talus, tributary debris fans, finer grained
alluvial and colluvial fans derived from smaller hillslope catchments, alluvial terraces, and localized eolian
dunes (Figure 2). The mainstem alluvial terraces include
both relatively common, finer grained Holocene fill terraces and higher Pleistocene gravelly fill terraces that
are preserved in the very widest reaches of the corridor.
The surficial geology in key-wide reaches of the corridor has been mapped, described, numerically dated, and
interpreted in detail in other studies (Hereford, 1996;
Hereford, Burke, & Thompson, 1998, 2000; Pederson
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et al., 2011). Based upon that work, the Holocene terraces of concern here are generally composed of silty very
fine to fine sand with well-preserved sedimentary structures indicating flood deposition in eddies or backwater settings. In many deposits, this alluvium interfingers
with pebbly colluvium or boulder-gravel debris fan sediment toward the valley margin (Hereford et al., 1996). Finally, vegetation-stabilized coppice dunes or active dunes
of eolian sand mantle other landforms, or they are commonly cored by alluvium (Hereford et al., 1993; Draut,
2012).
The arid river corridor at the bottom of Grand Canyon
receives 213 mm of mean annual precipitation and has a
mean annual temperature of 20.4o C (at Phantom Ranch).
Like topography, Grand Canyon’s precipitation regime
has two modes, with about half of the annual precipitation in the form of high-intensity summer and early
fall monsoonal storms and half in longer duration frontal
systems in the late fall and winter. Surficial processes in
the canyon link to this precipitation, and debris flows are
arguably the most important way that sediment is delivered from hillslopes to tributary drainages and to the
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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 boulder
and cobble-rich debris fans deposited at tributary junctions largely define the Colorado River’s channel geometry as well as the settings where historic and prehistoric
flooding has deposited alluvium flanking and between
debris fans (Schmidt, 1990; Figure 3). Eolian sediment
transport is highly variable across the river corridor, with
the dunes mantling debris fans and alluvial terraces derived largely from the reworking of unstabilized flood deposits along the channel margin (Draut, 2012; Figure 3).
Previous Geoarchaeological Studies in Grand
Canyon
Geoarchaeological work along the Colorado River corridor over the past few decades has been motivated by
erosion problems. The first monitoring of the erosion of
cultural sites along the river corridor came immediately
after the unexpected July 1983 flood release from Glen
Canyon Dam, which significantly reworked predam flood
deposits and negatively impacted some cultural sites. An
archaeological inventory was completed in May 1991 by
the NPS along 410 km of the Colorado River corridor
(Fairley et al., 1994). This led to the compilation of a
database of corridor sites, including the surface observations of archaeological identifications used in this study
(Table I) as well as the continued monitoring of sites with
degrading integrity.
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Many of the cultural sites of interest lie in the context of a suite of Holocene stream terraces, adjacent debris
fans, and capping eolian deposits. Although the chronostratigraphy of these Holocene deposits is not within the
purpose or scope of this particular study, a review of previous work related to both deposition and erosion is in
order. Studies of the Holocene stratigraphy by Hereford
and others (1993, 1996) found that archaeological sites
are frequently located in deposits they called the “alluvium of Pueblo II age” and “striped alluvium,” dating
from A.D. 700 to 1200 and 2500 B.C. to A.D. 300, respectively. These late Holocene deposits generally have
not been inundated by historic flows of the Colorado
River. Yet, flooding did create two other inset deposits,
the protohistoric “upper mesquite” and historic “lower
mesquite” alluvial terraces. Hereford and others noted
the possibility that apparent time gaps in the archaeological record actually result from the episodic erosion
evident in the corridor stratigraphy. Likewise, the Davis
et al. (2000) study at two sites in eastern Grand Canyon
found buried soils/cultural horizons overtopped and reworked by flooding and episodic erosion that may have
caused breaks in occupation due to unsuitable farming
conditions. Finally, Anderson and Neff (2011), in their
study of other Ancestral Puebloan sites in eastern Grand
Canyon, relate the changing position of cultural features
through time to modeled flood lines and interpret that
the Colorado River’s flood dynamics directly influenced
settlement patterns.
