The Role of Aeolian Sediment in the

Preservation of Archaeological Sites in the

Colorado River Corridor, Grand Canyon,

Arizona: Final Report on Research Activities,

2003–2006

U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2007-1001
The Role of Aeolian Sediment in the

Preservation of Archaeological Sites in the

Colorado River Corridor, Grand Canyon,

Arizona: Final Report on Research Activities,

2003–2006

Amy E. Draut and David M. Rubin

U.S. Geological Survey, Pacific Science Center, Santa Cruz, CA

U.S. GEOLOGICAL SURVEY

Open-File Report 2007-1001

2007

Any use of trade, firm, or product names is for descriptive purposes only and does not

imply endorsement by the U.S. Government.
U.S. Department of the Interior

Dirk Kempthorne, Secretary

U.S. Geological Survey

Mark Myers, Director

U.S. Geological Survey, Reston, Virginia 2007

Revised and reprinted: 2007

To download a copy of this report from the

World Wide Web: http://pubs.usgs.gov/of/2007/1001/

For more information on the USGS—the Federal source for science about the Earth,

its natural and living resources, natural hazards, and the environment:

World Wide Web: http://www.usgs.gov

Telephone: 1-888-ASK-USGS

To contact the authors:

Amy Draut, USGS Pacific Sciences Center, Santa Cruz CA 95060

831-427-4733; adraut@usgs.gov

David Rubin, USGS Pacific Sciences Center, Santa Cruz CA 95060

831-427-4736; drubin@usgs.gov

For more information on USGS science in Grand Canyon, Arizona through the Grand Canyon Monitoring

and Research Center: http://www.gcmrc.gov

Although this report is in the public domain, permission must be secured from the individual

copyright owners to reproduce any copyrighted material contained within this report.
CONTENTS

Executive Summary...............................................................................7

Introduction.........................................................................................11

Overview of wind, precipitation, and aeolian sediment-transport monitoring......14

Overview of sedimentary and geomorphic investigations..............................20

Identifying fluvial and aeolian deposits using sediment grain size..........28

Two types of aeolian deposits defined for the Colorado River corridor....31

Sedimentary environments—implications for restoration of archaeological

sites................................................................................35

Criteria for identifying site sensitivity to dam operations................................42

Three case studies......................................................................47

Effects of the November 2004 high-flow experiment on aeolian sediment

transport....................................................................................51

Site-specific findings.............................................................................56

24.5 mile....................................................................................56

Malgosa....................................................................................61

Palisades..................................................................................66

Comanche.................................................................................70

Forster......................................................................................74

202.9 mile.................................................................................77

Arroyo Grande...........................................................................79

Recommended future work....................................................................83

Conclusions........................................................................................86

Acknowledgements..............................................................................90
Appendix 1: Evaluation of aeolian sediment-transport models: can supply

limitation be discerned?................................................................91

Appendix 2: Estimation of historical changes in aeolian sand sources using a GIS

database developed at Utah State University..................................115

References.......................................................................................126

LIST OF TABLES

1. Summary of sedimentary processes affecting archaeological areas at

Palisades.

2. Summary of sedimentary processes affecting archaeological areas at

Lower Comanche.

3. Summary of sedimentary processes affecting archaeological areas at

Arroyo Grande.

4. Aeolian sand-transport models tested using high-resolution wind data from

Dumont Dunes (Death Valley, CA) and Malgosa (Grand Canyon).

5. Comparison of sand collected in sand traps during high-resolution

experiments at Dumont Dunes and Malgosa.

6. Reaches of the Colorado River, and corresponding river miles, analyzed

using a geomorphology GIS database developed by Utah State University.

7. Dates and river discharges of aerial photographs included in the USU GIS

database, and the available mapped reaches in each.

LIST OF FIGURES

1. Location map showing the Colorado River through Grand Canyon, with

study sites indicated.

2. Aerial photographs showing surficial geomorphology of the Palisades,

Comanche, and Arroyo Grande areas.

3. Comparison of grain-size distributions in fluvial and aeolian deposits.

4. Photographs showing examples of two categories of aeolian deposits

defined for the Colorado River corridor in Grand Canyon.

5. Strata mapped in detail at the (A) Palisades, (B) Lower Comanche, and

(C) Arroyo Grande study sites.

6. Aerial photographs of 24.5 mile in 1965 and 2004.

7. Aerial photographs of 23.5 mile in 1965 and 2004.

8. Aerial photographs of Malgosa in 1965 and 2004.

9. Photographs of different zones within the Malgosa dune field.

10. Aerial photographs of the Palisades area in 1965 and 2004.

11. Aerial photographs of the Comanche area in 1965 and 2004.
12. Aerial photographs of the area by the mouth of Forster Canyon in 1965

and 2004.

13. Aerial photographs of 202.9 mile in 1965 and 2004.

14. Aerial photographs of the Arroyo Grande area in 1965 and 2004.

15.Photograph showing the location of the experiment at Dumont Dunes.

16.Histograms of shear velocity (u*) measured during high-resolution

experiments at Dumont Dunes and Malgosa.

17.Sediment flux predicted for various shear velocities by eight aeolian

sediment-transport models.

18.Total areas of various types of sediment deposits mapped from aerial

photographs in the Glen Canyon (GL) reach.

19.Total areas of various types of sediment deposits mapped from aerial

photographs in the Lees Ferry (LF) reach.

20.Total areas of various types of sediment deposits mapped from aerial

photographs in the Redwall Gorge (RW) reach.

21.Total areas of various types of sediment deposits mapped from aerial

photographs in the Point Hansbrough (PH) reach.

22.Total areas of various types of sediment deposits mapped from aerial

photographs in the Little Colorado River (LCR) reach.

ADDITIONAL DIGITAL INFORMATION

For an online PDF version of this report, please see:

http://pubs.usgs.gov/of/2007/1001/

Regarding additional Grand Canyon research:

Grand Canyon Monitoring and Research Center: http://www.gcmrc.gov

For more information on the U.S. Geological Survey Western Region’s Coastal

and Marine Geology Team, please see:

http://walrus.wr.usgs.gov/

REPORT REFERENCE

Draut, A.E., and Rubin, D.M., 2006, The Role of Aeolian Sediment in the

Preservation of Archaeological Sites in the Colorado River Corridor, Grand

Canyon, Arizona: Final Report on Research Activities, 2003–2006: U.S.

Geological Survey Open-File Report 2007-1001, 141 p.
EXECUTIVE SUMMARY

This report summarizes a three-year study of aeolian sedimentary

processes in the Colorado River corridor, Grand Canyon, Arizona, and discusses

the relevance of those processes to the preservation of archaeological sites.

Findings are based upon detailed sedimentary and geomorphic investigations

conducted in three areas of the river corridor, continuous measurements of wind,

precipitation, and aeolian sediment transport at six locations for up to 26 months,

short-term field study at 35 other sites, examination of historical aerial

photographs, and review of data collected and analyzed by previous studies.

Detailed results of this study, which involved collaboration with scientists at the

Grand Canyon Monitoring and Research Center, National Park Service, Northern

Arizona University, the Hopi Tribe, and GeoArch, Inc., have been published

previously in topical USGS Open-File Reports (Draut and Rubin, 2005, 2006), a

USGS Scientific Investigations Report (Draut and others, 2005), and will be

discussed in two forthcoming journal articles. This report serves as an overview

of the results and contains new conclusions regarding aeolian sedimentary

processes in the Colorado River Ecosystem and their relevance to many

archaeological sites.

At many of the locations studied, aeolian sediment serves as a substrate

on which prehistoric cultural sites were formed (many sites are also formed on

fluvial and slope-wash sediment) and, commonly, as a surficial deposit that helps

to preserve cultural materials. Many cultural artifacts are exposed by wind

deflation of the surrounding sediment, forming a lag deposit on the land surface.

Cultural material is also vulnerable to erosion by gully incision. Incision of gullies

and arroyos into sediment deposits during rainfall runoff is an erosive process

counteracted by transport and deposition of aeolian sand, as gullies are natural

traps for wind-blown sand. The ability of aeolian sand transport to inhibit gully

formation and to heal small gullies (< 1 m wide and deep) is a major factor

controlling geomorphic evolution of Grand Canyon sediment deposits.

Precipitation and aeolian sand-transport rates measured during this study
showed that areas with heavy rainfall but abundant wind-blown sand may have

no gullies (which form episodically but are quickly filled by aeolian sand) whereas

areas of relatively low precipitation and little aeolian sand transport can be

eroded substantially by gullies and arroyos, destabilizing archaeological sites.

Many aeolian dune fields in Grand Canyon show evidence of reduced

sediment supply relative to some time in the past, with cryptogamic crust and

vegetation covering relict dune forms. Soil crusts and vegetation indicate a lack

of sediment deposition and decreased dune mobility compared to past times

when supply and transport of aeolian sand were abundant enough to generate

dune forms. Dunes with substantially reduced sediment supply (and with well

developed vegetation and soil crust) have insufficient active aeolian

sedimentation to counteract gully incision; large gullies and arroyos erode and

degrade such dune fields and many of the archaeological sites within them.

The availability and transport of aeolian sand has important implications

for management of Glen Canyon Dam operations, which regulate flow of the

Colorado River through Grand Canyon. Because fluvial sandbars now provide

the greatest source of aeolian sediment in the river corridor, the area of open

(unvegetated) dry fluvial sandbars directly affects entrainment and transport of

aeolian sand. Depending upon location and wind direction, fluvial-sandbar area

therefore can also substantially affect the transport of aeolian sand to dune fields

in which archaeological sites are covered by wind-blown sand. This relationship

between fluvial sandbars and many of the canyon’s aeolian deposits implies that

the availability of aeolian sand is affected by dam operations that increase or

decrease the area of open, dry fluvial sediment deposits. Dam operations can be

used to increase aeolian sand transport toward specific dune fields and

associated archaeological sites in locations with an appropriate dominant wind

direction. This report discusses criteria for evaluating the sensitivity of

archaeological sites to dam operations with regard to aeolian sedimentation.

We distinguish two types of aeolian deposits in the Grand Canyon river

corridor: (1) MFS (Modern Fluvial Sourced) deposits—those that formed as the

wind transported sand inland from a river-level sandbar, creating an aeolian dune
field downwind, and (2) RFS (Relict Fluvial Sourced) deposits—those that formed

as the wind eroded and redistributed sediment of extensive pre-dam flood

deposits. MFS deposits tend to be smaller and to occur more commonly in Grand

Canyon than RFS deposits. In RFS areas, the dominant wind direction may or

may not be appropriate to carry sand from the river (non-flood-stage fluvial

sandbars) toward local aeolian dune fields. Archaeological material is known to

occur in aeolian deposits of both types.

Both MFS and RFS aeolian deposits are sensitive to dam operations, but

for different reasons. MFS deposits, located directly downwind of fluvial

sandbars, receive a quantity of wind-blown sand that is a function of wind

velocity, the exposed dry fluvial sandbar area, and any barriers (such as

vegetation) that exist between the fluvial deposit and its associated aeolian

deposits. Fluvial sandbar area and riparian vegetation have both been shown, by

numerous studies, to respond to dam operations. Therefore, changes in either

fluvial sandbar area or riparian vegetation can affect the delivery of aeolian sand

to dune fields downwind. RFS deposits include aeolian coppice dunes (vegetated

sand mounds) that comprise the uppermost, wind-reworked part of pre-dam

terraces that are largely alluvial in composition, containing sediment deposited by

floods higher than any post-dam floods (up to and exceeding 5,660 m3 s-1

[200,000 ft3 s-1]). Because sediment-rich floods of that magnitude no longer occur

in this regulated river, the fluvial deposits from which extensive RFS aeolian dune

fields formed no longer constitute an active sediment source; no substantial new

deposition is likely to occur on those dune fields as long as the lack of large

sediment-rich floods continues. As vegetation and cryptogamic crust colonize

RFS coppice-dune fields over time, active aeolian sand transport decreases

within them such that these areas become increasingly unable to compensate for

precipitation-induced gully erosion. Large gully and arroyo systems can develop

in pre-dam terraces and associated RFS dune fields as a result.

Analysis of the effects of the November 2004 high-flow experiment on

aeolian sediment transport indicate that sediment-rich controlled high flows are

an effective means to restore sediment deposition in aeolian dune fields above
the high-flow elevation. Sandbar-building flows on the order of 1,270 m3 s-1

(45,000 ft3 s-1) have much more potential to enhance aeolian sedimentation on

MFS deposits than on RFS deposits, because of the direct link between fluvial

sandbars and MFS aeolian dunes downwind.

Our data suggest that restoration potential for cultural sites in aeolian

deposits can be optimized by using dam operations (controlled floods and postflood flows) that maximize the open sand area on fluvial sandbars during the

spring windy season (April through early June), when wind-borne sediment

transport is greatest. To provide the most benefit to aeolian deposits and

associated archaeological material, flows that follow sandbar-building high flows

would need to be managed to retain and maximize high-elevation, open, dry

sandbar area. In spring 2005, the first spring windy season that followed the

November 2004 high flow, the one study site with both a clear MFS-type link

between a fluvial sandbar and aeolian dunes and with substantial flood sediment

remaining in spring 2005 (having not been entirely eroded by high fluctuating

flows, as occurred at other study locations), wind-blown sand-transport rates

were significantly higher than in the 2004 spring windy season. At the same site,

gully erosion was effectively counteracted by deposition of aeolian sand derived

from the 2004 high-flow deposit. There would be a greater opportunity for windblown sand to fill gullies and counteract erosion of aeolian deposits and

associated archaeological sites if controlled high flows were followed by low

fluctuating flows or steady low flows rather than the experimental high daily flow

fluctuations that occurred in January–March 2005. Operating Glen Canyon Dam

under low fluctuating flows (or steady low flow) between the end of a controlled

high flow and the start of spring high winds is predicted to maintain the post-flood

sandbar size better than in winter 2005, leaving more sand available to be blown

into higher-elevation dune fields during the subsequent spring windy season.

Sedimentation patterns documented during this study indicate that aeolian

sediment deposits above the riparian zone can respond to dam operations, and

that archaeological sites located within aeolian deposits can be affected

consequently. “Sensitivity” of a MFS aeolian deposit and any associated cultural
sites to dam operations should be considered to have both negative and positive

connotations. If reduction of open, dry fluvial sandbar area causes reduced

transport to and deposition on an aeolian dune field downwind, then transport to

and deposition on that same dune field can be enhanced by managing river flows

to enlarge the fluvial sandbar that serves as a sand source for the dune field.

Increased aeolian transport in the dune field as a result of dam operations, as

shown by this study, can counteract deflation and gully incision there, leading to

greater preservation potential of associated cultural sites. Although preservation

of artifacts in Grand Canyon on geologic time scales is unrealistic due to

continual downcutting and backwasting of this bedrock canyon, cultural-site

protection could likely be enhanced on decadal to century time scales by

restoring fluvial and aeolian sedimentation to resemble more closely those

processes that occurred before the river was regulated by upstream dams,

including large, sediment-rich floods that left fluvial deposits from which sediment

was remobilized by wind.

INTRODUCTION

Since the closure of Glen Canyon Dam in 1963, the natural hydrograph

and sedimentary systems along the Colorado River in the reach through Grand

Canyon have changed significantly (for example, Andrews, 1986; Johnson and

Carothers, 1987; Webb and others, 1999a; Rubin and others, 2002; Topping and

others, 2003; Wright and others, 2005; Hazel and others, 2006a). The dam has

reduced the fluvial sediment supply at the upstream boundary of Grand Canyon

National Park by ~95 percent. Regulation of river discharge by dam operations

has important implications for storage and redistribution of sediment in the river

corridor. In the absence of floods, sediment is not deposited at elevations that

received sediment regularly before dam closure. Riparian vegetation has

colonized areas at lower elevation than in pre-dam time when annual floods
removed young vegetation (Turner and Karpiscak, 1980). Together, these factors

have caused a system-wide decrease in the size and number of subaerially

exposed fluvial sand deposits over the past four decades, punctuated by episodic

aggradation during exceptional high-flow intervals in 1983–1984, 1996, and

2004, and by sediment input from occasional tributary floods (Beus and others,

1985; Schmidt and Graf, 1987; Kearsley and others, 1994; Hazel and others,

1999; Schmidt and others, 2004; Wright and others, 2005).

When the Bureau of Reclamation sponsored the creation of the Glen

Canyon Environmental Studies (GCES) research initiative in 1982, research

objectives included physical and biological resources while effects on cultural

resources were not addressed (Fairley and others, 1994; Fairley, 2003). At that

time it was widely believed that, because few archaeological remains were

preserved within the river’s annual flood zone, cultural features would not be

greatly affected by dam operations. However, recent studies indicate that

alterations in the flow and sediment load of the Colorado River by Glen Canyon

Dam may affect archaeological sites within the river corridor, even above the

annual flood limit (Hereford and others, 1993, Yeatts, 1996, 1997; Thompson and

Potochnik, 2000; Draut and others, 2005). (The annual flood zone is defined here

by the mean annual pre-dam flood, 2,410 m3 s-1 [85,000 ft3 s-1]; the ‘pre-dam

flood limit’, the highest elevation at which fluvial deposits are locally present, was

roughly equivalent to a rare, major event of 8,500 m3 s-1 [300,000 ft3 s-1; Topping

and others, 2003]). Of the ~500 cultural sites documented between Glen Canyon

Dam and Separation Canyon (255 river miles), more than 330 are considered to

be within the Area of Potential Effect (APE) of dam operations designated by the

National Park Service (NPS) to be below the 8,490 m3 s-1 (300,000 ft3 s-1) stage

(Fairley and others, 1994; Neal and others, 2000; Fairley, 2005).

Archaeological research and monitoring in Grand Canyon National Park

focus increasingly on the potential effects of Glen Canyon Dam operations on the

landscape in which these cultural sites exist. Many cultural sites located in or on

sediment deposits are now eroding due to aeolian deflation and incision by

gullies (Leap and others, 2000; Neal and others, 2000; Fairley, 2003, 2005).
Hereford and others (1993) first suggested that gully incision of sediment

deposits, and the base level to which small drainage systems respond, were

linked to dam operations; they hypothesized that pronounced arroyo incision,

which occurs during rainfall runoff, was caused by lowering of the effective base

level at the mouths of ephemeral drainages to meet the new, post-dam elevation

of high-flow sediment deposition, ~3–4 m below the lowest pre-dam alluvial

terraces. Thompson and Potochnik (2000) modified that hypothesis to include

restorative effects of fluvial deposition in the mouths of gullies and arroyos,

presumed to raise effective base level at least temporarily, and new aeolian

deposition on pre-dam alluvial deposits as wind reworks flood sand. Thompson

and Potochnik (2000) concluded that sediment deprivation and lack of floods,

caused by dam operations, reduce the potential for new deposition that could

heal gullies formed by precipitation runoff. Neal and others (2000) suggested that

aeolian deposition in incipient gullies is “one of the strongest restorative forces

operating at archaeological sites.” Repeated high-resolution mapping conducted

after the 1996 controlled-flood experiment (1,270 m3 s-1 [45,000 ft3 s-1]; see

Webb and others, 1999b; Schmidt and others, 2001) confirmed that high flows

can deposit sediment in arroyo mouths, and also suggested that those deposits

can be a source for wind-blown sand that accumulates at higher elevation,

causing sediment accretion above the flood-stage elevation (Yeatts, 1997; Hazel

and others, 2000).

This study began in 2003 as a collaborative effort by the U.S. Geological

Survey (USGS) and National Park Service (NPS), funded by the U.S. Bureau of

Reclamation through the Grand Canyon Monitoring and Research Center

(GCMRC), to investigate the role of aeolian sediment transport in the

preservation of archaeological sites in the river corridor through Grand Canyon,

and thereby to clarify the effects or potential effects of dam operations on aeolian

deposits and any associated archaeological sites. Participants in this research

included geologists and archaeologists from the USGS (including GCMRC),

NPS, Northern Arizona University (NAU), GeoArch, Inc., the Hopi Tribe, the

Hualapai Nation, and Western Area Power Administration (WAPA).
Observations made by this research group during reconnaissance work in

May 2003 formed the basis for more detailed sedimentary and geomorphic

studies the following year and led to the selection of locations at which weather

stations were deployed to measure wind, rainfall, and aeolian sand transport

continuously between late 2003 and early 2006. This report summarizes the

results of the aeolian-sediment study and provides a final analysis and

conclusions drawn from the data. Detailed presentation, figures, and discussions

of data are not included with this report but have been published previously in

USGS Open-File Reports (Draut and Rubin, 2005, 2006), a USGS Scientific

Investigations Report (Draut and others, 2005), are in press with the journal

Geomorphology (Draut and others, submitted 6/05) and in review as a chapter in

a volume to be published by the University of Arizona in fall 2007 (Draut and

Rubin, submitted 12/05).

OVERVIEW OF WIND, PRECIPITATION, AND AEOLIAN

SEDIMENT-TRANSPORT MONITORING

Wind, precipitation, and aeolian sand-transport data collected during this

study at six locations constitute the only continuous weather record available

from the river corridor between November 2003 and January 2006 with the

exception of temperature and rainfall measurements made at Phantom Ranch

(near river-mile 88) by NPS (locations in the river corridor are commonly referred

to by distance in miles downstream of Lees Ferry, Arizona [Fig. 1]; this report

follows that convention while using metric units for other measurements. River

miles are those provided by the internet map server operated by GCMRC;

http://www.gcmrc.gov/products/ims/ims.htm).

This study provided the first measurements at this temporal and spatial

resolution of seasonal and regional variability in wind intensity and direction, and

resultant aeolian sediment transport, as well as precipitation patterns within the

interior of Grand Canyon. High-resolution records from these instrument stations
can be used to identify rainfall events that caused gully incision and to predict

aeolian sediment redistribution, thus aiding other sedimentary and geomorphic

studies of sediment deposits in the river corridor. Quantifying wind magnitude

and direction was essential for identifying sediment-transport pathways in the

vicinity of the study locations, information used subsequently to evaluate the

potential sensitivity of certain sites to dam operations. Detailed discussions of the

data collected at the weather stations, as well as electronic data files, were

published by Draut and Rubin (2005, 2006).

Nine weather stations operated in the Colorado River corridor in Grand

Canyon as part of the aeolian sediment-transport investigations. Locations of

instrument stations are shown in Figure 1. Three sites located in eastern Grand

Canyon, at 24.5 mile, Malgosa (river-mile 57.9), and Palisades (river-mile 66.1),

were equipped with two weather stations between November 2003 and January

2006. One instrument station operated at each of three additional sites:

Comanche (river-mile 68.0) in eastern Grand Canyon, and Forster (river-mile

123.0), and 202.9 mile in western Grand Canyon, between April 2004 and

January 2006. The weather stations were deployed in areas known to contain

cultural artifacts but located such that the presence and operation of equipment

would not disturb the sites. Analysis of aerial photographs indicated that all six

study sites had experienced a reduction in open sand area after Glen Canyon

Dam was constructed, and that all sites experienced new deposition as a result

of the 1996 high-flow experiment (Webb and others, 1999b). These criteria were

intended to allow monitoring of the effects of similar high-flow experiments in the

event that any occurred during the two-year operation of the weather-stations;

the effects of the November 2004 high-flow experiment were studied as part of

this project. Digital wind and rainfall data were recorded at 4-minute intervals at

all stations; aeolian sand-transport measurements were made based on

collection of sand in sand traps emptied during maintenance visits every 4–8

weeks (Draut and Rubin, 2005, 2006).

At all study locations, the most common incidence of high wind velocities,

and greatest potential for sediment transport (as well as actual measured sand-
0 km 50

113 ° 112° 37°

36°

Palisades

Comanche

202.9

mile

Lake Mead

Lake

Powell

Glen

Canyon

Dam

ria

ARIZONA

Grand Canyon National Park

or

Lees Ferry

24.5

mile

Malgosa

FIGURE 1. Location map showing the Colorado River through Grand

Canyon, with study sites indicated. Weather stations collected data at

24.5 mile, Malgosa, Palisades, Comanche, Forster, and 202.9 mile.

Sedimentary profiles were studied at Palisades, Comanche, and

Arroyo Grande. Lees Ferry, Phantom Ranch, and Diamond Creek are

shown for reference. The reach of the canyon between Lees Ferry and

the Little Colorado River is known as Marble Canyon.

Forster

Phantom

Ranch

Arroyo

Grande Diamond Creek
transport rates), occurred during the spring windy season. In 2004, seasonal

sediment transport peaked in April and May; the peak in 2005 was somewhat

later, in May and early June. During the spring windy seasons of 2004 and 2005,

maximum winds were locally as high as ~25 m s-1, and sand-transport rates

locally exceeded 100 g cm-1 day-1. At all stations, rates of sand transport during

the spring windy season were generally 5–15 times higher than at other times of

year.

Wind direction was observed often to change radically over very short time

intervals; wind velocities and dominant directions commonly varied on a diurnal

cycle, with velocities typically highest in the afternoon. At 24.5 mile, Malgosa, and

Palisades, where two weather stations operated at each site, wind velocities and

aeolian sand-transport rates were consistently higher at the higher-elevation

station than at river level. This is attributed in part to vegetation near the river,

which reduces wind velocity and therefore aeolian sediment entrainment; sand

transport has been shown in many cases to decrease approximately

exponentially as vegetation cover increases (Olson, 1958; Bressolier and

Thomas, 1977; Ash and Wasson, 1983; Wasson and Nanninga, 1986; Buckley,

1987; Bauer and others, 1996; King and others, 2005). Wind can also attain

higher velocity at higher elevation given the reduced interaction of air flow with

the canyon walls, although local topography can cause significant local variations

in wind speed and direction.

Variations in sand-transport rates between study sites are related not only

to variable wind strength but also to other local factors that affect the potential for

sand entrainment. Sand transport is likely reduced near the river because

moisture in the sediment that comprises fluvial sandbars limits entrainment of

sediment there by wind because (sand becomes cohesive when damp; see

Svasek and Terwindt, 1974; Sarre, 1988, 1989; McKenna Neuman and Nickling,

1989; Namikas and Sherman, 1995; Selah and Fryrear, 1995; Wiggs and others,

2004). In Grand Canyon, sandbars inundated by daily fluctuating flows often do

not dry thoroughly before they are inundated again the following day. Although

the sand traps used in this study did not allow resolution of sand transport on the
daily scales of river flow fluctuations (no automated sand traps are available

commercially that record sand transport at that resolution), this study

documented a local increase in aeolian sand transport adjacent to a new dry

(subaerial) sandbar deposited during the 2004 high-flow experiment (Draut and

Rubin, 2006).

The highest rates of sediment transport were measured in “active” dune

fields. Where active aeolian sediment transport is common, dune crests and

sand shadows are well defined, while vegetation and cryptogamic (cryptobiotic)

crust are rare. At active dune fields, particularly the Malgosa and Forster study

sites, sand transport occurred by saltation and suspension at rates commonly on

the order of 10 g cm-1 day-1 but reached 100–1000 g cm-1 day-1 during the spring

windy season. In contrast, less-active aeolian dune fields (those with substantial

vegetation cover and cryptogamic crust, such as Palisades and Comanche) had

sediment-transport rates at least an order of magnitude lower (0.1–1 g cm-1 day-1

during the non-windy season and 1–10 g cm-1 day-1 during the windy season).

Those measured differences are consistent with the findings of previous studies

that have demonstrated lower transport rates on sparsely vegetated dunes and

over surfaces with cryptogamic crust (Leys and Eldridge, 1998; Belnap, 2003;

Goossens, 2004). The dune field at 24.5 mile, which has both active and

relatively inactive areas, had transport rates more similar to those of Palisades

and Comanche (order 1 g cm-1 day-1) than to the very active Malgosa and Forster

sites. With the lowest transport rates of any sites, the weather station near the

river at Palisades (the station named Pal L; see Draut and Rubin, 2005, 2006 for

locations), located on a relict fluvial cobble-boulder bar, typically recorded

sediment-transport rates on the order of 0.01 g cm-1 day-1. Such low transport

rates at that location are apparently a function of a low supply of available sand,

and cannot be accounted for only by lower wind speeds at Station Pal L as

compared to those within the Palisades dune field (Station Pal U; Draut and

Rubin, 2006).

Precipitation data collected throughout the 26-month study interval

showed considerable spatial and seasonal variation. Daily rainfall at the upper
and lower rain gauges at a single location commonly differed by several tenths of

millimeters, and daily rainfall totals by up to several mm. Rain gauges recorded

strong, spatially isolated storms, particularly during the summer and early fall

monsoon season. Total annual rainfall varied by more than a factor of two over

distances of ~10 km in 2005 (Draut and Rubin, 2006), with Malgosa and

Palisades receiving the highest and lowest precipitation measured during this

study, respectively, a difference attributed to local topographic influence on the

movement of weather systems. During the unusually wet winter of 2005, western

Grand Canyon received approximately twice as much rain as the average of the

eastern Grand and Marble Canyon study sites. The degree of measured spatial

variation in precipitation indicates that future sedimentary and geomorphic

studies in the canyon would benefit substantially from continued or expanded

data collection at multiple locations along the river corridor, because rainfall

records collected by NPS at Phantom Ranch (near river-mile 88) are evidently

not valid for other areas of the canyon.
OVERVIEW OF SEDIMENTARY AND GEOMORPHIC INVESTIGATIONS

In May 2003, reconnaissance work was performed by personnel from the

USGS, Hopi and Hualapai Tribes, GeoArch Inc., and the Western Area Power

Administration (WAPA), led by NPS archaeologists J. R. Balsom, J. L. Dierker,

and L. M. Leap. This group of scientists visited 38 archaeological sites at which

aeolian sediment was believed to be important either as the material underlying

the site or forming a protective cover. Depositional environments of sediment

around these archaeological sites were determined where possible through

identification of sedimentary textures that are characteristic of waterlain (fluvial or

tributary flow) or aeolian deposition (e.g., Walker, 1963; Hunter, 1977; Rubin and

Hunter, 1982, 1987; Gladfelter, 1985; Rubin, 1987; Rubin and others, 1990;

Schmidt, 1990; Southard and Boguchwal, 1990). This initial 2003 survey, while

noting that the selected sites were likely not typical of all sites in the river

corridor, determined (through digging shallow pits into sediment to expose threedimensional sedimentary structures) that much of the sediment beneath or above

these particular archaeological sites had been transported by wind from fluvial

deposits at the river margins (Draut and others, 2005). Several of the sites were

built on or buried by sediment deposited directly by large floods of the Colorado

River. Observations made during the reconnaissance work in May 2003 formed

the basis for more detailed sedimentary and geomorphic studies at a selected

sub-set of the 38 sites the following year, and provided the basis for the

establishment of the weather stations discussed above.

During the May 2003 field work, the research group observed that aeolian

sand transport to archaeological sites is apparently limited largely by sediment

supply and vegetation rather than by wind. Reduction in the open sand area on

sandbars, and submergence of sandbars by daily flow fluctuations, can be

expected to reduce aeolian sand transport to nearby dune fields that contain

archaeological sites (cf. Ash and Wasson, 1983; Buckley, 1987; Namikas and

Sherman, 1995; Wiggs and others, 2004). Reduced aeolian transport, in turn,

can be expected to facilitate the growth of existing arroyos and establishment of
new ones, as discussed by Thompson and Potochnik (2000). At many of the

sites visited in May 2003, erosion of archaeological sites by arroyo incision was

apparent; repeat photography at those sites by NPS over the past decade

supports this observation (for example, Leap and others, 2000). Some

archaeological sites are affected by small drainages that might be repaired by

modest increases in aeolian sand deposition; in some locations, filling of small

gullies (<1 m wide) by aeolian sand would aid preservation of archaeological

features that have already been stabilized by construction of check-dams. Other

sites are compromised by very large drainages that could not reasonably be

expected to fill with aeolian sediment given the current local geomorphology and

observable sand supply.

Sedimentary and geomorphic work conducted in May 2004 focused on the

Palisades, Lower Comanche, and Arroyo Grande areas of Grand Canyon (Fig.

2). These reaches of the river corridor are characterized by alluvial terraces that

represent multiple episodes of floodplain aggradation within the pool-and-drop

bedrock canyon of the Colorado River. The highest alluvial terraces at each site

contain deposits left by pre-dam flood events of over 5,660 m3 s-1 (200,000 ft3 s-1;

Hereford and others, 1993; Topping and others, 2003). The terraces at all three

sites contain arroyo networks (sensu Patton and Schumm, 1981; Waters and

Haynes, 2001; Webb and Hereford, 2001) up to several meters deep and wide

resulting from incision by local precipitation runoff. Coppice dunes partially cover

the terrace surfaces at all three sites. Talus piles are commonly present at the

base of bedrock walls, and debris fans occur at the mouths of side-canyon

tributaries. The work conducted by this research group in 2004 complemented

several previous studies, particularly at Palisades. Geomorphic mapping by

Hereford (1993, 1996) and by Hereford and others (1993, 1996) at Palisades

generated detailed interpretations of the surficial geology and radiocarbon dates;

the same location was one area used by Hereford and others (1993) and

Thompson and Potochnik (2000) to formulate the base-level hypotheses

discussed above. Grams and Schmidt (1999) used historical photographs to

document reduction in the extent of surficial sand deposits at Palisades since
C 50 m

River flow

1

11

10 9

8

3

4

7

6

2

5

T

T

T

T

T

T

T

T

T

T

T

T

N

B

3

1 2

4

5 6 T

T

T

T

T T

River

flow

A 50 m

3

1

2

4

T

T

T

T

T

T

T

T

T T

T

N N
FIGURE 2. Aerial photographs showing surficial geomorphology of the

Palisades, Comanche, and Arroyo Grande areas. General geomorphology is

shown in colored overlays and the locations of studied sedimentary exposures

are indicated with white circles. Circles with numbers are sedimentary profiles

that were measured in detail and described by Draut and others (2005); circles

labeled with a ‘T’ are test pits at which sedimentary properties were studied but in

less detail and to shallower depth than at the numbered exposures. A. The

Palisades area, with geomorphology based both on this study and mapping by

Hereford (1993). The area outlined in pale green in the northern part of this

image is a debris fan on which Hereford (1993) documented evidence of multiple

debris flows. The darker green strip along the shoreline is a bar containing well

rounded, well sorted boulders and cobbles that likely represents relict fluvial

reworking of a debris-flow deposit. The broad area shown in light blue is a

terrace surface mapped by Hereford (1993) as a relict (pre-dam) alluvial feature.