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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 recent erosion of cultural sites, Hereford et al. (1993) and
Thompson and Potochnik (2000) documented that gully
incision increased dramatically between 1973 and 1984,
based on analysis of historic aerial images and repeat photographs of sites. Hereford et al. (1993) also studied precipitation records and proposed that a period of more intense precipitation from the late 1970s through the 1990s
drove accelerated erosion. These and subsequent empirical studies indicate that gullying is the most acute erosion process at cultural sites, driven by infiltration-excess
overland flow in this semiarid to arid landscape with
high-intensity precipitation events (Pederson, Petersen, &
Dierker, 2006).
An erosion process of secondary importance, but which
is ubiquitous across the canyon, is creep, especially
through rainsplash and bioturbation. Although creep
processes are incremental and therefore subtle, they have
a strong cumulative effect on site integrity and preservation over the centuries. An empirical study at one site in
eastern Grand Canyon indicates that particles can creep
downslope at rates of 5–10 cm/yr, rapidly taking out of
context any artifacts that have emerged onto the land
surface (Tressler & Pederson, 2010). Recent studies have
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also focused on quantifying eolian sediment transport as
a process affecting cultural sites. Draut’s (2012) research
at selected sites highlights the very strong spatial variability of wind, moisture, and eolian transport along the river
corridor, making linkages between increased erosion, reduced eolian deposition, and reduced sediment supply
from sandbars in the postdam era.
METHODS
Observational Field Data
Data from 227 cultural sites are presented here; 16 of
them have two spatially distinct loci and so n = 243.
This large sample of sites was assessed in the field during five Grand Canyon river trips in 2006 and 2007 as
well as one trip upstream into the Lower Granite Gorge
from Lake Mead. Systematic observations were recorded
using a standardized form designed to capture specific geomorphic attributes (Table II). With the exception of restricted information on site location and archaeological
identification, the full data illustrated and discussed here
are available in Supplementary Table 1. Sites frequently
extend across more than one landform and have a variety
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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
0%
Eolian sand
Alluvial sand
Slopewash
Bedrock
Talus
1–25%
26–50%
51–75%
76–90%
>90%
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
identificationb
Archaic
Preformative
Formative
Protohistoric
Historic
Area covered by
soil crust
Characteristics of
soil crust
a
b
See O’Brien and Pederson (1994) for detailed erosion-ranking criteria.
Pre-existing data provided by NPS (see Fairley et al. 1994, for example).
of substrate and surface characteristics. Our approach was
to rank landforms, substrates/deposits, and surface processes according to their predominance within the official
site areas. In the case of subequal landforms that transition across a study site, they were ranked from bottomup stratigraphically. Each site was also assigned an overall
ranking expressing stability or the degree of erosion evident (Table II). Details of the criteria used for each subcategory of field observations can be found in O’Brien and
Pederson (2009), beyond the basic explanation provided
here.
In terms of landforms, “bedrock” includes canyon
walls, cliffs, shelters below ledges, and caves or alcoves
(Table II). We make a distinction between debris fans and
colluvial/alluvial fans. The former are steep and coarse
fans of debris-flow diamicton that constrict the Colorado
River at tributary junctions, whereas the latter are underlain by finer sediment from overland flow off smaller
hillslope catchments onto the valley bottom in wider
reaches. Our observations include the condition and coverage of biotic-soil crust, which is ubiquitous throughout
the corridor and influences both sediment cohesion and
infiltration (Pederson, Petersen, & Dierker, 2006). These
range from insipient rainsplash crusts to dark and rugged
biological-soil crusts.
Many distinct geomorphic processes were documented
in the field, which we have categorized into five groups
here: (1) “overland flow” includes slopewash, rilling, piping, and gullying (Figure 3); (2) “diffusive” processes
recorded include soil and particle creep, rainsplash, bioturbation, and in situ physical weathering; (3) “eolian”
processes include both deposition and deflation; (4) “visitation effects” are mostly erosion caused by trailing, but
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include rare instances of artifact relocation and graffiti;
and finally, (5) “depositional” processes were noted in
the rare instances of significant alluviation within cultural
site areas (Table II).
One of our priorities in the field was to document at
each site how acute and prevalent erosion was, or conversely, how stable the site was. To make this systematic,
we utilized an erosion ranking of 1–5, ranging from “stable,” where little or no erosion is documented across a
site, to “mild,” “intermediate,” “serious,” and finally “severe” where acute erosion was destroying much of the
cultural value of the site (Table II). It is important to recognize that our erosion ranking reflects the condition of
the site area and cultural features at the time the observations were made, which may not accurately reflect the
stability over longer timescales or in the archaeological
record.
Archaeological Identifications
We combined into our dataset the site archaeological
identifications provided by the NPS, as determined by
archaeologists from surface features or artifacts apparent
during the original survey of sites (Fairley et al. 1994),
as well as during subsequent monitoring visits. To enable
first-order analysis of a greater number of each, we summarized the more detailed archaeological identifications
applied by the NPS into general categories of Archaic,
Preformative, Formative, Protohistoric, and Historical
(Tables I and II). These associations are not mutually
exclusive, such that about two-thirds of sites have multiple affiliations, and about 8% of sites are undetermined.
These surficial archaeological determinations inevitably
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underrepresent cultural features that are buried within
deeper stratigraphy, as well as generally older cultures
that are less preserved. Examples of this are illustrated
in the relative dearth of Preformative (1000 B.C. to
A.D. 800) associations, which have only been found in
the subsurface in this setting, as well as older Archaic
associations that may be buried or poorly preserved
(Table I). Despite these caveats, we can still draw out
broad patterns of the overall frequency of recorded
archaeological identifications across the corridor.
routes of human access from the canyon rim to bottom (Euler & Chandler, 1978; Fairley et al., 1994). Settlement patterns may also reflect susceptibility to flooding and preservation potential, as well as the presence
of key terrace landforms and a broader riparian landscape that could be more intensively utilized by people
(Fairley et al., 1994; Fairley, 2003). A comparison of site
density and our measurements of canyon-bottom width
confirm these trends (Figure 4). The narrow bedrock
gorges, where the canyon bottom is hardly wider than the
river channel itself, are strongly associated with low cultural site density. Conversely, the abundant cultural sites
of the Furnace Flats and western Grand Canyon reaches
correspond with canyon bottoms that are twice the average width of the narrow gorges, providing accommodation space for the distinct landforms and resources of
those areas.
An instructive exception to this trend is the upper Marble Canyon reach (river miles 0–35), which has moderate
to high average canyon-bottom width, but relatively few
surveyed cultural sites (Figure 4). This reach is marked by
deeply entrenched bedrock meanders, and although the
canyon bottom does have some accommodation space,
the river corridor is also typically hemmed in by nearly
vertical cliffs of Paleozoic bedrock. Thus, in these upper
reaches, the metric of canyon-bottom width fails to capture some key controlling factor, such as accessibility of
the canyon bottom by foot (Fairley et al., 1994). This upper Marble Canyon exception also argues against their
being significant roles played by preservation potential
relative to floods and space for resources. That is, in upper Marble Canyon, the relatively wide canyon-bottom
prevents wholesale flood scouring and it allows for more
valley-bottom resources, yet the density of sites is relatively low for some other reason not discernable with our
data focused along the river corridor.
The distribution of different landforms and their associated deposits varies along the river corridor, as recorded
at any ranking of prevalence at the study sites (Figure 4C). For first-order trends, we group landform types
into those lining the river banks along the “valleybottom” axis (alluvial terraces, eolian dunes, debris fans)
versus the “canyon-slope” landforms at the edges of the
corridor (talus, colluvial/alluvial fans, bedrock; Figures 2,
3). The relative percentage of these landform groups illustrates an expected and strong trend of axial valley-bottom
landforms being more predominant in wider reaches with
more accommodation space for such deposits, generally matching Figure 4A. This includes the lower Marble Canyon, Furnace Flats, and western Grand Canyon
reaches, and the predominance of riparian valley-bottom
landforms is therefore associated with a greater number
of recorded cultural sites (Figure 4B).