The areas in darker blue are modern fluvial sandbars at an elevation within the

river’s usual dam-controlled fluctuations (below a discharge of 566 m3 s-1 [20,000

ft3 s-1]; in all three photographs in this figure, the flow is 226 m3 s-1 [8,000 ft3 s-1]).

Areas in yellow are coppice-dune fields. The area in orange is a playa-like

surface where water forms small ponds after recent rain; slope-wash and

colluvial sediment accumulates there. An arroyo drainage, which trends roughly

perpendicular to the river, is outlined in white. B. The Lower Comanche area,

with the same general color scheme as in A. The green area is a boulder bar

probably reworked from a debris fan several hundred meters upstream of the

area shown. The blue overlay shows the outline of fluvial deposits that form a

narrow terrace, although the landward border is approximate because vegetation

obscures the exposure. The large yellow area contains coppice dunes at the

modern land surface. The orange area is a talus and colluvial slope drained by

an ephemeral stream and arroyo network outlined in white. C. The Arroyo

Grande area. Two alluvial terrace surfaces are apparent, one at higher elevation

(shown in light blue) and one at lower elevation (darker blue). The boundary

between these terraces is approximated here because vegetation obscures

some of the exposure. Aeolian coppice dunes, shown in yellow, are present

locally on the surface of the upper terrace and to its east. The area in orange is

talus and colluvium with some bedrock exposed. The dendritic arroyo after which

this site is named is outlined in white.
1890. High-resolution mapping by Yeatts (1996) and Hazel and others (2000)

indicated aggradation of fluvial sand deposits at Palisades as a result of the 1996

controlled flood released from Glen Canyon Dam, and inferred aeolian migration

of sediment to higher elevation over the following year.

After viewing natural sediment exposures, the May 2004 research group

chose to focus on those that were most complete vertically and that offered good

spatial coverage throughout variable terrain. Data were obtained through detailed

examination of sedimentary profiles and numerous shallow test pits in alluvial

terraces, coppice-dune fields, and, at Palisades, near the distal margin of a

debris fan from a tributary canyon (Fig. 2). Sediment exposures studied during

this work were located in the general area of archaeological features but no

cultural artifacts were exposed or collected. The presence of arroyos facilitated

the exposure of vertical faces; at Palisades and Lower Comanche, three of the

sections studied in detail were exposed by digging pits into areas without cultural

artifacts. Thicknesses of sedimentary units were measured and sedimentary

structures, clast lithology, and color of the sediment were described. At selected

locations, sediment samples were collected for grain-size analysis using a

Coulter laser particle-size analyzer at the GCMRC laboratory in Flagstaff,

Arizona. Sedimentary structures were used to identify the number and thickness

of fluvial deposits, aeolian reworking of fluvial sediment, and interaction of those

processes with local runoff sedimentation. Geomorphic and sedimentary

processes that affect each studied archaeological site are summarized for

Palisades, Lower Comanche, and Arroyo Grande in Tables 1, 2, and 3,

respectively; detailed discussions of each site have been published by Draut and

others (2005).
TABLE 1. Summary of sedimentary processes affecting archaeological areas at

Palisades. Archaeological-site numbers are those assigned by the National Park

Service (NPS). Detailed discussions of processes in these areas may be found in

Draut and others (2005). Prehistoric sites have been dated by radiocarbon

methods and artifact identification to the Pueblo I and Pueblo II (PI and PII)

periods (Fairley and others, 1994; Hereford, 1996) although there is some

evidence for Preformative occupation (Dierker and Downum, 2004). The historic

site is associated with mining activity and was occupied from A.D. 1890–1910.

Site

Number

Age and

Affiliation

of Site

Sediment on which

site was originally

built

Sediment

overlying

site

Sedimentary and

geomorphic

characteristics

C:13:033 Uncertain;

probable

Pueblo I–III

Debris-fan cobbles,

gravel

Aeolian

(minor)

Dominated by debris-flow

deposition

C:13:098 Historic Debris-fan cobbles,

gravel

None Remains of 19th century cabin

built on debris-fan sediment

C:13:099 Pueblo I–II Fluvial; slope-wash

sand and gravel

Aeolian; distal

debris-flow

material

Interbedding of fluvial, aeolian,

debris-flow and slope-wash

sediment

C:13:100 Pueblo I–II Fluvial Fluvial/aeolian Fluvial sediment modified by

wind

C:13:101 Pueblo I–II Rounded cobbles, likely

fluvial

Aeolian

(minor)

Coppice dunes overlying

rounded cobbles

C:13:272 Pueblo I–II Debris-fan cobbles,

gravel

Fluvial and

aeolian

Fluvial sediment modified by

wind into coppice dunes;

bioturbated

C:13:334 Pueblo I–II Fluvial; ponded slopewashFluvial Eroding out of apparent fluvial

deposit

C:13:336 Pueblo I–II Likely fluvial (artifacts

not in original position)

Fluvial/aeolian Fluvial sediment reworked by

wind into coppice dunes

C:13:355 Pueblo II Fluvial; distal debrisflowFluvial and

aeolian

Area dominated by debris fan,

smaller volumes of fluvial and

aeolian sediment
TABLE 2. Summary of sedimentary processes affecting archaeological areas at

Lower Comanche. Site numbers have been assigned by NPS. Most cultural

features in this area date to the Late PI – Early PII Formative period (A.D.

~900–1000; Fairley and others, 1994; NPS, 2004). At least one site contains

artifacts related to late prehistoric and early historic use by the Hopi Tribe (Fairley

and others, 1994). The Early Formative age given for Site C:13:273 is based on

radiocarbon ages obtained from a hearth feature by Yeatts (1998).

Site

Number

Age and

Affiliation of Site

Sediment on

which site was

originally built

Sediment

overlying

site

Sedimentary and

geomorphic

characteristics

C:13:273 Early Formative

(Basketmaker III)

Fluvial, aeolian;

colluvial

Aeolian

(minor)

Fluvial and aeolian

deposits interbedded with

slope-wash colluvium

C:13:274 Pueblo I–II Aeolian None Aeolian substrate overlies

distal debris-fan sediment

C:13:333 Pueblo I–II Aeolian Aeolian

(minor)

Site destabilized by aeolian

deflation and dune

migration

C:13:335 Pueblo I–II Aeolian Aeolian Exposed by aeolian

deflation

C:13:337 Pueblo I–II Aeolian None Aeolian dunes; site in

interdune area

C:13:373 Prehistoric to Early

Historic Hopi

Aeolian Aeolian Site destabilized by aeolian

deflation and dune

migration
TABLE 3. Summary of sedimentary processes affecting archaeological areas at

Arroyo Grande. Site G:03:064 includes 15 separate features. In general these

are associated with the protohistoric and early historic-era Pai culture related to

the Paiute and Hualapai Tribes. Many features were built on aeolian sediment

directly, but this relatively thin aeolian substrate is underlain by volumetrically

more substantial flood deposits.

Site

number

Site age and

affiliation

Sediment on

which site was

originally built

Sediment

overlying

site

Sedimentary and

geomorphic characteristics

G:03:064,

F1

Protohistoric to

Early Historic Pai

Aeolian Aeolian (minor) Coppice dunes on upper terrace

surface

G:03:064,

F2

Protohistoric to

Early Historic Pai

Aeolian Aeolian (minor) Coppice dunes on upper-terrace

surface

G:03:064,

F3

Protohistoric to

Early Historic Pai

Aeolian Aeolian (minor) Coppice dunes on upper-terrace

surface

G:03:064.

F4

Protohistoric to

Early Historic Pai

Aeolian Aeolian (minor) Interdune area in coppice-dune

field; arroyo incision affects

feature stability

G:03:064.

F5

Protohistoric to

Early Historic Pai

Aeolian Aeolian (minor) Interdune area in coppice-dune

field; affected by aeolian

deflation

G:03:064.

F6

Protohistoric to

Early Historic Pai

Terrace surface

(dominantly fluvial)

None Arroyo incision affects terrace

surface in the vicinity of this

feature.

G:03:064,

F7

Protohistoric to

Early Historic Pai

Terrace surface

(dominantly fluvial)

None Surface of upper terrace

G:03:064,

F8

Protohistoric to

Early Historic Pai

Terrace surface

(dominantly fluvial)

None Surface of upper terrace

G:03:064,

F9

Protohistoric to

Early Historic Pai

Terrace surface

(dominantly fluvial)

None Surface of upper terrace

G:03:064,

F10

Protohistoric to

Early Historic Pai

Terrace surface

(dominantly fluvial)

None Surface of upper terrace

G:03:064,

F11

Protohistoric to

Early Historic Pai

Terrace surface

(dominantly fluvial)

None Surface of upper terrace

G:03:064,

F12

Protohistoric to

Early Historic Pai

Aeolian Aeolian

(partially

covers feature)

Small coppice dunes on upperterrace surface. Feature affected

by arroyo incision.

G:03:064,

F13

Protohistoric to

Early Historic Pai

Aeolian None Small coppice dunes on upperterrace surface. Feature affected

by arroyo incision.

G:03:064,

F14

Protohistoric to

Early Historic Pai

Interbedded

fluvial/aeolian/collu

vial

Aeolian Exposed in small drainage within

upper terrace

G:03:064,

F15

Protohistoric to

Early Historic Pai

Colluvium None Near landward edge of upper

terrace, in area dominated by

slope-wash sedimentation

Not

assigned

Unknown Slope-wash

colluvium and

aeolian

Fluvial,

aeolian, and

colluvial

Hearth feature exposed in arroyo

wall within upper terrace
Identifying Fluvial and Aeolian Deposits Using Sediment Grain Size:

In addition to describing depositional units using sedimentary structures,

we investigated the use of grain-size analysis to distinguish fluvial from aeolian

material. Previous studies have shown that grain size and moment statistics are

of little use for identifying fluvial and aeolian deposits when comparing samples

from multiple geographic regions (Ahlbrandt, 1979; Tucker and Vacher, 1980;

Gladfelter, 1985) due to variations in clast composition and weathering

conditions. We analyzed particle-size distributions of 100 sediment samples from

pre-dam fluvial deposits, post-dam fluvial deposits, and aeolian deposits

collected only within the Colorado River corridor of Grand Canyon (Fig. 3), to

assess whether enough consistency is present within this sedimentary system to

use grain size to classify samples of unknown origin also collected within this

same river corridor. (We refer here only to fine-grained fluvial deposits at floodstage elevation that are relevant to archaeology; deeper in the river channel,

fluvial deposits include sediment up to boulder sizes to which this discussion

does not apply).

Although there is a range of grain sizes within Colorado River fluvial

deposits (a function of sediment availability, flow velocity, and proximity to

flooding tributaries), this study found essentially no overlap between textural

characteristics of pre-dam fluvial (Fig. 3a) and mature aeolian deposits (Fig. 3c)

in Grand Canyon, such that the finest aeolian deposits are coarser than the

coarsest pre-dam fluvial units sampled (Fig. 3e; see also Burke and others,

2003). The lack of overlap indicates that although the mature aeolian deposits

are assumed to have been derived largely from pre-dam fluvial deposits, enough

of the finer fluvial sediment is removed by wind that the grain-size distribution of

the remaining (winnowed) sand in mature aeolian dunes is characteristically

coarser than the original fluvial deposit (see also Rubin and others, 2006). Grainsize distributions of aeolian samples do overlap, however, with those from postdam high-flow deposits (Fig. 3f). The post-dam fluvial samples shown (deposited

by the November 2004 high flow) are coarser, as a population, than the pre-dam
20

40

60

80

100

0.01 0.1 1

20

40

60

80

100

0.01 0.1 1

20

40

60

80

100

0.01 0.1 1

20

40

60

80

100

0.01 0.1 1

20

40

60

80

100

0.01 0.1 1

E

Pre-dam

fluvial

deposits

Aeolian

deposits

20

40

60

80

100

0.01 0.1 1

A B C

D F

Grain size (mm)

Grain size (mm)

Pre-dam

fluvial

deposits

Post-dam

(November 2004)

fluvial deposits Aeolian deposits

Pre-dam

fluvial

deposits

Post-dam

(November

2004) fluvial

deposits

Post-dam

(November

2004) fluvial

deposits

Aeolian

deposits
FIGURE 3. Comparison of grain-size distributions, obtained by laser particle-size

analysis, in fluvial and aeolian deposits. In all plots, values on the vertical axis

indicate the cumulative percent of the sediment sample that is finer than the

corresponding grain diameter shown on the horizontal axis. Distributions are

shown for a grain-size range of 0.037 mm (coarse silt) to 1.000 mm (the

boundary between coarse sand and very coarse sand in the Wentworth

classification). A. Grain-size distributions of 27 sediment samples collected from

10 different fluvial strata, all of which were deposited by Colorado River floods

that pre-dated the closure of Glen Canyon Dam and associated changes in the

downstream sediment load. These samples were collected in May 2004 from

strata at Palisades, Lower Comanche, and Arroyo Grande identified as fluvial

deposits by the presence of unequivocal sedimentary structures. Multiple

samples were collected from some flood deposits because grain size commonly

varies vertically within each deposit (Rubin and others, 1998). B. Grain-size

distributions of 63 samples collected from 10 different deposits of the

experimental flood conducted on the Colorado River through Grand Canyon in

November 2004 (Topping and others, 2006). C. Grain-size distributions of

samples collected at 10 aeolian deposits in Grand Canyon in 2003 and 2004. D.

Comparison of the pre-dam fluvial deposits from A, shown with a light gray field

outlined in a solid line, with the post-dam (November 2004) deposits of B,

outlined with the darker gray field and dashed line. Deposits of the November

2004 flood are coarser, and show a tighter distribution, than the pre-dam

deposits. This is consistent with previous findings (Rubin and others, 1998;

Topping and others, 2000a, b) that the sediment transported and deposited by

this river coarsened after closure of Glen Canyon Dam, in response to limitation

of the upstream sediment supply and winnowing of the fine sediment already in

the river channel. E. Comparison of the pre-dam fluvial deposits of A, the lightgray field, with the aeolian deposits of C, the dark gray field. A distinct grain-size

separation is apparent, with the finest aeolian deposits being coarser than the

coarsest pre-dam flood deposits. Minor overlap occurs in the two fields at the

very coarsest and very finest grain sizes measured, but the fields are otherwise

discrete. F. Comparison of the post-dam, November 2004 flood deposits of B, the

medium-gray field with dashed border, with the aeolian deposits of C, the dark

gray field. The November 2004 experimental-flood deposits show grain-size

distributions that partially overlap with the aeolian field, the coarsest fluvial

samples being coarser than the finest aeolian samples.
flood deposits (Fig. 3d). This is consistent with previous analyses of other postdam flood deposits and reflects the fact that the sediment transported and

deposited by this river coarsened after closure of Glen Canyon Dam, in response

to limitation of the upstream sediment supply and winnowing of the fine sediment

already in the river channel (Rubin and others, 1998; Topping and others, 2000a,

b).

Grain size therefore apparently can be used to distinguish pre-dam flood

deposits (which underlie and overlie archaeological sites in Grand Canyon, unlike

the younger, lower-elevation post-dam deposits) from mature aeolian deposits

within this river corridor if sedimentary structures are absent. Particle size as a

diagnostic tool should still be used with caution even within one sedimentary

system unless the range of grain sizes for known aeolian and fluvial deposits in

that system is well defined. The difference in grain-size distribution between a

fluvial deposit and its winnowed, aeolian counterpart depends on the initial

composition of the flood sediment and on the wind conditions affecting the

sediment. Colorado River fluvial sediment that has undergone some wind

reworking might not yet plot within the mature aeolian field of Figure 3c, limiting

these diagnostic applications. The patterns in Figure 3 indicate that sand in the

mature aeolian deposits sampled was derived almost entirely from extensive,

pre-dam flood deposits with substantially less contribution from the smaller,

lower-elevation deposits left by rare post-dam high flows.

Two Types of Aeolian Deposits Defined for the Colorado River Corridor:

Findings from the sedimentary and geomorphic investigations of 2004,

combined with quantified seasonal wind directions and sediment-transport

potential, indicate that aeolian deposits in the Grand Canyon river corridor fall

into two categories referred to here as Modern Fluvial Sourced (MFS) and Relict

Fluvial Sourced (RFS). The two categories are distinguished by the relative

locations and orientations of aeolian deposits, modern (<1,270 m3 s-1 [45,000 ft3
s-1]) fluvial deposits, and dominant wind direction causing sediment transport in

each setting.

MFS aeolian deposits are those located directly downwind from recent

(post-dam) fluvial sediment deposits. These aeolian dune fields formed as the

wind transported sand inland from a river-level sandbar, creating the dune field

downwind. An example of a MFS aeolian deposit, at the mouth of Forster

Canyon (river-mile 123.0), is shown in Figure 4a. At this location the wind

direction (measured during this study) is consistently from the north-northeast,

dune configuration indicates net sediment transport toward the south-southwest,

and substantial wind velocities (>20 m s-1 during the spring windy season) thus

result in the transport of sediment away from a river-level sandbar to form a dune

field downwind. Within this very active dune field, sand-transport rates on the

order of 100 g cm-1 day-1 were common in 2004 and 2005, and dune-surface

elevations were observed to increase or decrease locally by tens of centimeters

between equipment-maintenance visits made every six to eight weeks.