Site Distribution and Terrain Metrics
The database is partly analyzed with respect to site location, specifically by river mile as measured downstream
from Lee’s Ferry (Figure 1), extending to river mile 260
in the western end of Grand Canyon. The calculation
and extraction of terrain metrics for each site was conducted in ESRI ArcMap software. Mean values of topographic slope and aspect (azimuth direction of slope, or
the direction a landform faces, Figure 3) were extracted
for the area of each NPS site polygon. For slope, this utilized a 1-m terrain model of the river corridor developed
through photogrammetry and provided by the Grand
Canyon Monitoring and Research Center. In the case of
aspect, mean values were more accurately obtained using less detailed 30-m digital elevation models (DEMs)
from the U.S. Geological Survey, calculated across those
site polygons that have an overall slope. Finally, calculations of valley-bottom width are from Mackley (2005)
and were made at 500-m intervals along the river, normal to the channel, utilizing a 10-m DEM. Valley-bottom
width in this study is the distance from hillslope/bedrock
wall to the opposite hillslope/bedrock wall, 5 m above the
channel, just above the level of the flood plain and most
Holocene terrace deposits (Figure 3).
RESULTS AND DISCUSSION
Geoarchaeological Patterns Along the River
Corridor
This study involves about half the total recorded Colorado
River corridor cultural sites, and the spatial distribution
of our large sample matches the overall pattern of all
sites (O’Brien & Pederson, 2009). Archaeological sites are
not evenly distributed through the corridor, with concentrations found in the Furnace Flats and western Grand
Canyon reaches, and relatively few in the intervening
gorges (Figure 4).
It has been hypothesized that this uneven distribution is linked to broader geomorphic controls; for example, the steepness of surrounding terrain as it dictates
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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 clustering of sites in Furnace Flats and western Grand Canyon and a correspondence to valley-bottom width
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.
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Looking more closely at the distribution of the individual valley-bottom landforms that tend to contain sites,
alluvial terraces and debris fans comprise a relatively predictable component wherever valley-bottom landforms
are significant, whereas eolian dunes at cultural sites are
more variable in occurrence, with no systematic trend
apparent at this scale. At those cultural sites found on
the valley-bottom suite of terraces, alluvial terraces are a
consistent component at 20–40% of them in both eastern and western Grand Canyon. Debris fans are a more
dominant component (40% or more) along the narrower
inner gorge reaches, and they also dominate cultural sites
recorded from river miles 20–40, including the reach
known as the “roaring 20s” by whitewater rafters for the
frequent rapids caused by debris-fan constrictions. Yet,
important for our investigation into broad trends along
the corridor, the distribution of landforms at cultural sites
does not show any particular distinction between eastern
and western Grand Canyon. In the main Furnace Flats
and western Grand Canyon reaches, both with a healthy
proportion of valley-bottom landforms, the alluvial terraces, debris fans, and eolian dunes all comprise a subequal proportion of site areas.
Finally, a somewhat unexpected spatial pattern exists
regarding the current erosional condition of sites, seemingly matching the overall distribution of sites and valleybottom landforms along the river corridor (Figure 4D).
Of course, in reaches with a greater number of cultural
sites to begin with, one will find more sites with erosion
problems. But even when our ranking of site erosion is
averaged over 10-mile bins, and we include only those
bins with more than two sites to reduce this frequency
bias, more intense erosion of sites appears to mirror the
occurrence of valley-bottom landforms and therefore also
the overall density of sites along the corridor (Figure 4).
For example, there is a higher proportion of unstable sites
within Furnace Flats and western Grand Canyon, with
erosion rankings of serious (4) or severe (5) clustered in
those reaches (O’Brien & Pederson, 2009). This supports
the idea that these wider areas with their distinctive landforms are more active in terms of erosion processes, particularly gullying, as discussed below.
Basketmaker III affiliations are not included because of
their low overall frequency in the dataset). The primary
trend is that “Formative” or Puebloan is an association
notably more abundant in the eastern river corridor,
whereas Protohistoric site associations are recorded
with increasing dominance in western Grand Canyon.
Although Archaic sites appear to be more abundant in
a short intermediate reach within the Upper Granite
Gorge (Figure 5), this is uncertain because the river mile
100–110 bins contain only three sites total.
There are many possible reasons for this distinct trend
in the human record of the corridor, including those that
are purely cultural or territorial and others that relate to
changing climate and plant resources across the canyon.