RFS aeolian deposits formed as the wind eroded and redeposited

sediment that comprised extensive pre-dam flood deposits, generating aeolian

dunes essentially ‘in place’ over parts of fluvial terraces. Whether or not RFS

deposits are located downwind of modern fluvial sandbars, their most significant

sediment source is not the modern, presently active sandbars (those containing

fluvial sediment transported by river flows at or below ~1,270 m3 s-1 [45,000 ft3 s1

]) but larger pre-dam flood deposits. The coppice-dune field at Palisades, shown

in Figure 4b, is an example of a RFS deposit. This study and Hereford and others

(1993, 1996) showed that the Palisades area is underlain by extensive pre-dam

fluvial deposits; the aeolian dune area lies atop part of the fluvial terrace. The

dominant wind direction measured at Palisades over more than two years comes

from the south-southeast (Draut and Rubin, 2006). If that net direction is

representative of long-term local weather conditions, then it is unlikely that

significant quantities of sediment are transported by wind from river-level

sandbars to the aeolian dune field at Palisades. The aeolian dunes apparently

formed instead from wind erosion and redeposition of sediment that comprised
N

A B

50 m

*

50 m

*

*

*

N

(ephemeral

sandbar)

FIGURE 4. Photographs showing examples of two categories of aeolian deposits defined for the Colorado River corridor in Grand

Canyon. Green arrows indicate the dominant local wind direction capable of transporting sand (obtained from vector sums of fourminute wind velocity measurements over many months; see Draut and Rubin, 2005, 2006). Aeolian dune fields are outlined with

black dashed lines. Red asterisks indicate nearby fluvial sand deposits. A. Modern Fluvial Sourced (MFS) aeolian dune fields,

such as this example at the mouth of Forster Canyon (river-mile 123.0) are located directly downwind from fluvial sand deposits.

Here the wind direction is consistently from the north-northeast, transporting sand away from a river-level sandbar to form a dune

field downwind. B. Relict Fluvial Sourced (RFS) aeolian deposits, such as this example from Palisades (river-mile ~66) formed as

the wind reworked sediment deposited by large pre-dam floods, generating aeolian dunes essentially ‘in place’ over parts of fluvial terraces. The dominant wind direction at Palisades is from the south-southeast. Modern fluvial sand deposits (shown by red

asterisks) at Palisades are not directly upwind of the aeolian dune field, and so little sediment from those fluvial sandbars reaches

the dunes.
the pre-dam fluvial deposits. Some migration of aeolian dunes is apparent today

in the northern part of the Palisades dune field, but in general this dune field is

relatively inactive with grasses, mesquite, and cryptogamic crust well

established. Although some new sediment is probably transported into the dune

field on occasion from river-level sandbars (Yeatts, 1996) given appropriate

short-term wind conditions, the net wind direction measured at Palisades by this

study indicates that wind likely causes a net loss of sediment from this dune field

toward the northwest (where it can be lost into the river) at rates on the order of

~1 g cm-1 day-1. Continued erosion of both aeolian and fluvial deposits by gullies

causes substantial additional loss of sediment at Palisades. Because the

mechanism of formation of this RFS aeolian deposit was linked inextricably with

deposition of fluvial sediment during large, sediment-rich pre-dam floods, it is

unlikely that substantial new sediment will be supplied to the Palisades dune field

without similar large, sediment-rich floods in the future.

MFS aeolian deposits tend to be smaller and to occur more commonly in

Grand Canyon than RFS deposits. Aeolian deposits of both types are known to

contain archaeological material. Both MFS and RFS dune fields should be

considered sensitive to Glen Canyon Dam operations, but for different reasons.

MFS deposits, located directly downwind of fluvial sandbars, receive an amount

of wind-blown sand that is a function of wind velocity, exposed dry fluvial sandbar

area, and sediment-trapping vegetation that exist between the fluvial deposit and

its associated aeolian deposits. Fluvial sandbar area and riparian vegetation

have both been shown, by numerous studies, to respond to dam operations.

Therefore, changes to either have the potential to affect the delivery of aeolian

sand to MFS dune fields downwind.

The length and width of fluvial sandbars relative to the local wind direction

control the supply of wind-blown sediment inland that forms MFS aeolian

deposits and potentially covers local archaeological sites. For example, a narrow

(2 m wide) sandbar extending 50 m along the river bank will be able to supply

more wind-blown sand inland, and to a longer strip of shoreline, downwind than

will a sandbar 10 m wide and 10 m long assuming equilibrium sand transport is
attained during wind events for each. Sensitivity of MFS aeolian deposits to dam

operations is therefore a function not only of post-dam changes in the area of

open, dry sand on fluvial sandbars but also of the dimensions of the remaining

open, dry sandbar and its location relative to aeolian deposits (and any

archaeological sites) away from the river’s edge.

RFS deposits include aeolian coppice dunes that comprise the uppermost,

wind-reworked part of pre-dam terraces that are largely alluvial in composition,

containing sediment deposited by floods higher than any post-dam floods have

been (up to and exceeding 5,660 m3 s-1 [200,000 ft3 s-1]). Sediment-rich floods of

that magnitude no longer occur in this regulated river and the relict fluvial

deposits no longer constitute an active sediment source. Therefore, no

substantial new deposition is likely to occur on RFS dune fields as long as the

lack of large sediment-rich floods continues. As vegetation and cryptogamic crust

grow on RFS coppice-dune fields over time, active aeolian sand transport

decreases within them such that these areas become increasingly unable to

compensate for precipitation-induced gully erosion. Large gully and arroyo

systems can develop in pre-dam terraces and associated RFS dune fields as a

result (Figs. 2a, 2c).

Sedimentary Environments—Implications for Restoration of

Archaeological Sites:

Fluvial deposits are the most common substrate underlying archaeological

sites at Palisades and Arroyo Grande. Fluvial deposition was found generally to

decrease with increased elevation and distance from the river as locally derived

slope-wash and colluvial sediment became more prominent (Fig. 5). The

proportion of fluvial sediment does not decrease regularly away from the river,

however, in part because aeolian modification can occur across much of an

alluvial terrace almost irrespective of distance from the river (although wind

velocities are lowest at the river margin, especially in the post-dam era, where

vegetation is commonly thick). In the Colorado River corridor through Grand
1

2

5

6

7

8

9

10

11

4

428

429

430

431

432

27 29 36 45 55 59 60 63 72 95

427

Distance from river (m)

C

Fluvial sediment

Aeolian sediment

Slope-wash and

colluvial sediment

Sedimentary

textures

inconclusive

Distal debris-flow

sediment

1

798.898

2

800.742

4

800.619

3

800.972

800

46 106 117 137

798

799

801

Distance from river (m)

A

KEY

797

796

795

60 72 85 96 175

794

793

792

796.147

2

Distance from river (m)

795.874

4

5

793.395 6

795.525

797.480

3

B
FIGURE 5. Strata mapped in detail at the (A) Palisades, (B) Lower Comanche,

and (C) Arroyo Grande study sites. Horizontal axes indicate cross-shore distance

from the river. The vertical axes indicate elevation (m) referenced to the

NAD83(1999) datum for the profiles; numbers above each profile are the

elevation of the land surface, in meters. The 1,270 m3 s-1 (45,000 ft3 s-1) stage is

equivalent to an elevation of ~796 m at Palisades, ~789 m at Comanche, and

~425 m at Arroyo Grande (stage-discharge relationships for Palisades and

Comanche are given in Draut and others [2005] and Hazel and others [2006b];

values for Arroyo Grande are an estimate only). Numbers beneath each profile

are the stratigraphic section numbers by which these profiles are identified in a

detailed unit-by-unit discussion in Draut and others (2005). Individual

sedimentary units within each measured profile are shown; colors of strata

correspond to inferred depositional environment (assigned on the basis of

sedimentary structures, clast composition, and, in the case of approximately 10

individual strata, grain-size distribution). The color key shown in A applies to

parts A, B, and C: blue units are fluvial deposits, yellow are aeolian, orange are

slope-wash and colluvial sediments derived from local bedrock and talus, green

are distal debris-flow deposits, and blank (white) units are those for which a

depositional setting was not assigned, either because sedimentary textures were

inconclusive or because those parts of the sedimentary profiles were not

accessible. Width of each sedimentary unit shown in the figure is a schematic

representation of its grain size (the wider the unit, the coarser the sediment);

some units coarsen upward or fine upward. Grain-size properties were

determined primarily by visual inspection and use of grain-size-comparator

charts. Selected strata were also sub-sampled for laser particle-size analysis,

accounting for around 10% of the total number of strata described. In general, at

the three study sites, the proportion of fluvial sediment decreases with distance

from the river, as aeolian sediment and material derived from slope-wash and

colluvial processes become more volumetrically significant away from the river

and with increasing elevation. This trend is to be expected because areas farther

from the river and at higher elevation would be inundated by only the largest

floods, and to a shallower depth than low-elevation sites near the main channel.

The decrease in recognizable fluvial deposits away from the river is not a uniform

progression, however, because local topography affects all relevant depositional

processes to some extent and because aeolian reworking of subaerially exposed

flood sand can occur at virtually any elevation or distance from the river.

Modification of fluvial deposits into aeolian deposits thus gradually reduces the

proportion of recognizable “fluvial” sediment after a flood recedes.
Canyon, wind speed (and aeolian modification of fluvial sediment) is typically

greatest during the spring windy season, as discussed above. Although aeolian

dunes are present on terrace surfaces at Palisades and Arroyo Grande, aeolian

sediment was found to be more important as a protective cover of sites at these

locations than as the substrate on which cultural features were formed. At Lower

Comanche, in contrast, five of the six archaeological sites were formed on and

are partially buried by aeolian sediment; however, the largest and most extensive

site at Lower Comanche is not in the aeolian dune field but is on a lowerelevation terrace constructed of fluvial, aeolian, and slope-wash deposits. In all

three study areas, geomorphic processes that affect the condition of cultural sites

today include arroyo incision, wind action, and episodic local deposition of slopewash and debris-flow sediment. Arroyo incision and aeolian deflation contribute

to exposure and erosion of artifacts in all three areas. Aeolian deflation exposes

artifacts that may then be washed away by rainfall runoff, and dune migration

(particularly at Lower Comanche) causes down-slope movement of artifacts as

the sand surface shifts under them. Episodic deposition of flood sediment at

Palisades and Arroyo Grande no longer occurs at elevations high enough to

affect archaeological sites under the current dam-controlled flow regime of the

Colorado River.

Many archaeological sites, including most of those at Palisades, Lower

Comanche, and Arroyo Grande, were formed on terraces that consist largely of

Colorado River alluvium. Many of those sites are preserved today by having

been buried by flood deposits and aeolian dunes. From prehistoric times until the

closure of Glen Canyon Dam in 1963, the greatest fluvial deposition in this river

occurred during spring snowmelt floods that regularly exceeded even the highest

post-dam discharge; the highest post-dam flow of 2,750 m3 s-1 (97,000 ft3 s-1)

occurred in 1983, while the pre-dam eight-year flood level was 3,540 m3 s-1

(125,000 ft3 s-1; Topping and others, 2003). Annual sediment-rich floods occurred

concurrently with or receded during the spring windy season, when wind would

have remobilized sand from recent flood deposits to form aeolian dune fields.

The modern absence of sediment-rich floods eliminates the mechanism for
depositing sediment at the elevations of prehistoric alluvium and also eliminates

the source of new sediment to aeolian deposits. Dam operations have thus

substantially altered both the fluvial and aeolian processes that acted on the

prehistoric, inhabited landscape. This alteration potentially hinders the

preservation of tens to hundreds of cultural sites, including most of those in this

study, by limiting the fluvial and aeolian sediment available to bury and cover

those sites (NPS lists several hundred sites as potentially affected by dam

operations but we refer here only to those that were studied at this level of

sedimentary detail for this project). Enhanced site protection could likely be

achieved, therefore, by restoring fluvial and aeolian deposition processes to

resemble more closely those that occurred before the river was dammed: large,

sediment-rich floods that left fluvial sand deposits from which sand was

remobilized by wind.

Fluvial deposits form the most extensive base on which archaeological

sites at Palisades and Arroyo Grande are located, spanning hundreds of square

meters in area (see also Hereford and others, 1996). Moderate-scale (~1,270 m3

s-1; 45,000 ft3 s-1) controlled Colorado River high flows, as were conducted in

1996 and 2004, can build sandbars that supply sand to aeolian deposits covering

archaeological sites at higher elevation. However, to effect large-scale

restoration of the fluvial terraces in these and other areas, a flow of >4,810 m3 s-1

(170,000 ft3 s-1) would likely be needed, based on stage-discharge relationships

developed for Palisades from historical flood debris (Draut and others, 2005;

Hazel and others, 2006b; we recognize that stage-discharge curves are

somewhat variable due to episodic constriction of the river channel by debris

flows). In order to cause deposition instead of erosion, flood waters would also

require very high suspended-sediment concentrations (Hazel and others, 2006a;

Topping and others, 2006). Suspended-sediment concentrations measured for

pre-dam flows >1,000 m3 s-1 (35,000 ft3 s-1) were within an order of magnitude of

0.1 g l

-1 (Topping and others, 2000a); although it is not known what the sediment

concentration would be in a post-dam flood of 4,810 m3 s-1 (170,000 ft3 s-1). In

general, the annual average post-dam sediment load is around five percent of
average pre-dam values. An experimental flow of the magnitude needed to

inundate pre-dam fluvial terraces at Palisades, ~4,810 m3 s-1 (170,000 ft3 s-1),

would be nearly four times greater than the highest experimental flows currently

under consideration by the Glen Canyon Dam Adaptive Management Program. It

is likely, also, that sediment concentrations would be too low in such a high flow

to have a substantial restorative effect unless the flood were to follow an

exceptionally large input of tributary sediment or unless sediment is added

artificially downstream of Glen Canyon Dam. Were a sediment-rich, highdischarge dam release to occur, a large volume of new high-elevation fluvial

deposits could then act as a source for aeolian sediment, which could be

transported to Modern Fluvial Sourced (MFS) dune fields and reworked to form

or replenish Relict Fluvial Sourced (RFS) dune fields. Increased transport of

aeolian sand to dune fields is expected to inhibit erosion both by filling small

gullies, which act as natural traps for wind-blown sand (documented in filled

paleo-gullies by Thompson and Potochnik [2000] and observed during this study

in several paleo-gully exposures and in modern dune fields; Draut and Rubin

[2006]), and by providing additional protective cover to archaeological sites.

Generating a major new fluvial deposit as a source for aeolian sand is

anticipated to produce a much greater benefit to aeolian deposits than creating

smaller sources of aeolian sand by simpler, local methods (such as removing

vegetation from selected channel-margin sand bars). Controlled dam releases of

~1,270 m3 s-1 (45,000 ft3 s-1) in 1996 and 2004 deposited sand in arroyo mouths

at Palisades and elsewhere (Yeatts, 1996; Hazel and others, unpublished data,

2004; Topping and others, 2006), which may have increased the preservation

potential of some sites by temporarily raising the effective base level to which

arroyos incise (Hereford and others, 1993). This effect is likely short-lived,

however, as runoff from local rainstorms can quickly remove the new deposits in

arroyo mouths and lower the effective base level once again. Relatively little data

is available concerning the effects of sediment deposition during controlled

floods, and its ability to control arroyo base level on various time scales, that

would definitively support or refute the still-debated hypotheses of Hereford and
others (1993) and Thompson and Potochnik (2000). Local climate, drainage

area, substrate composition, and river-related sediment and base-level effects all

contribute to arroyo incision and healing (Waters and Haynes, 2001; Webb and

Hereford, 2001), although arroyo incision remains incompletely understood.

Arroyo-incision processes in the Colorado River ecosystem could be clarified by

repeated high-resolution mapping of gullies and affected archaeological sites to

quantify local erosion and deposition rates, and by quantifying precipitation and

other climate parameters that vary widely within the complex canyon topography.

It is noteworthy that Palisades and Lower Comanche, where large arroyos erode

multiple archaeological sites, were the two regions with the least rainfall out of

the six monitored during this 26-month study (Draut and Rubin, 2006). If our data

are representative of longer-term regional precipitation variability, then enhanced

local precipitation might be eliminated as a possible cause of recent preferential

arroyo development at those sites.

In general, deposition of slope-wash and debris-flow sediment has less

potential to be affected by dam operations than are fluvial and aeolian deposits.

However, deposits resulting from local runoff will vary in their extent and location

from the pre-dam condition if the base level onto which local sediment is

delivered is lowered. For example, if the configuration of aeolian dunes changes,

or if a new gully breaches the dune field (as at Palisades), slope-wash events

may cause additional gully incision that drains to the river instead of allowing

slope-wash and colluvial sediment to accumulate in ponded areas. Loss of slopewash sediment would have the greatest effect farthest from the river, where the

proportion of such sediment is highest. At our study sites, slope-wash and

colluvial sediment was volumetrically less significant in site preservation than the

protective cover provided by thicker fluvial and aeolian deposits. However,

multiple exposures that we observed in other areas of the river corridor include

interbedded aeolian and local sediment; a thin stratum of poorly sorted debrisflow sediment can form a resistant cap that protects the more erodible aeolian

sediment beneath. Therefore, reduced accumulation of the cap-forming slope-
wash sediment (caused by reconfiguration of local slope-wash drainage) could

lead to increased erosion of aeolian sediment.

CRITERIA FOR IDENTIFYING SITE SENSITIVITY TO DAM OPERATIONS

This study combined analysis of prehistoric sedimentary and geomorphic

environments with that of modern aeolian processes to assess the sensitivity of

specific cultural sites to dam operations. Site-specific evaluation of cultural sites

is very important because the relative importance of fluvial, aeolian, and other

sedimentation processes, as well as local wind and precipitation patterns, differs

widely among sites in the same region of the river corridor. We proposed a series

of steps to determine the role of aeolian sediment in preserving any given

archaeological site, and then to assess the effect of dam operations on aeolian

sedimentation at that site (Draut and others, 2005); three case studies using

those criteria were presented by Draut and Rubin (University of Arizona Press, in

review). Here we summarize those site-evaluation criteria and case studies

briefly. These guidelines were formulated specifically to apply to aeolian

sedimentation, though our approach could be modified to address a wider range

of processes and their interaction with archaeological features. We reiterate that,

although the guidelines below refer only to aeolian sedimentation and specifically

to MFS aeolian deposits, pre-dam fluvial terraces and associated cultural sites

(as well as all RFS aeolian deposits, for reasons outlined above) should be

considered sensitive to dam operations because the lack of sediment-rich floods

on the order of 2,830 m3 s-1 (100,000 ft3 s-1) eliminates the mechanism by which

those deposits formed and received significant quantities of new sediment.

An essential first step in evaluating factors that contribute to cultural-site

preservation is to determine the depositional context of sediment on which the

site was built and that which has buried (protected) the site. Depositional settings

can be inferred by examining geomorphic features, sedimentary structures on the

land surface, and sedimentary structures in the subsurface viewed in vertical
section. Field investigations can be supplemented by laboratory analysis of

sediment particle-size distribution, as discussed above, and by radiocarbon

dating of materials that contain organic matter or charcoal (whether cultural or

non-cultural). Radiocarbon dating not only provides information on the age of

cultural occupation but can also be used to quantify recurrence intervals of floods

and debris flows in the study area, events that may have affected occupation by

prehistoric cultures (for example, Hereford and others, 1996, 2000).

If it has been determined, through field study of surface geomorphology

and sedimentary horizons, that aeolian deposition has buried and thus protected

a given cultural site, it is then necessary to establish whether aeolian sediment

that previously covered the site has been lost. Recognizing whether aeolian-sand

cover has been lost, by wind deflation or gully incision, is a step in assessing the

risk of impending cultural-site degradation or the degree to which degradation

has already occurred. If no loss of aeolian sediment is suspected at a particular

site, that site may not be at immediate risk of artifact loss. Loss of aeolian

sediment at a site could be documented most accurately using repeated highresolution mapping to quantify changes in the elevation of a deflating land

surface or the depth and width of growing gullies. If quantitative survey methods

are not available, loss of sediment can be qualitatively identified using repeated

ground-based photography and geomorphic evidence of deflation. Evidence for

deflation includes pedestal development (although erosion by rainfall may also

leave small-scale pedestal features), winnowed lag deposits on the land surface,

and exhumation of plant roots.

Deflation of aeolian deposits represents a net loss of sediment from an

area that had formerly experienced net deposition. Such a situation would arise

from a change in the balance between the sediment supply and the sedimenttransport capacity of local winds. If local wind conditions are assumed not to

have changed significantly and a decrease in sediment supply is likely, the next

step in this evaluation is to identify the source of aeolian sediment that buried the

archaeological site. Source areas for aeolian sand are most readily identified by

quantifying local wind conditions. The longer the time interval over which winds
are measured, the more accurate the resulting sediment-transport predictions will

be. In Grand Canyon, the most prominent sources of new sediment from which

the wind can mobilize sand are fluvial sandbars. Records of wind speed and

direction, such as those obtained during this study, can be used to generate

vector sums that demonstrate the potential for aeolian transport of sediment over

time (Draut and Rubin, 2005, 2006). If long-term weather monitoring is not

feasible, recent local wind conditions can be inferred in the field from the

orientation of ripple structures, sand shadows (which form in the lee of

obstacles), and dune morphology (slip faces).

The presence of cryptogamic soil crusts also indicates that aeolian

deposits now receive little or no active sand deposition. Cryptogamic crusts,

dark-colored microbiotic soils that comprise up to 70 percent of desert soils in the

southwestern United States, are dominated by cyanobacteria that live

symbiotically with mosses and lichens. The crusts begin to establish in 6–24

months and require five or more years without disturbance to form a fully

developed community. Although cryptogamic crust may survive after being

subjected to very shallow burial, as the biota extend sheaths to the land surface

to resume photosynthesis, deeper burial (> 0.5 cm in quartz sands) will kill the

crust (Belnap and Eldridge, 2003; J. Belnap, written comm., 2006). Therefore, if

an area with aeolian dune-forms on the land surface also contains well

developed cryptogamic crust, this implies that although sediment accumulated

there in the past (forming the dunes) it has been years since any substantial

quantity of wind-blown sand was last deposited on that surface. Similarly,

abundant vegetation on the surfaces of aeolian dunes implies that the dune

forms are now relatively immobile and not undergoing rapid deposition or dune

migration. Note that vegetation and cryptogamic crust on dunes are not

themselves direct indicators of deflation (loss of aeolian sediment), only of an

absence or near absence of deposition in an area that had formerly experienced

it. In an aeolian dune field where sediment supply becomes limited, the land

surface can first deflate (as wind transports more sand out of the dune field than

into it), then after several years of limited sediment input and deposition,
cryptogamic crust and grasses will colonize the deflated land surface and act to

reduce additional deflation by armoring the relict dune forms (Leys, 1990; Leys

and Eldridge, 1998; Belnap, 2003).

In such a dune field where dunes become less mobile over time (and

where vegetation and cryptogamic crust have grown), dune “stability” should not

be confused with “stability” of archaeological sites. An archaeological site located

in a deflated, vegetated dune field is at risk of erosion via gully incision, as is

commonly observed in Grand Canyon dune fields (such as those at Palisades,

Comanche, Tanner/Cardenas [RM ~70], Arroyo Grande, 202.9 mile), and in such

dune fields where dunes are relatively immobile and not migrating, gully incision

proceeds with little or no counteraction by aeolian sediment deposition, thereby

ultimately destabilizing dunes and associated cultural sites.

After local wind conditions have been used to identify a sand source, it

must be asked whether, and to what degree, the area of that sand source has

decreased. Aerial photography and repeated high-resolution mapping can and

have been used to document historical changes in sandbar area and structure

throughout the Colorado River corridor in Grand Canyon (for example, Schmidt

and others, 2004). Oblique, ground-based photography can effectively document

other changes in source-area characteristics that might affect the ability of that

sandbar to serve as a source for aeolian sediment. Important changes in

sediment-source quality might include a decrease in area caused by sandbar

erosion, growth of vegetation, increased moisture content of the exposed sand

area (more frequent inundation by flow fluctuations than in the past), greater

surface exposure of rocks, driftwood, or other obstacles that inhibit wind, or the

development of cryptogamic soil crust on the land surface.

If the aeolian supply of sediment to a particular site in a MFS dune field

appears to have declined, it is necessary to judge whether renewed deposition of

aeolian sand could be expected to have a substantial restorative effect on

protection of the archaeological site in question. Sites at which increased aeolian

deposition would substantially aid the preservation of cultural features are those

at which the greatest degradation occurs via deflation of sediment cover by wind,
or from incision by gullies small enough to be healed by wind-blown sand (<1 m

wide and deep, judging by occasional exposures of filled gullies observed during

this study and by Thompson and Potochnik, 2000). Aeolian sedimentation is

unlikely to prevent the loss of archaeological features that are threatened more

immediately by incision of a major arroyo or side-canyon channel than by

deflation.

These steps thus provide a method for evaluating the sensitivity of a

particular MFS aeolian deposit (and any associated archaeological sites) to dam

operations by assessing how closely the condition of the aeolian deposit is linked

to the condition of nearby fluvial sandbars, which in turn are greatly affected by

the dam-controlled flow and sediment content of the river (for example, Wiele

and others, 1996; Webb and others, 1999a; Schmidt and others, 2004; Topping

and others, 2006). In the case of cultural sites that could be better preserved with

the increased cover and gully-filling ability of additional aeolian sand, there are

several ways to accomplish this. At some sites, erosion control by checkdams

can keep gullies small enough to be partially or fully healed by windblown sand.

Localized mitigation efforts by NPS and the tribes with whom NPS collaborates

may substantially restore site conditions in such cases (Leap and Coder, 1995;

Balsom and others, 2005). In many areas, however, and especially if a largerscale solution is desired, restoration of cultural sites that rely on aeolian sediment

cover can be achieved only by restoration of the source areas from which the

aeolian sediment is derived. Because Colorado River sand deposits constitute

the largest source of new sediment mobilized by wind, increasing open, dry

sandbar area is the most effective way to enhance the potential for transport to

and deposition on MFS aeolian dune fields and associated archaeological sites.

Sandbar restoration using controlled high flows under sediment-enriched

conditions therefore holds the most promise as a means to restore aeolian

deposits and cultural sites above the high-flow stage (Yeatts, 1997; Fairley,

2005; Draut and Rubin, 2006).
Three Case Studies:

This study identified archaeological sites at which erosion is apparently

linked to dam operations, sites at which erosion can be shown not to be linked to

dam operations, and some sites where the connection between dam operations

and site degradation remains unclear. Case studies of each situation are

presented in this section. Locations of the sensitive archaeological sites cannot

be disclosed, for obvious reasons; they are referred to here as Sites 1–3.

Site 1 constitutes an example of an archaeological site that displays

considerable sensitivity to dam operations. Based on geomorphic observations

and a shallow test pit dug into sediment near this site in May 2003, it was

determined that aeolian sediment forms both the substrate on which the site was

built and the cover that has kept the roasting feature and artifact scatter intact.

This site, located near the landward, upper-elevation end of an aeolian dune

field, has experienced loss of aeolian sediment manifested by downslope

movement of artifacts as the land surface has deflated (based on years of

monitoring by NPS). Although the dune field shows evidence of active sand

transport in the vicinity of Site 1, a large area of the dune field ~10 m away has

undergone pronounced deflation and cryptogamic-crust growth, indicating little or

no active sand deposition at the present time. The source of the aeolian

sediment that partially covers Site 1 is a fluvial sandbar ~80 m directly upwind of

the site. This source was identified based on wind conditions measured at an

instrument station that operated nearby for more than a year. Comparison of

oblique historical photographs demonstrates a major decrease in the area and

volume of that same river-level sandbar from pre-dam (1920s) to post-dam

(1970s) time (Turner and Karpiscak, 1980), a trend confirmed by comparison of

aerial photographs of this location taken in the 1960s and 1990s. This suggests

that the loss of the fluvial sandbar since dam closure is very probably related to

the deflation and limited sand transport in the dune field ~80 m downwind

(identified therefore as a MFS dune field). Site 1 is presently intact enough that it

could be better preserved given additional aeolian sediment cover—the site and
its surroundings are not affected by gully incision, visitor trails, or any other

processes of degradation.

The apparent sensitivity of Site 1 to dam operations implies that

restoration of the fluvial sandbar by a sediment-rich controlled high flow could

enhance wind-blown sediment delivery to the dune field, helping to cover

artifacts. This sandbar received substantial new sediment during the 1996 flood

experiment (based on pre- and post-flood aerial photographs) and again during

the November 2004 high-flow experiment (based on pre- and post-flood site

visits). Aeolian sand-transport rates measured in this area before and after the

2004 flood showed greater sand flux during the spring 2005 windy season

compared to those of the previous spring. This provides an example, therefore,

of dam operations rebuilding a sandbar from which sand transport to an

archaeologically significant aeolian deposit was then enhanced.

Site 2 shares many characteristics with Site 1 in that it contains cultural

features built on and buried by aeolian sediment. A natural cross-section

exposure reveals debris-flow sediment underlying the aeolian dune field in the

area of Site 2. Advanced deflation and well developed cryptogamic crust are

apparent throughout the upper dune field. Although no quantitative

measurements of wind conditions are available from this immediate area,

orientation of dunes and sand shadows in the lower, more active part of the dune

field, which was photographed during four visits over multiple seasons, indicate

that the source of the aeolian sediment is a fluvial sandbar situated ~70 m

upwind of Site 2. Aerial photographs show a pronounced decrease in the open

sand area of that sandbar between the 1960s and 1990s, and increased

vegetation cover on the aeolian dune field. Thus far, evidence suggests that

deflation and reduced sand mobility on the dune field are linked to post-dam

shrinkage of the river-level sandbar that serves as the sediment source for that

MFS dune field. However, although it is likely that the general condition of the

dune field responds to dam operations, the rapid degradation of archaeological

features in Site 2 cannot be linked to Glen Canyon Dam because other factors

unrelated to wind-blown sand affect the site on shorter time scales. A large
tributary channel known to have experienced major floods and debris flows in

historic time has incised into aeolian sediment at the edge of the dune field.

Cultural features of Site 2 erode more substantially from episodic tributary activity

than from processes that could be reduced or reversed with more wind-blown

sand (aeolian deflation or formation of small gullies).

In contrast to Sites 1 and 2, where cultural-site sensitivity to dam

operations can be either confirmed or refuted, respectively, Site 3 presents a

situation in which the effect of dam operations remains unclear even after

detailed study. Similar to the first two examples, Site 3 was built on and buried by

aeolian sediment; this site is located in a large, sparsely vegetated dune field.

Individual features show evidence for some loss of aeolian sediment that

previously covered them, including deflation, slumping, and down-slope

movement of artifacts associated with dune migration. Minor cryptogamic crust is

present in some areas, but most of the dune field appears to undergo active

sediment transport. The dominant local wind direction, measured near Site 3 for

more than a year, indicates that the most likely source of sediment for this dune

field is a large, pre-dam fluvial terrace upwind. The modern morphology of that

terrace suggests substantial loss of sediment from this source area: the terrace is

incised by an arroyo network meters deep and meters wide, and its surface has

been colonized by abundant vegetation and cryptogamic crust. These

observations are consistent with the fact that no post-dam floods have deposited

sediment on or near this terrace.

Although conditions in the aeolian dunes may be related to the post-dam

reduction of sediment from the fluvial terrace upwind, it is not clear whether Site

3 would be better preserved if deposition from a very large, sediment-rich flood

were to enhance sand transport from the terrace to the dune field. Aeolian

processes are responsible for the disturbance of artifacts at Site 3, but the dune

migration that affects them is a natural process endemic to active dune fields. In

a natural system, continued dune migration would be expected eventually to

cover Site 3. Over time scales of years to decades, migrating dunes in a natural,

unaltered system would expose and cover any given part of the dune field
repeatedly, and down-slope movement of artifacts would be expected as the

sand shifts under them. Dunes near this site would need to be surveyed

repeatedly over decades to determine whether dune migration will continue as it

would in a natural system with plentiful sediment, or whether the dune field will

show signs of exacerbated sediment-supply limitation in the future (increase in

vegetation, cryptogamic crust, or winnowed lag deposits; decreased mobility of

dune forms). The latter situation might indicate that deprivation of sediment

caused by dam operations had prolonged the exposure of these artifacts, thereby

increasing the risk of their exposure, damage, and loss.

In the manner outlined above, it can be determined through careful, sitespecific field study whether and to what degree any given location is sensitive to

the effects of Glen Canyon Dam operations. Although preservation of cultural

artifacts in situ on geologic time scales is unrealistic due to continual downcutting

and backwasting of this bedrock canyon, cultural-site protection could likely be

enhanced on decadal to century time scales by restoring fluvial and aeolian

sedimentation to resemble more closely those processes that occurred before

the river was regulated by upstream dams, including large, sediment-rich floods

that left fluvial deposits from which sediment was remobilized by wind.
EFFECTS OF THE NOVEMBER 2004 HIGH-FLOW EXPERIMENT ON

AEOLIAN SEDIMENT TRANSPORT

Archaeological sites that depend on a supply of wind-blown sediment from

fluvial sandbars for their continued preservation could benefit from restoration of

sandbars by high-flow releases from Glen Canyon Dam. Sandbar restoration was

the primary goal of the high-flow experiment conducted on the Colorado River in

November 2004, the second experiment of its kind. To ensure a greater supply of

sand in the upstream reach than was available in the first flood experiment (in

March 1996; Webb and others, 1999b; Patten and others, 2001; Schmidt and

others, 2001), the 2004 controlled high flow took place after substantial quantities

of sand had recently been supplied to upper Marble Canyon by the Paria River

and other tributaries, and after an interval of lower dam releases designed to

minimize sediment export. Conditions during the 2004 high flow therefore

represented sediment enrichment in the river channel relative to conditions

during the 1996 flood (Topping and others, 2006).