Our task here is to investigate any possible relations to
geomorphology. It has been hypothesized that systematic
east to west trends in both the type and the age of deposits
control what archaeologists record at the surface compared with what may be hidden in the subsurface (Fairley, 2003). Furthermore, beyond stratigraphy and preservation, cultural patterns in the landscape also may follow
underlying geomorphic trends in landforms and deposit
types, as past cultures may have adapted to certain corridor settings.
Although landform distribution relates most broadly to
valley-bottom width (Figure 4), and the relative proportion of the specific terrace and debris-fan landforms most
associated with sites changes little across the corridor,
there still may be a geomorphic explanation for the eastto-west trends in archaeological identifications. A fruitful approach is to take each generalized archaeological
identification and inquire about the landform types it is
associated with at sites, regardless of location along the
corridor. When the dominant (first-ranked) landforms
recorded at sites are plotted according to associated archaeological identifications (Figure 6), the relatively few
sites with Archaic affiliation have the strongest trend.
Fully 43% of sites dominated by debris fans include an
Archaic component, whereas this is true for only 5% of
sites dominated by alluvial/colluvial aprons and eolian
dunes. This disproportionate appearance of Archaic sites
at debris fans may be due to greater stability of debris-fan
substrates and preservation of older archaeology there
(e.g., Hereford et al., 1996), or it could possibly reflect a
real preference for these features by mobile Archaic populations.
Regarding the primary pattern of more eastern Grand
Canyon Formative sites and more western Grand Canyon
Protohistoric sites along the river corridor, the two archaeological identifications have very similar and equable
distributions among dominant landforms, with the exceptions of alluvial/colluvial aprons and eolian dunes
(Figure 6). Formative sites occupy alluvial/colluvial
Trends in Archaeological Identifications
The distribution along the corridor of the generalized
archaeological affiliations recorded for the study sites
illustrates a long-recognized trend across the canyon
(Fairley et al., 1994; Figure 4). In order to remove
the bias from varying site density across the canyon,
we illustrate archaeological identifications as a relative
percentage of those recorded within 10-river-mile bins
(note that “Preformative” or Early Agricultural and
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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 archaeological associations. In fact, the majority of alluvial/colluvial
aprons in our dataset lie in the wider Furnace Flats
reach where Formative sites are clustered. Similarly, Protohistoric sites occupy eolian dunes in relatively higher
proportions. To some degree, this is stratigraphically inevitable because both Protohistoric sites and eolian dunes
generally occupy the tops of other landforms along the
corridor.
A final possible link between culture and landscape
across the length of the river corridor lies in topographic
aspect—the direction a landform or surface dips or faces.
A rose diagram of all study sites where aspect could be
measured (not including rock art sites and those without significant slope) illustrates strong variability and a
slight tendency for site landforms to face either eastsoutheast or west-northwest (Figure 7A). This reflects
the trend of the river and canyon itself in the Furnace Flats and western Grand Canyon reaches where
most cultural sites lie (Figure 1), considering that landforms along the canyon bottom tend to slope toward
(normal to) the river channel (Figure 3). A comparison 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.
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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 western
Grand Canyon) reveals a contrast. Formative sites in our
sample are set on landforms that preferentially face either
northwest or southeast, while Protohistoric sites tend to
face a wider range of aspects, but with a disproportionate
number in a southwesterly direction (Figure 7B and C).
The aspects of Formative sites are consistent with their
dominance in the Furnace Flats reach where the river
trends northeast to southwest (only 18% face southwest). Yet, the river in western Grand Canyon has the
same trend, and therefore such an explanation cannot
account for the fact that 29% of Protohistoric sites face
southwest and only 15% northeast. Although not a major pattern, this observation suggests Protohistoric people
may have preferentially utilized the southwest-facing aspects 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, debris fans and alluvial terraces are the most common firstranked landforms, each dominating about 20% of sites.
Eolian dunes are less common as the first-ranked landform (12% of sites), yet they appear as subsidiary, lower
ranked landforms at nearly half of the study sites. Of
the 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 ledges
or shelters as well as rock art localities.