The design of the 2004 high-flow experiment included a 60-hour steady

flow of 1,160 m3 s-1 (41,000 ft3 s-1) released from Glen Canyon Dam on

November 22 and 23. Post-flood surveys indicated that the area and volume of

sandbars in Marble Canyon above river-mile ~40 were significantly greater than

before this high flow, and that approximately half of the surveyed sand deposits

were much larger than they had been immediately after the 1996 controlled flood

(Hazel and others, 2005; Topping and others, 2006). Below river-mile ~40,

localized deposition and erosion were documented and a consistent pattern of

sandbar aggradation could not be demonstrated, implying that in future high-flow

experiments more sand must be present in the river channel to achieve

deposition on sandbars throughout the canyon (Topping and others, 2006).

In addition to the sandbar surveys that followed the 2004 high-flow

experiment (Hazel and others, 2005), fluvial sand deposits at sites associated

with this study were photographed immediately before and immediately after the

high flow (November and December 2004, respectively), and again multiple
times during 2005; pre- and post-flood photographs have been published

previously by Draut and Rubin (2005, 2006). The duration of this study included

one year of pre-flood data spanning each season, and slightly more than one

year of post-flood data. The sites in this study were chosen in part because they

had experienced new fluvial sand deposition during the 1996 high-flow

experiment. Therefore, it was predicted that similar deposition would occur at

these locations during the November 2004 high flow. Based on wind conditions

measured in the year before the high-flow experiment (Draut and Rubin, 2005)

the greatest potential for redistribution of new, flood-deposited sediment was

predicted to occur during the spring windy season of 2005.

At all six weather-station sites (Fig. 1), major deposition of new fluvial

sand occurred as a result of the November 2004 controlled flood (Draut and

Rubin, 2005, 2006). The substantial deposition in those areas during a sedimentrich Colorado River high flow implies that such flows have excellent potential as a

management tool for restoring the sandbars that comprise the source areas of

sediment for MFS aeolian deposits. In addition to depositing a substantial

quantity of new sand on many sandbars, the November 2004 high flow also

facilitated aeolian sand entrainment because the new sediment covered

vegetation, rocks, and driftwood, decreasing the roughness of the land surface

and thereby inhibiting entrainment of sediment by wind (Lettau, 1969; Marshall,

1971; Raupach and others, 1993; Gillies and others, 2000; King and others,

2005).

However, if maximizing aeolian redistribution of sediment is one of the

management goals of a controlled high flow, river flows that occur between the

time of the high flow and the next windy season must be managed to maximize

open, dry sandbar area during that windy season (April through early June).

Much of the 2004 flood sand was removed by high daily flow fluctuations that

ranged from 142 to 566 m3 s-1 (5,000 to 20,000 ft3 s-1) from January to March

2005, before the spring windy season began (Draut and Rubin, 2006). The most

dramatic changes were observed at the Malgosa study site (river-mile 57.9),

where a new sand deposit 1.5 m thick and more than 10 m wide formed during
the high flow but was eroded almost entirely over the following four months. Sites

a short distance downstream of the Little Colorado River (LCR), such as

Palisades and Comanche, 7.4 and 10.4 km below the LCR confluence,

respectively, showed less-pronounced loss of sand during the January–March

2005 flow fluctuations than that observed in Marble Canyon, presumably

because flooding of the LCR in February 2005 (~70 m3 s-1 [2,500 ft3 s-1]) provided

additional sand to these areas.

At sites where the high-flow deposits were entirely removed before the

2005 windy season began, windy-season aeolian sand-transport rates in 2005

were comparable to or lower than those in the 2004 windy season, given similar

wind conditions. However, at 24.5 mile, where approximately half of the flooddeposited sand remained at the start of the windy season in spring 2005, sandtransport rates measured near the river during the spring 2005 windy season

were approximately double those in spring 2004 (Draut and Rubin, 2006). This is

interpreted as a result of the increased area of available sand upwind and

decreased roughness of the land surface where the 2004 flood sediment was still

present. No similar increase in sand flux relative to the previous year was

measured at the upper-elevation station in the same dune field.

The dominant wind direction at 24.5 mile transports sand from westsouthwest toward east-northeast (from river level toward the upper dune field;

Draut and Rubin, 2006). Sand-transport measurements there suggested that,

although the influence of the new flood sediment was felt near river level (at the

lower of the two weather stations) in spring 2005, its effects had not yet

propagated to the upper part of the dune field. In many areas of the river corridor,

new sand would need to reach the highest parts of aeolian dune fields in order to

benefit archaeologically significant locations. Because the sand flux measured at

our upper weather station at 24.5 mile exceeded that measured at the lower

station even with the new flood sand present (Draut and Rubin, 2006), the net

flux of sand into this dune field was still negative even after the high-flow

experiment. It is possible that, had the sandbar near the lower station at 24.5

mile still been as large during the spring 2005 windy season as it was
immediately after the November 2004 high flow, the sand flux into the dune field

might have balanced or exceeded the flux out of the dune field, resulting in net

sediment gain. At this same site, the restorative function of aeolian sand derived

from the flood deposit was still apparent more than a year after the high-flow

experiment. In January 2006, a new gully was photographed that had recently

begun to incise into the aeolian dune field at 24.5 mile. The orientation of the

gully and of wind ripples in the sand indicated that sediment filling the lower part

of this gully had blown inland from the November 2004 high-flow deposit. This is

consistent with the dominant wind direction recorded at 24.5 mile over the

preceding 26 months (Draut and Rubin, 2006). The ability of gullies to act as

natural traps for wind-blown sand in this manner could contribute to slowing or

cessation of erosional processes that affect the stability and preservation of

archaeological sites in aeolian deposits (for example, Thompson and Potochnik,

2000).

Effects of the November 2004 high-flow experiment on aeolian sediment

transport therefore indicate that sediment-rich controlled high flows are an

effective means to restore sediment deposition in aeolian dune fields above the

flood-stage elevation. Sandbar-building floods have much more potential to

enhance aeolian sedimentation on MFS-type deposits than on RFS deposits as

defined above (Fig. 4), because of the direct link between active fluvial sandbars

(at or below the 1,270 m3 s-1 [45,000 ft3 s-1] stage) and MFS aeolian dunes

downwind. To promote substantial sediment restoration at RFS dune fields would

require sediment-rich floods on the scale of pre-dam spring snowmelt floods,

twice as high as those conducted experimentally thus far. It is therefore expected

that any future controlled high flows on the scale of the 2004 experiment could

cause substantial enhancement of aeolian deposition on MFS dune fields

(provided that post-flood flows are managed to maintain the size and condition of

new fluvial deposits through the next windy season) but that those same floods

would have little effect on aeolian sediment flux into RFS dune fields.

The high daily flow-fluctuation schedule (142 to 566 m3 s-1 [5,000 to

20,000 ft3 s-1]), which itself comprised experimental flows, was conducted from
January–March of 2003, 2004, and 2005 to suppress non-native fish populations.

Each of those high-fluctuating-flow intervals exported approximately twice as

much fluvial sediment as under Modified Low Fluctuating Flow (MLFF)

operations of Glen Canyon Dam for comparable volume months and comparable

sediment influx (Wright, 2006). Therefore, substantially more fluvial sand would

be retained and available for subsequent aeolian transport to dune fields if a

sediment-rich high flow were followed by low fluctuating flows rather than by high

daily flow fluctuations. Operating under low fluctuating flows (or steady low flow)

between the end of a controlled flood and the start of spring high winds is

predicted to maintain much more of the post-flood sandbar size until the wind can

redistribute flood sediment into higher-elevation aeolian deposits during the

subsequent spring windy season.
SITE-SPECIFIC FINDINGS

24.5 mile:

An aeolian dune field on the downstream side of a debris fan at 24.5 mile

was the site of two weather stations deployed during this project: one near river

level (Station 24.5 L) and one at the upper end of the dune field (Station 24.5 U).

The region of the dune field between the two weather-station locations (Draut

and Rubin, 2005, 2006) undergoes active aeolian sand transport and has little

vegetation or cryptogamic crust, while an area of approximately equal size at the

northern (upstream) end of the dune field is relatively inactive, with well

developed cryptogamic crust and evidence of deflation of the land surface.

Comparison of aerial photographs taken in 1965 with more recent images (Fig. 6)

shows a marked decrease in the size of river-level sandbars and increased

riparian vegetation at 24.5 mile in post-dam time. Notably, a river-level sandbar

at the western end of the aeolian dune field is clearly visible in 1965 photographs

(marked ‘a’ in Fig. 6) but is much smaller or absent in recent photographs (see

also Fig. 39 of Turner and Karpiscak, 1980). This sandbar was temporarily

enlarged by the 1996 flood experiment (based on examination of aerial

photographs) and again by the November 2004 high-flow experiment (Draut and

Rubin, 2006) after which it was partially eroded by daily flow fluctuations. A

second fluvial sand deposit (labeled ‘b’ in Fig. 6) is now smaller than in 1965 and

is partially covered by vegetation; Figure 6 shows that riparian vegetation is

much more abundant today than in 1965.

Wind velocity and direction measured at the two weather stations at 24.5

mile between November 2003 and January 2006 reveal a very consistent pattern

of wind blowing from the west-southwest. Calculated vector sums of wind data

sampled continuously at four-minute intervals, excluding those intervals during

which wind velocities were below the threshold of motion needed to transport

sand, yielded a net potential sediment-transport direction of 244 degrees at both

weather stations during calendar-year 2005; data from 2004 were very similar
25 m

N

A B

dune field

Dominant wind direction

(from 244 degrees in both

2004 and 2005)

FIGURE 6. Aerial photographs of 24.5 mile in 1965 and 2004. River discharge in the May 1965 photograph (A) is

between 680 and 792 m s (24,000–28,000 ft s ). Discharge in the May 2004 photograph (B) is 226 m s (8,000

ft s ). Locations ‘a’ and ‘b’ mark fluvial sand deposits. The modern extent of the aeolian dune field is indicated in

B. Dominant wind direction (the direction of net sediment transport) is from the west-southwest at 24.5 mile (see

Draut and Rubin, 2005, 2006 for details on calculation of net sand-transport direction). Note increased riparian

vegetation and decreased open fluvial sandbar area since 1965.

3 -1

3 -1 3 -1 3 -1
(see Draut and Rubin, 2005, 2006 for all data and additional discussion of

method and results)1. The aeolian dune field at 24.5 mile is therefore located

directly downwind of the river-level sandbar marked ‘a’ in Figure 6 and is an

example of a MFS aeolian deposit as defined above.

Rates of aeolian sediment transport were typically on the order of 1 g cm-1

day-1 in this dune field; transport rates at the upper end of the dune field were

consistently higher than those measured near river level by a factor of two to

four, consistent with higher wind velocities measured at Station 24.5 U (Draut

and Rubin, 2006). Given the projected strong link between sediment comprising

the fluvial deposit ‘a’ and the aeolian dune field, it was not surprising that

replenishing sediment on the sandbar ‘a’ during the 2004 high flow was followed

by a measured increase in aeolian sediment transport near river level (recorded

at Station 24.5 L) in the spring windy season of 2005, compared to the spring

windy season of 2004. As discussed above, however, no similar increase in sand

flux relative to the previous year was measured at the upper-elevation station.

Because the sand flux measured at the upper station at 24.5 mile exceeded that

at the lower station even with the new flood deposit present, the net flux of sand

into this dune field was still negative, indicating deflation of its land surface.

Assuming wind conditions similar to those measured in 2005, the sediment flux

at the lower station would need to be ~50 percent higher than that measured in

2005 to equal the sand flux at the upper station; such a situation would indicate

that the dune field experienced no net gain or loss of sediment. We speculate

 1 Dominant wind directions given in this report were calculated using vector sums of the 4-minute

wind measurements at each weather station. Complete documentation of all data and methods

used in these and other net wind directions mentioned were published by Draut and Rubin (2005,

2006). Vector sums (magnitude and direction of wind, using only data points in which wind

velocity exceeded the critical threshold of motion needed to transport sand of the grain size found

at each site) for each month of data at each weather station were reported by Draut and Rubin

(2005, 2006). Error in each wind measurement is estimated to be no more than 10 degrees for

the wind’s direction (5 degrees is the manufacturer’s stated accuracy for the anemometers, and

allowing another 5 degrees for operational error). For the magnitude component of wind velocity,

analyses include error of 0.5 m s

-1 for velocities <17 m s-1 and ±3% for velocities between 17 and

30 m s-1 (manufacturer’s stated accuracy). Draut and Rubin (2005, 2006) published both the

magnitudes and directions of net (dominant) wind and potential sediment transport for each

month of data at each station. Measurement errors are expected not to affect net (dominant) wind

calculations significantly except in cases where wind blew often from opposing directions, such as

at 202.9 mile. For all other sites, wind direction was consistent enough that measurement errors

would not significantly affect the reported dominant wind and sediment-transport direction.
that, if the sandbar near the lower station at 24.5 mile had still been as large

during the spring 2005 windy season as it was immediately after the November

2004 flood (having been reduced to half its post-flood size by high daily flow

fluctuations between January and March 2005), the wind-blown sand flux into the

dune field might have balanced or exceeded that measured at the upper station,

resulting in net sediment gain in the dune field.

Archaeological sites are known to occur in the vicinity of 24.5 mile that are

built on and buried by aeolian sediment. Because this is a MFS aeolian system in

which an aeolian deposit receives sediment from a fluvial sandbar located

directly upwind, changes in the amount of sediment available on the fluvial

sandbar that serves as a sand source can be expected to affect the stability and

preservation potential of archaeological material in the aeolian deposit to which

this fluvial sediment is linked. It is very probable that the relatively ‘inactive’

northern area of the 24.5-mile dune field has experienced deflation as a result of

the decreased sand source upwind (the fluvial sandbar marked ‘a’ in Fig. 6) since

the 1960s. The growth of the same fluvial sand deposit during the 2004 high-flow

experiment, consequent increased aeolian sand flux into the dune field, and the

filling of a small gully by wind-blown sand derived from the 2004 high-flow

deposit (Draut and Rubin, 2006) clearly indicate that sediment-rich floods of the

magnitude conducted in 2004 (1,160 m3 s-1 [41,000 ft3 s-1]) can significantly

increase aeolian sediment transport to and deposition on MFS aeolian deposits

and archaeological sites associated with them.

It may be possible to extrapolate the dominant west-southwest wind

direction (and same projected net sediment-transport direction) measured at 24.5

mile for some short distance upstream and downstream. Because wind

conditions are very variable with local topography, however, it is not

recommended to extrapolate these results for great distances and any

extrapolations that are made should be considered speculative. However,

because the orientation of the canyon around 24.5 mile remains approximately

constant (trending northeast and varying by no more than 10 degrees between

river-miles 22.5 and 25.0), similar net aeolian sediment movement may be
N

A B

25 m

Dominant

wind direction

at 24.5 mile

(from WSW)

dune field

FIGURE 7. Aerial photographs of 23.5 mile in 1965 and 2004. River discharge in the May 1965 photograph (A) is between 680 and 792 m3 s-1 (24,000–28,000 ft3 s-1). Discharge in the May 2004 photograph

(B) is 226 m3 s-1 (8,000 ft3 s-1). A small aeolian deposit is indicated in B. Note the decreased area of open

sand, and increased vegetation, in the 2004 photograph relative to 1965. If the same dominant wind

direction is assumed to occur at 23.5 mile as at 24.5 mile, the aeolian dune field is directly downwind of a

fluvial sandbar.
anticipated for other sediment deposits between river-miles 22.5 and 25.0. The

largest fluvial sandbar that presently exists in that reach of the river is located at

river-mile 23.5. Aerial photographs show that the 23.5-mile sandbar is now

smaller and that the area contains more vegetation than in 1965 (Fig. 7), similar

to changes apparent at 24.5 mile. If a dominant west-southwest wind direction is

assumed for 23.5 mile, this implies that the small aeolian deposit immediately

northeast of the fluvial sandbar there is likely also a MFS aeolian deposit in which

sedimentation is linked to sand supply from the 23.5-mile fluvial sandbar. The

aeolian dune field at 23.5 mile is approximately half the size of that at 24.5 mile

and was not studied directly during this project.

Malgosa:

The aeolian dune field at the downstream, south side of the Malgosa

Canyon debris fan (river-mile 57.9; Fig. 8) is one of the most active dune fields

observed during this study. Wind gusts exceeding 20 m s-1 were not uncommon

during 2004 and 2005 in the upper dune field; aeolian sand transport rates on the

order of 10–100 g cm-1 day-1 were typical, and in May 2004 a two-day event was

recorded during which transport rates exceeded 4000 g cm-1 day-1. Wind

direction in this dune field aligned with the orientation of the main canyon,

blowing from either south-southeast or north-northwest. Wind records from

Malgosa commonly show daily fluctuations in wind strength and direction, with

wind at higher velocity and more consistently from the south-southeast in the

afternoons (Draut and Rubin, 2005, 2006). Wind velocities high enough to

transport sediment of the grain size found in the Malgosa dune field (> 2 m s-1)

come most often from the south-southeast at this site, such that net sediment

transport is directed upstream with respect to the Colorado River. This is

consistent with dune morphology on the southern side of the Malgosa debris fan,

which includes north-facing slip faces that indicate northward dune migration.

The primary sediment source for this dune field is therefore found to be the fluvial
sandbar immediately upwind of the dune field, marked ‘a’ in Figure 8. A second

sandbar located across the river and ~200 m downstream, labeled ‘b’ in Figure 8,

occasionally also provides a minor amount of sediment to the Malgosa dune field

(in May 2004, sand was observed to blow from sandbar ‘b’ across the river and

into the Malgosa dune field upstream of sandbar ‘a’). The orientation of the

aeolian dunes with respect to fluvial sandbars, together with the dominant wind

direction measured from November 2003 to January 2006, implies that the

Malgosa dune field is a MFS aeolian deposit in which aeolian sedimentation is

directly linked to sediment supply from a fluvial sandbar (or sandbars) upwind.

The higher river discharge in the 1965 aerial photos compared to those

taken since the mid-1990s and the generally low elevation today of sandbars ‘a’

and ‘b’ make it difficult to compare fluvial-sandbar area between the two sets of

photographs in Figure 8. Aerial photographs do however show greater vegetation

cover along the riparian zone and in the Malgosa dune field itself in 2004

compared with 1965 (Fig. 8). Increasing vegetation in the Malgosa dune field

since the 1960s suggests that the dunes have become less mobile over that

time. Dunes on the north side of Malgosa Creek (a tributary of the Colorado River

that is dry except after recent heavy rain) can be considered relatively ‘inactive’

compared with those on the south side of Malgosa Canyon and have well

developed grass cover and cryptogamic crust. Photographs in Figure 9

demonstrate the difference in dune surfaces between the north and south sides

of Malgosa Creek. These differences are attributed to the Malgosa Canyon

channel trapping aeolian sand between the two parts of the dune field—the

southern dunes migrate north but their sand is removed by episodic runoff in

Malgosa Canyon before significant quantities of sand reach the northern dune

field. Only sediment carried in suspension during high wind velocity can therefore

be expected to reach the northern dune field; sand moving north by saltation or

reptation will be trapped in the tributary drainage and unavailable to the northern

dune field. Aeolian sediment-transport rates were not measured in the dunes to

the north of Malgosa Canyon during this study. That substantial sand dunes exist

at all on the northern part of the Malgosa debris fan, although they are now fairly
50 m

N

A B

dune

field

studied

Dominant wind

direction (from 132

degrees in 2004,

from 156 degrees in

2005)

Fig. 9A

FIGURE 8. Aerial photographs of Malgosa in 1965 and 2004. River discharge in the May 1965 photograph (A) is between

680 and 792 m3 s-1 (24,000–28,000 ft3 s-1). Discharge in the May 2004 photograph (B) is 226 m3 s-1 (8,000 ft3 s-1). Locations ‘a’ and ‘b’ mark fluvial sand deposits. The most-active area of the aeolian dune field on the downstream side of the

Malgosa Canyon debris fan is indicated in B. The box in B shows the area that appears in the photograph in Figure 9A.

Dominant wind direction (the direction of net sediment transport) is from the south-southeast at Malgosa (see Draut and

Rubin, 2005, 2006 for details on calculation of net sand-transport direction). Note increased riparian vegetation since 1965.
inactive, suggests that in the past there was likely a greater supply of sediment to

this northern dune area than occurs today.

Malgosa experienced not only the highest aeolian sand-transport rates

measured at any site during this study but also the highest rainfall. Annual

precipitation at Malgosa totaled nearly 270 mm in 2005, a factor of two greater

than that measured at the next-nearest study site (Palisades, ~12 km

downstream). High rainfall at Malgosa compared to other sites was common on

an event-by-event basis. The marked differences between rainfall at Malgosa

and elsewhere are attributed to local topography, but this interpretation is not well

constrained and only 26 months of rainfall data are available from this study,

which may not accurately represent long-term climate at either site. The Malgosa

Canyon tributary trends northeast and drains the eastern side of the Walhalla

Plateau, located southeast of the main Kaibab Monocline. It is possible that the

orientation of Malgosa Canyon and drainage-basin geometry affect movement of

local weather systems, perhaps creating a funneling effect for eastward-moving

storms that could account for the significantly higher rainfall recorded near river

level at Malgosa compared to other sites (see Draut and Rubin, 2006).

It is noteworthy that although Malgosa experiences unusually high rainfall

compared to other areas of Grand and Marble Canyons, the active dune field

there is not incised by gullies. Small ephemeral gullies have been observed there

on occasion that were soon filled by wind-blown sand and did not grow large

enough to cause any significant erosion at this site. This relationship among

precipitation, gully incision, and the restorative effects of aeolian sand contrasts

with the conditions at the Palisades site, discussed below, where major gully

incision occurs even in an area that received unusually low precipitation

compared to other sites during this study, but where wind-blown sand transport

was be at least an order of magnitude lower than at Malgosa and is apparently

unable to keep pace with gully formation.

The November 2004 high-flow experiment resulted in deposition of a

major new sandbar at Malgosa, at the location marked ‘a’ in Figure 8, that was

up to 2 m thick and more than 10 m wide. High daily flow fluctuations for the first
A

B

FIGURE 9. Photographs of different zones within the Malgosa dune field. The photo in A

was taken facing north (upstream with respect to the Colorado River) from the top of a

dune crest just south of the Malgosa Creek tributary (mid-ground of photograph). In the

background are dune forms north of the tributary; these dunes are covered with abundant

grass and other vegetation; cryptogamic crust also occurs there, indicating little active aeolian sand deposition there. In contrast, B shows the southern part of the Malgosa dune field

(south of Malgosa Creek); there, vegetation is much more sparse and sand transport rates

reach hundreds of g cm-1 day-1.
three months of 2005 eroded this sand deposit almost entirely, however, and by

May 2005 the location ‘a’ appeared almost identical to its pre-flood condition

(photographs published by Draut and Rubin, 2006). Because this sandbar had

eroded so significantly before the spring 2005 windy season began, it was not

surprising that aeolian sand-transport rates measured at the Malgosa weather

stations in spring 2005 were no higher than in spring 2004.

Palisades:

This research group studied sedimentary profiles in detail in the Palisades

(Palisades of the Desert) area near river-mile 66, in addition to deploying two

weather stations there that measured wind, precipitation, and aeolian sand

transport from November 2003 to January 2006. Detailed discussions of the

measured strata and surface morphology studied at Palisades (Figs. 2a, 4a)

were published by Draut and others (2005).

The Palisades region forms the upstream part of the geomorphic reach of

the canyon described by Schmidt and Graf (1987) as the Furnace Flats reach. As

discussed in the Overview of Sedimentary and Geomorphic Investigations

section above, alluvial terraces in this area represent multiple episodes of

floodplain aggradation in pre-dam time, represented by distinct fluvial

sedimentary structures in the studied profiles. The areal extent of fluvial deposits

demonstrated that the entire terrace area in the Palisades region was submerged

episodically during pre-dam high flows (cf. Hereford, 1996; Hereford and others,

1993, 1996). No post-dam flows (since 1963) have inundated the majority of this

terrace. A stage-discharge relationship developed from driftwood deposits

indicated that the terraced area was almost entirely inundated by a flow of ~5,940

m3 s-1 (210,000 ft3 s-1; Draut and others, 2005; Hazel and others, 2006b), a flood

level reached most recently in 1884. Reworking of fluvial sediment by wind was

common in pre-dam time, evidenced by aeolian deposits between fluvial
deposits; aeolian material was also found interbedded with ‘ponded’ sediment

deposits where slope-wash runoff had formed small, isolated pools at the eastern

side of the terrace. Aerial photographs show that the amount of vegetation on the

terraced area has increased somewhat since 1965 while vegetation in the

riparian zone has increased substantially (Fig. 10). The now-substantial arroyo at

Palisades (Fig. 2a) has deepened since the 1960s and is believed to have

increased after this local drainage breached pre-dam alluvium exposures and

aeolian dunes in the late 1970s or early 1980s and commenced to drain directly

to the Colorado River (Hereford and others, 1993; Thompson and Potochnik,

2000).

Six of the nine archaeological sites at Palisades were formed in or on

fluvial sediment, some of which had been reworked by wind (Tab. 1; see also

Draut and others, 2005). Five of the nine sites are preserved at least in part by a

cover of aeolian sediment, with minor contributions from aeolian sedimentary

cover at two additional sites. Erosion associated with arroyo incision affects two

of the sites. Exposure of artifacts can be attributed at least in part to aeolian

deflation at five archaeological sites.

The relative abundance of fluvial sediment compared to aeolian sediment

in vertical profiles indicates that the aeolian coppice dunes on the terrace surface

apparently formed by wind-reworking of the extensive pre-dam flood deposits

that underlie the Palisades area, defining the Palisades dune field as a RFS

aeolian deposit (Fig. 5b; Hereford, 1996). Wind tends to align with the northsouth orientation of the river in this area and may blow either from north or south,

although the dominant wind direction at Palisades (at velocities strong enough to

transport sediment) comes from the south-southeast (Draut and Rubin, 2005,

2006). Aeolian sediment-transport rates within the coppice dunes were on the

order of ~1 g cm-1 day-1 while on a cobble deposit at the river’s edge, rates were

an order of magnitude lower. Given the net wind direction from the southsoutheast, it is unlikely that significant quantities of sediment are transported by

wind from river-level sandbars (the locations of which are indicated in Fig. 10)

into the coppice dunes at Palisades. Some migration of aeolian dunes is
50 m

N

A B

dune

field

Dominant

wind direction

(from 150

degrees in

2004, 180

degrees in

2005)

(ephemeral

sandbar)

FIGURE 10. Aerial photographs of the Palisades area in 1965 and 2004. River discharge in the May 1965 photograph (A) is between 680 and 792 m3 s-1 (24,000–28,000 ft3 s-1). Discharge in the May 2004 photograph (B) is 226

m3 s-1 (8,000 ft3 s-1). Dominant wind direction (the direction of net sediment transport) is from the west-southwest at

Palisades (see Draut and Rubin, 2005, 2006 for details on calculation of net sand-transport direction). Locations ‘a’,

‘b’, and ‘c’ mark fluvial sand deposits. The most-active area of the Palisades dune field is indicated in B. Note

increased riparian vegetation and decreased open fluvial sandbar area since 1965.
apparent today in the northern part of the dune field, but in general the dunes are

relatively inactive with grasses, mesquite, and cryptogamic crust well

established. Although some new sediment is probably transported into the dune

field on occasion from river-level sandbars (Yeatts, 1996), the net wind direction

measured during this study indicates that there is probably a net loss of sediment

from this dune field toward the northwest (where it can be lost into the river). As

discussed above, gully and arroyo incision causes episodic additional erosion of

sediment at Palisades. Because the formation of this RFS aeolian deposit was

linked to deposition of fluvial sediment during large, sediment-rich pre-dam

floods, it is unlikely that substantial new sediment will be supplied to the

Palisades dune field without similar large, sediment-rich floods in the future.

Controlled Colorado River high flows on the order of ~1,270 m3 s-1 (45,000 ft3 s-1)

can enlarge the fluvial sand deposits labeled in Figure 10; substantial deposition

there was documented after high flows in 1996 (Yeatts, 1996) and 2004 (Draut

and Rubin, 2006). However, flows of that magnitude are unlikely to promote

substantial aeolian sand transport into the Palisades dune field given the

dominant wind direction measured during this study and the location of sandbars

‘a’, ‘b’, and ‘c’ relative to the location of the aeolian dunes.

As mentioned in the previous section, during this study the annual

precipitation at Palisades was significantly lower than that measured ~12 km

upstream at Malgosa (in 2005, the ~135 mm recorded at Palisades was less than

half of the rainfall at Malgosa). Palisades in fact received the lowest total annual

rainfall of any site studied during this project, though it was similar to that

measured nearby at Comanche (river-mile 68, where 138 mm of rain fell in

2005). The much-studied arroyo at Palisades has therefore reached its advanced

state of terrace incision despite being located in a region of the river corridor that

experienced unusually low rainfall, at least during the 26-month duration of this

study. Although many factors contribute to gully incision (including long-term

climate, drainage-basin geometry, substrate composition, and effective base

level), we infer that the arroyo incision would not have progressed as far as it has

if aeolian sand movement within the Palisades terrace area were an order of
magnitude greater than it is today. A short distance upstream, gullies are typically

absent from the Malgosa dune field—a site with twice the rainfall, but ten times

the aeolian sand-transport rates, of Palisades. Active aeolian sediment

movement at Malgosa more than accommodates the episodic gully incision

there, and gullies (which act as natural traps for wind-blown sand) are quickly

healed, similar to the situation observed at 24.5 mile in January 2006 (Draut and

Rubin, 2006).

Comanche:

As discussed above, sedimentary profiles were analyzed at Lower

Comanche, near river-miles 68–69, in May 2004. One weather station measured

wind, precipitation, and aeolian sand transport in the northern part of this study

area between April 2004 and January 2006. Detailed discussions of the

measured strata and surface morphology studied in the Comanche area were

published by Draut and others (2005); see Figures 2b and 4b.

Stratigraphy of the southern, arroyo-incised part of the Comanche area

indicated multiple episodes of fluvial sedimentation followed by reworking at the

land surface by wind and local runoff that follows rainfall events. Aeolian climbing

ripples were present in some of the deposits; much of the silt and fine sand did

not contain preserved diagnostic sedimentary structures. Colluvium (recognized

by sandstone and shale clasts derived from local bedrock) was present in all six

profiles studied at Lower Comanche. Five of the six cultural sites in the vicinity

are located within aeolian dunes (Tab. 2). Of those, four were originally situated

on aeolian sediment, with the fifth built on an interdune, playa-like surface. Four

sites were at least partially buried by aeolian sediment; two of those four have

only minor sediment cover (<10 cm thick). Three of the five sites within the dune

field are affected by active aeolian deflation and dune migration. The one cultural

site not located in or among the large coppice dunes at Lower Comanche (Site
C:13:273; Tab. 2) was constructed on a terrace that contained slope-wash

sediment interbedded with lighter-colored, better-sorted fine sand and silt with

poorly preserved sedimentary structures that appeared to grade laterally into

both fluvial and aeolian deposits.

Based on field relationships between fluvial and aeolian deposits, the

aeolian dunes in the Comanche area most likely originated by reworking of large,

pre-dam flood deposits—some in place and some that occur to the immediate

southeast (upwind) of the dune area—in a similar manner to processes

described above for Palisades. The large aeolian dune field at Comanche is

defined therefore as primarily a RFS deposit. The net wind direction (net potential

sand-transport direction) identified for Comanche in 2004 and 2005 comes from

the southeast, directed upstream. A substantial component of potential sediment

transport does come from the south, however (Draut and Rubin, 2005, 2006),

implying that some sediment from non-flood-stage fluvial deposits may reach the

aeolian dunes. If sand does blow from river-margin sandbars (in the area labeled

‘a’ in Fig. 11) toward the aeolian dunes, it is probable that this supply of sediment

has diminished since the 1960s because the area of open sand has decreased

and the riparian vegetation increased between the sandbar and aeolian dunes

over that time (Fig. 11). Decreased open sand area and increased vegetation are

similarly apparent at area ‘b’ in Figure 11, several hundred meters downstream

and to the south-southeast of ‘a’. The November 2004 high flow caused a

substantial increase in the open sand area on sandbar ‘a’. The new flood sand

was eroded rapidly by high daily flow fluctuations in early 2005, such that six

months after the flood the beach was essentially identical to its pre-flood state

(photographs of the Comanche sandbar ‘a’ appear in Fig. 28 of Draut and Rubin,

2006). Post-flood aeolian sand-transport rates during the spring windy season in

2005 were higher, however, than in spring 2004, by up to a factor of two

(transport rates in this dune field are typically on the order of 0.1 g cm-1 day-1,

reaching >1 g cm-1 day-1 during spring winds).

Given the proximity of the Palisades and Comanche sites (~5 km apart),

the consistent north-south orientation of the main canyon between them, and the
50 m

N

A B

dune

field

Dominant wind

direction

(from 98

degrees in

2004, from

153 degrees in

2005)

FIGURE 11. Aerial photographs of the Comanche area in 1965 and 2004. River discharge in the May 1965 photograph (A) is between 680 and 792 m3 s-1 (24,000–28,000 ft3 s-1). Discharge in the May 2004 photograph (B) is

226 m3 s-1 (8,000 ft3 s-1). Locations ‘a’ and ‘b’ mark fluvial sand deposits. The modern extent of the aeolian dune

field is indicated in B. Dominant wind direction (the direction of net sediment transport) is from the southwest at

Comanche (see Draut and Rubin, 2005, 2006 for details on calculation of net sand-transport direction). Note

increased riparian vegetation and decreased open fluvial sandbar area since 1965.
similar south-southeast dominant wind directions measured at each, we consider

it reasonable to extrapolate similar net aeolian sand transport pathways for the

reach of the river corridor between these two studied areas. There are few fluvial

sandbars on river right (the west bank of the river) in that reach, with the

exception of the small deposit at the Lava-Chuar camp site (river-mile 65.3). On

river left, there is more open area with some fluvial deposits from which sand is

likely remobilized and transported, predominantly, toward the north-northwest.

The dominant transport direction calculated for both Palisades and Comanche is

oriented such that new fluvial sand deposits left by flows on the order of 1,270 m3

s-1 (45,000 ft3 s-1) will probably not act as major sand sources for adjacent dune

fields on river left; instead, at those sites and presumably along the ~5 km of the

river corridor between them, the net transport from fluvial sandbars on the east

side of the river (river left) is directed upstream and toward the river, where windblown sand would become entrained in the water.