Related substrate materials should have a control on
erosion processes through physical resistance measured
by the caliber and cohesion of sediment. At our study
sites, sandy alluvium, eolian coppice, slopewash fines and
Geoarchaeology: An International Journal 29 (2014) 431–447
gravels, and coarser debris-fan sediments all exist as the
dominant substrates in subequal proportions. In contrast,
sites underlain by bare bedrock or talus, or that have
been stabilized by vegetation and soil crusts are all relatively rare as primary substrate or surface cover. Yet,
resistant biological soil crusts appear very frequently as
a subsidiary surface cover, recorded at some ranking at
nearly half of sites. In fact, soil crust is different from the
other substrate categories of the database, in that it develops on any of the other fine-grained substrates, given
some surface stability.
More directly responsible for erosion than landforms or
substrate are surface processes. Our ranking of processes
active at sites indicates that throughout the corridor,
overland flow (including gullying, rilling, slope wash, and
piping) is indeed the dominant class of process at nearly
half of cultural sites, and it is also the most pervasive
across all sites when tallied at any ranking (Figure 8). Diffusive processes of creep, rainsplash, bioturbation, and in
situ weathering are first-ranked less often (28%), but they
are similarly pervasive, with creep specifically being the
single most pervasive individual surface process, recorded
at more than half of all sites at some ranking. The
incremental but persistent nature of creep results in great
cumulative and detrimental effects, causing nearly all exposed site features and artifacts to move out of context.
Finally, eolian processes and visitation impacts—typically
human trailing, which breaks soil crusts and promotes
channelized overland flow—are present to a lesser yet
still significant degree (Figure 8).
Erosion end members
Does the problem of erosion of cultural sites along
the Grand Canyon corridor exhibit patterns relative to
these principal landforms, substrates, and geomorphic
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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 (Table II), when tallied for all study sites, reveals patterns
that provide important insight into the causes and potential mitigation of the erosion. First, half of the sites in this
dataset are documented as stable or only mildly affected
by erosion (Figure 9). More than a quarter of sites are
ranked intermediate, and only 14% and 7% of sites are
ranked as having serious or severe erosion, respectively.
On the other hand, these last two categories reflect acute
erosion at nearly 50 sites of critical concern along the cor-
ridor just in our sampled dataset. These are cultural sites
where resources and information are actively being lost.
A first-order expectation regarding the stability of sites
under geomorphic processes is that the gentlest, lowest gradient settings will be more stable and those in
the steepest places will be the most acutely eroded. Interestingly, this trend does not occur in Grand Canyon
(Figure 9); sites with serious or severe erosion actually
have a lower mean gradient than other sites. Those that
are stable have the highest mean gradient, even when
Figure 9 Percentage of study sites with each erosion ranking (see Table II for ranking criteria), and the mean slope gradient within 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.
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excluding the 12 rock-art sites in the dataset, which lie
upon anomalously steep ledges. The fact is that the erosion of cultural sites in Grand Canyon is not a simple story
that can be encapsulated by a basic metric such as gradient. Instead, an end-member analysis is useful in understanding the more complex relation of cultural-site stability and trends in the landforms, substrates, and processes
at each site. Sites are grouped into those that are stable
or mildly eroded and those that are seriously or severely
eroded, while ignoring those with intermediate erosion.
In terms of landforms, it is intuitive that sites on highly
resistant bedrock walls and within bedrock shelters are
mostly stable (Figure 10). Other landforms with largely
stable sites are coarse talus and debris fans, but as one
approaches the valley-bottom axis with sites on alluvial
terraces, stability is much less frequent. Nearly half of
sites with alluvial terraces as the primary landform exhibit acute erosion, and colluvial/alluvial fans are similarly unstable (Figure 10). Thus, a first-order pattern is
that relatively stable cultural sites are found on landforms of resistant substrate farther from the river, while
the least stable sites are on landforms with fine-grained
sediment nearer the river. Sites in fine-grained eolian
dunes, which are nearer the river axis but neither particularly stable nor acutely eroded, are an exception. We
note that the very high infiltration rate of eolian sand
serves as a buffer to overland flow in this setting (Pederson, Petersen, & Dierker, 2006). Indeed, the stability
trend is linked most directly to parallel trends in substrate
caliber and cohesion, not landform position (O’Brien &
Pederson, 2009). Bedrock and coarse, poorly sorted talus
and debris-fan sediment is mechanically more resistant to
erosion than silty-sand alluvial deposits. These relations
explain the inverted trend with gradient noted above
(Figure 9); the relative stability of steeper talus and
debris-fan landforms is largely due to them being underlain by much coarser and more resistant sediment.
Regarding the five categories of geomorphic processes,
there are very strong trends with stable and acutely
eroded sites (Figure 11). Because there are about twice
as many stable sites as acutely eroded sites along the
corridor (Figure 9), a relatively high proportion of sites
are also stable for most process categories. Yet, sites dominated by overland-flow processes are acutely eroded in
by far the highest proportion, nearly half of them. Acute
erosion is also somewhat common at sites where human
visitation is the dominant process, partly because trails
may become channels for overland flow. Not surprisingly, stable sites are found in the highest percentages
either where incremental diffusive/creep processes are
dominant or where there is actual deposition rather than
erosion.
A final interesting, but less intuitive result of the endmember analysis relates to the topographic aspect of
sites. Within the overall population of sites shown in
Figure 7A, a disproportionately high number of acutely
eroded sites face (or slope) either northwest or eastsoutheast, whereas almost no acutely eroded sites face
northeast or southwest (Figure 12). First, we have established that the many sites lying along the Furnace
Flats and western Grand Canyon reaches, where the river
trends northeast to southwest (normal to the trend of Figure 12A), more frequently lie within the context of alluvial terraces and weak substrates subject to erosion by
overland flow. A possible secondary influence could be
a meteorological phenomenon of more intense moisture
being focused where canyon topography lies parallel to
the prevailing storm tracks from southwest to northeast.
This was suggested by Griffiths, Webb, and Melis (2004)
to account for modeled tributary debris-flow frequency
being notably higher in reaches of Grand Canyon with
this same aspect trend.
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CONCLUSIONS
Our goal has been to explore the most basic patterns between the diverse and dynamic river corridor landscape
and the archaeological and historic record, including its
erosion and preservation. The unique geology and environment of Grand Canyon makes such inquiry especially pertinent, and this represents the first systematic,
canyon-wide exploration of such patterns and linkages.
Yet, this dataset warrants further development, updating, and certainly statistical analysis, as a tool for landmanagement and broader inquiry.
Patterns of Landscape Context
The clustering of cultural sites into eastern and western
Grand Canyon reaches tracks the width of the valleybottom landscape. Our metric of valley-bottom width is
intended to capture the Colorado River’s lateral erosion
and widening over geologic timescales, and is inversely
correlated with bedrock strength (Mackley, 2005). This
background geologic control ultimately results in specific reach properties pertinent to geoarchaeology. The
greater accommodation space of wider reaches results in
a greater proportion of valley-bottom alluvial terrace and
debris-fan landforms. It also provides the potential for
greater access by foot, resources and habitability, and better preservation in the face of flooding, when compared
to the narrow gorges (Fairley et al., 1994). This pattern in
Grand Canyon parallels that identified by Nials, Gregory,
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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 patterns across the broader alluvial valleys of the basin and
range of southern Arizona and New Mexico. There, cultural site density is greater near fluvial-reach boundaries
where geologic and geomorphic conditions create larger
floodplain areas and more available surface and groundwater for irrigation agriculture.
In contrast, clear trends between broad geomorphology and recorded archaeological identifications along the
Colorado River corridor are lacking in our data, and this
may be partly due to the reconnaissance-survey nature
of the cultural data. For example, the more frequent Protohistoric affiliation of sites in western Grand Canyon
and Formative in eastern Grand Canyon has no clear
link to geomorphic differences from eastern to western
Grand Canyon in landforms, substrates, or active geomorphic process. Also, the chronostratigraphic record of
the river corridor indicates there is no discernable trend
in deposit ages or preservation across the canyon that
would account for this trend in archaeological identifications (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.
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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 southwesterly aspects, and Archaic sites tend to be set upon debris fans. Such patterns are worthy of further investigation, and it is possible they may relate to past people’s
choices regarding resources, seasonal activity, and sun
exposure. Regardless, our results generally suggest that,
although geology and geomorphology set the broad patterns of where sites occur along the Grand Canyon corridor, the specific trends within that cultural landscape are
instead mostly controlled by cultural, territorial, or biological drivers.
Erosion and Stability of Cultural Sites
Preserved cultural sites are more prevalent in reaches
where there are greater proportions of axial valleybottom landforms, such as in the Furnace Flats and western Grand Canyon reaches. Unfortunately, our data indicate that those valley-bottom landforms and weak substrates also host a disproportionate number of sites with
acute erosion problems. This is not due to steep slopes
along the valley bottom; instead, the steeper landforms
that are generally farther from the river and underlain by
coarser and more cohesive sediment are associated with
relatively stable sites.
A linkage of site stability specifically to substrate resistance makes sense, considering that the primary erosion
process at sites with acute problems is gullying, which
hinges upon the entrainment of grains by the flow of water. Although there is the secondary possibility of meteorological controls, the preferential northwest-southeast
aspect of acutely eroded cultural sites likewise must be a
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result linked to the dominant process of overland flow.
The finer, weaker alluvial substrates are more prevalent in the wider, densely populated areas of eastern
and western Grand Canyon, and the river trends normal
to those aspects in those reaches. Although gullying accounts for most acute erosion issues, human visitation is
the next most significant process, and it is one that perhaps can be more easily managed. Finally, diffusive-creep
processes are pervasive across sites, and in the long run,
they play an insidious role in the degradation of sites in
Grand Canyon.
The driving land-management question behind this
study and other geoarchaeological research in Grand
Canyon is how these erosion issues may relate to the
operation or presence of Glen Canyon Dam, which has
changed the balance between flooding and sediment supply downstream. The goal of this study is to provide
spatial-geomorphic context, not to address process linkages to the presence or operation of the dam. Yet, this
large dataset quantitatively confirms that the dominant
erosion process at sites is overland flow. In this, the Colorado River corridor shares the irony of most dryland
settings—despite water deficit defining the landscape, it is
flowing water that dominates surface processes. Gullying
relates most directly to local topography and runoff, as
controlled by weather patterns and climate shifts (Hereford et al., 1993; Pederson, Petersen, & Dierker, 2006).
Although the dam does not play into either of these direct controls, overall sediment depletion in the river corridor may still have a role. Our data indicate erosion is
most acute in weak, fine-grained substrates proximal to
the river. With the loss of fine-grained sediment and lack
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of replenishing floods along the river margin, the erosion of such deposits is unhampered, even if the process
linkage is indirect, as through lower local baselevel for
gully systems (Hereford et al., 1993) or a decrease in eolian reworking of flood deposits (Draut, 2012). Although
our field-survey data indicate eolian landforms and processes, with some exceptions, are not predominant factors at most sites, the current state we have recorded may
be altered from previous conditions.
Grand Canyon serves to highlight these issues of geoarchaeology and human impacts, but there are many
dammed rivers with downstream landscapes where cultural sites are a management concern. We submit that the
most robust patterns evident along the Colorado River
corridor between geology, river-corridor topography, and
site distribution, and between alluvial landforms, weak
substrates, and site stability, are trends that should be
expected elsewhere. Though these are rather intuitive,
quantifying and confirming such patterns provides the
groundwork for making predictions, including for management purposes. Thus, inquiry with spatial datasets can
provide part of the context for the human record, as we
have done here, but we hope it may also help assure its
preservation.
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This project was supported by the Cultural Program of the Grand
Canyon Monitoring and Research Center, and we thank director Helen Fairley. National Park Service archaeologists Jennifer
Dierker, Lisa Leap, and Amy Horn were essential for direction
and interpretation of cultural sites. Two anonymous reviews
and the comments of the editors made this a clearer and better contribution. Thanks go to many student field assistants from
Utah State University, including Chris Tressler, Susannah Erwin,
Mike Keller, Ben DeJong, Alex Steely, and Erin Tainer, Michelle
Summa, and Jonathan Harvey for both field help and data reduction. Finally, thanks to the boatmen and women who conveyed us downriver: Brian Dierker, Chris Mengel, Bruno, Annie
Anderson, and Brian Hansen.
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