Immediately downstream of Comanche the canyon widens considerably

and makes several tight bends in the Tanner area (river-miles 69–71). Given the

significant topographic variations between Comanche and the Tanner area, we

believe it is not reasonable to extrapolate a similar south-southeast dominant

aeolian transport direction for this reach below Comanche; wind conditions there

would need to be measured independently to identify transport pathways for that

area.

Because many of the archaeological sites in the Comanche area are

associated with aeolian sediment, and because the local aeolian sediment is

apparently sourced from pre-dam and, to a lesser extent, post-dam fluvial

deposits, the presence or absence of sediment-rich floods is linked to

sedimentary processes in the dune field and can likely affect the preservation of

cultural material there. As with several other large aeolian dune fields in Grand

Canyon, however (see discussion of Site 3 in the Three Case Studies section

above), it is uncertain whether archaeological sites at Comanche would be better

preserved if new flood sediment were available to be blown into the dune field

because many of the sites are exposed by dune migration, which occurs
naturally as aeolian dunes migrate over time. If the Comanche dunes are

significantly affected by sediment-supply limitation in the future (in other words,

by the effects of dam operations), the dune field will experience an increase in

vegetation cover and cryptogamic crust, both of which indicate decreased

mobility of the aeolian sediment comprising the dune forms. Continued NPS

monitoring and topographic surveys of dunes would indicate whether, and how

rapidly, dune migration and sand transport occurs. As aeolian sediment transport

becomes less and less active in a sediment-supply-limited dune field, and rates

of dune migration correspondingly decrease, the risk of archaeological-site

destabilization by dune migration diminishes. However, although artifacts will be

less likely to be disturbed by dune migration in that case, the risk of site erosion

by gully incision increases because with less aeolian sand transport, gullies that

form during rain runoff will have less opportunity to be filled by aeolian sand.

Forster:

The study site at the mouth of Forster Canyon (river-mile 123.0) was an

aeolian dune field located south-southwest of a small fluvial sand deposit

opposite the tail waves of Forster rapid (Fig. 12; fluvial deposit labeled ‘a’).

Higher river discharge in the 1965 aerial photographs compared to those taken in

the 1990s and 2000s makes it difficult to determine accurately changes in the

size of sandbars ‘a’, ‘b’, and ‘c’ at Forster (Fig. 12); however those photographs

clearly show an increase in riparian vegetation covering formerly open sandbar

area.

A weather station in the Forster dune field from April 2004 to January

2006 suffered multiple equipment failures related to adverse conditions caused

by high winds and abundant wind-blown sand. During its operation, the station

recorded winds almost exclusively from the north and northeast (oriented up the

axis of Forster Canyon, perpendicular to the trend of the Colorado River at this

location), with a dominant wind direction very consistently from the north-
50

m

B

dune

field

A

N

Dominant

winddirection

(21degrees

in 2004, 26 degrees in

2005)

C

N

FIGURE 12.Aerial photographsof the area by themouth of

Forster Canyon in 1965 and 2004. River discharge in the

May 1965photograph(A)is between680 and 792m

3

s-1

(24,000–28,000 ft

3

s-1). Discharge intheMay2004photograph (B) is 226 m3 s-1 (8,000 ft3 s-1). Locations ‘a’, ‘b’, and

‘c’ mark fluvial sand deposits (‘c’ismostly obscured byhigh

dischargeinthe 1965photo).Themodern extentofthe

aeolian dune fieldis indicated inB. C. Oblique view of the

same areafacing southwest toward river left near sandbar

‘b’. Light-colored aeoliandeposits are visible on the slopes

abovethe river, downwind of the fluvial sand deposits. Photograph C was taken in April 2004. The dominant wind

direction (thedirection of net sedimenttransport)isvery

consistently from the north-northeast at Forster (see Draut

andRubin,2005,2006fordetailsoncalculationofnet

sand-transport direction). Note increased riparian vegetation and decreased open fluvial sandbar area since 1965.
northeast (net potential sediment transport came from 26 degrees during 2005;

Draut and Rubin, 2006). The aeolian dune field at Forster (Figs. 5a, 12b) is thus

located directly downwind of sandbar ‘a’, and is defined as a MFS aeolian

deposit. Sand transport rates within this dune field were among the highest

measured during this study (commonly > 100 g cm-1 day-1), and the sand surface

was observed to aggrade and deflate repeatedly at the site of the weather station

by tens of centimeters on time scales of weeks. Forster was the site of the

highest wind speed measured during this study, a gust in July 2004 of 29 m s-1.

Aside from the main dune field at Forster aeolian sediment also occurs at

elevations tens of meters above the river, where sediment is deposited on a

steep, vegetated, sloping canyon wall on the south side of the river (left bank).

These aeolian deposits on the slope, visible at the bottoms of aerial photographs

in Figure 12a and b and shown in an oblique view in Figure 12c, are downwind of

fluvial deposits ‘a’, ‘b’ and ‘c’ and are assumed to have been sourced from them.

Controlled sediment-rich flows on the order of 1,270 m3 s-1 (45,000 ft3 s-1) are

apparently capable of depositing significant new sediment in at least three

locations on river left at Forster (sand deposits ‘a’, ‘b’, and ‘c’) and are therefore

expected to replenish sediment in the dune field and on aeolian deposits of the

adjacent canyon-wall slope.

Aerial photographs taken before and after the 1996 flood experiment

showed an increase in the size of sandbar ‘a’ at the upstream end of the Forster

dune field. Although the schedule of flood-related field work did not permit this

beach to be photographed immediately before or immediately after the 2004 high

flow, preliminary observations including a shallow sedimentary profile dug in

early 2005 indicated that new sand was deposited there as a result of the 2004

high flow. High daily flow fluctuations from January to March 2005 caused partial

erosion of the new flood deposit until tamarisk roots were exposed by March

2005 that had not been exposed prior to the November 2004 flood. Transport

rates during the spring windy season of 2005 were significantly higher than those

measured in spring 2004, but sand-trap malfunctions during spring 2004 and
anemometer malfunctions in spring 2005 prevent confident comparison of preand post-flood spring wind conditions and sand transport at Forster.

Known archaeological sites in the Forster area include several that are

built on and buried by aeolian sediment. Dune migration causes many of the

artifacts to be exposed and covered repeatedly. Because this is a very active

MFS aeolian system with frequent high winds that move significant quantities of

sand, the supply of sediment to the Forster dunes is strongly linked to the

amount of available sediment at fluvial sandbar ‘a’ (Fig. 12). Because the Forster

study site is located in a section of Grand Canyon where the canyon orientation

changes substantially within several km of this site, it is not recommended to

assume that a north-northeast wind direction can be extrapolated for other areas

upstream and downstream.

202.9 mile:

One weather station operated on the west side of the river (river right) at

river-mile 202.9 from April 2004 to January 2006. This site includes an aeolian

dune field near river level that is covered by trees and abundant other vegetation.

The heavy vegetation cover has grown on the dune field and on a large riverlevel sand bar just downstream of the dune field since the 1960s (labeled ‘a’ in

Fig. 13), and now covers a large area of previously open sand. Immediately

upstream from the instrument site, an aeolian dune field shows evidence of

erosion by deflation, some cryptogamic crust, and gully incision. The purpose of

instrumenting this site was to identify the sand source for that eroding, higherelevation dune field by documenting the dominant wind direction that could cause

sand transport. Knowing this could establish to what degree vegetation

encroachment downstream of the site may have affected the condition of the

upper dune field by limiting aeolian transport from its sand source.

Data collected between April and December 2004 at 202.9 mile indicated

that the dominant sand transport would be directed north (from downstream;
50 m

N

A B

dune

field

Dominant wind

direction in 2004

(194 degrees)

N

Dominant wind

direction in 2005

(283 degrees)

FIGURE 13. Aerial photographs of 202.9 mile in 1965 and 2004. River discharge in the May 1965 photograph (A) is between 680

and 792 m3 s-1 (24,000–28,000 ft3 s-1). Discharge in the May 2004 photograph (B) is 226 m3 s-1 (8,000 ft3 s-1). Location ‘a’ marks a

large fluvial sand deposit. Note greatly increased riparian vegetation and decreased open fluvial sandbar area since 1965.The modern extent of the aeolian dune field, on the downstream (south) side of a small debris fan, is indicated in B. Dominant wind direction

is highly variable at 202.9 mile, with calculations yielding net potential sediment transport from the south-southwest in 2004 and

from the west in 2005 (see Draut and Rubin, 2005, 2006 for details on calculation of net sand-transport direction).
Draut and Rubin, 2005). Those initial data supported the idea that the large

sandbar ‘a’ formerly provided a sand source to the dune field on the debris fan

(Fig. 13). This in turn implied that reduced sand-entrainment potential from the

newly vegetated sandbar had affected or could affect the condition of the aeolian

deposits downwind (upstream) of this sandbar. Wind conditions there in 2005,

however, indicated a more complex situation. Although upstream and

downstream winds nearly balanced each other at that site in 2005, a tertiary

component from the northwest yielded a vector sum showing net sediment

transport toward the river (Fig. 22 of Draut and Rubin, 2006). Thus the longerterm record now available from this site seems to suggest that although loss of

the potential sand source (the now-vegetated sandbar) downstream of this site

may affect the condition of that dune field, the availability of sand from other

sources could also affect aeolian deposition and erosion processes there.

Arroyo Grande:

The Arroyo Grande site is located on land managed by both the Hualapai

Nation and Grand Canyon National Park. The area studied there spanned ~250

m by ~70 m; the nearest side-canyon tributaries enter the Colorado River >1 km

upstream and 300 m downstream of the study area (Fig. 2c). A large eddy is

present on river left in the Arroyo Grande area even at non-flood stage. An

arroyo system up to 5 m deep has incised two levels of pre-dam alluvial terraces

in this area (the arroyo is shown in Fig. 2c and is indicated by the white arrow in

Fig. 14). Aeolian coppice dunes, most of which are now relatively inactive with

cryptogamic crust and sparse vegetation, are present on the terraces. Terrace

surfaces are deflated, indicated by development of pedestals holding pebbles

and cultural artifacts at heights up to 5 cm above the surrounding land surface.

Four archaeological-site complexes are present in alluvial-terrace and tributarydelta regions of the Arroyo Grande area. The largest of these, Site G:03:064, is
affected by the arroyo. This site contains 15 documented cultural features on the

land surface with several additional features exposed within arroyo walls.

Colorado River flood deposits dominate the stratigraphic record around

the extensive archaeological site G:03:064 at Arroyo Grande. Profiles within the

upper terrace (Fig. 2c) showed the proportion of fluvial sediment generally

increasing toward the river. The record of flood deposition is best preserved in

sedimentary profiles closest to the river, with 15 individual flood events evident in

one profile (Draut and others, 2005). All profiles showed evidence of subaerial

reworking and incorporation of colluvial and slope-wash sediment between

floods. The repeated occurrence of fluvial deposits overlain by subaerial

sediment led to the description of a ‘flood couplet’ facies consisting of a lower

fluvial and upper subaerial member (Draut and others, 2005). Bioturbation and

the presence of lithic clasts in the upper parts of flood couplets (derived from the

local gneiss bedrock) indicated reworking of the flood sediment at the land

surface. Similar to the Lower Comanche site, channel-fill deposits exposed at

Arroyo Grande indicate multiple episodes of gully formation and filling during past

subaerial exposure. Aeolian sediment constituted a relatively minor volume in the

measured sections. The most likely sediment source for small, deflated coppice

dunes visible on the land surface today is reworking of the extensive fluvial

deposits that created the terrace morphology in this area. The aeolian dunes at

Arroyo Grande, as at Palisades, can thus be considered to be relict features that

formed by in situ aeolian reworking of large, pre-dam flood deposits, and so we

define this dune field as a RFS aeolian deposit.

Abundant charcoal and ash material in many strata at Arroyo Grande is

most probably attributable to grass fires during times of subaerial exposure. It is

likely that these fires were deliberately set by the inhabitants of the area,

although lightning strikes could also have caused occasional fires. The Hualapai

and Southern Paiute Tribes, both with ancestral ties to the Arroyo Grande region,

have cultural traditions that include the deliberate setting of grass fires to initiate

seed germination and to prevent other, larger fires from starting (L. Jackson and

I. Bullets, oral commun. with L. M. Leap, 2004). The setting of grass fires was
50 m

A B

N

dune

field

100 m

arroyo

river

flow

FIGURE 14. Aerial photographs of the Arroyo Grande area in 1965 and 2004. River discharge in the May 1965 photograph (A) is between 680 and 792 m3 s-1 (24,000–28,000 ft3 s-1). Discharge in the May 2004 photograph (B) is 226 m3

s-1 (8,000 ft3 s-1). Extensive fluvial sand deposits are visible in the riparian zone in the 1965 photographs that are no

longer present as open sand areas in the 2004 photo despite the lower river discharge; note also the greatly increased

riparian vegetation since 1965.The modern extent of the aeolian dune field, on the downstream (south) side of a small

debris fan, is indicated in B. The location of the arroyo is shown (see also Fig. 2c).
practiced at certain prescribed times of year, and continues today. Other possible

reasons for deliberately set fires include hunting and warfare (Powell, 1878; Kelly

and Fowler, 1986; Boyd, 1999).

Of the 16 cultural features we examined at Arroyo Grande, 15 are

exposed at the land surface (Tab. 3). Fourteen were formed on the surface of the

upper terrace, of which seven are located within coppice dunes on the terrace

surface. Seven of the 14 features on the terrace surface are partially covered by

aeolian sediment. Of the two features not on the upper terrace surface, one was

built on colluvium near the landward edge of the terrace and the other, for which

no site name or affiliation has been assigned, was exposed within an arroyo wall.

Comparison of aerial photographs taken in 1965 and 2004 shows a

substantial decrease in open sandbar area along several hundred meters of the

north side of the river channel (river left) in post-dam time, even though the 2004

photographs were taken at much lower flow (Fig. 14). Riparian vegetation has

also increased substantially over the same time interval. A stage-discharge

relationship was not developed for the Arroyo Grande area but we estimate that

a flood of ~5,660 m3 s-1 (200,000 ft3 s-1), a level last reached in 1884, would

probably inundate the upper terrace (recognizing that stage-discharge relations

are episodically altered when debris flows constrict the river channel and cause

water to pool upstream of the constriction). Because the Arroyo Grande aeolian

dune field is defined as a RFS system based on sedimentary profiles through the

dunes and underlying terrace, the post-dam lack of sediment supply to this

terrace by floods has removed the major source of sediment to the aeolian dunes

as well. Although we do not know the extent to which lower-elevation fluvial

sandbars (those at or below the 1,270 m3 s-1 [45,000 ft3 s-1] stage) contributed

wind-blown sand to the dune field in pre-dam time, it is unlikely that any aeolian

sediment reaches the dune field from the small river-level sandbars that still exist

in this area given the very thick vegetation now separating the sandbars from the

dune field. It is therefore unlikely that a high-flow experiment of 1,270 m3 s-1

(45,000 ft3 s-1) would deposit new sediment that would blow into the aeolian dune

field. We believe this to be the case even though the dominant wind direction in
the Arroyo Grande area was not quantified during this study, because such

dense vegetation separates the dune field from the river in essentially any

direction. Dominant wind capable of transporting sediment is likely from the east,

directed upstream, based on the slip-face orientation of a large, active dune at

the eastern edge of the study area (Draut and others, 2005).

RECOMMENDED FUTURE WORK

In order to evaluate the degree to which the presence and operation of

Glen Canyon Dam may influence the stability of archaeological features in the

Grand Canyon river corridor, site-specific stratigraphic and geomorphic

knowledge is essential. Establishing the local importance of fluvial, aeolian, and

other processes in pre-dam and post-dam time is an important prerequisite for

accurate assessments of dam effects, and site sensitivity to dam operations must

be evaluated on a site-by-site basis because the relative roles of various

sedimentation processes differ widely between sites, even comparing sites within

the same reach of the river. Future sedimentary and geomorphic studies could

assess the sensitivity to dam operations of additional archaeological sites not

included in this study. It would be highly informative to include field analysis of

vertical sediment profiles such as those conducted here (identifying depositional

environments using sedimentary structures) in any future excavation of cultural

sites conducted in the river corridor.

The 26 months of wind, precipitation, and sand-transport data collected by

this study span a variety of conditions and weather patterns but still are probably

not representative of long-term climate in the Grand Canyon region. A quasipermanent weather-monitoring program would allow weather patterns and

potential sand-transport measurements to be refined with greater accuracy. To

represent weather conditions at high spatial resolution over the entire river

corridor in Grand Canyon, many stations would be required. To represent spatial

variability of weather in the canyon, a minimum of five (but preferably more)
stations should be distributed between Lees Ferry and Diamond Creek (river-mile

226; Fig. 1). Although the stations used in this study monitored weather that was

directly relevant to archaeological sites (that is, aeolian sand transport and

precipitation), longer-term weather monitoring could utilize additional equipment

to measure air temperature, relative humidity, barometric pressure, and other

variables to generate a broader-scale record of climate in the Colorado River

corridor useful for physical, biological, and cultural research. The measured

broad spatial variation in rainfall documented during this study indicates that

future sedimentary and geomorphic studies in the canyon would benefit

substantially from continued or expanded weather monitoring at multiple

locations along the river corridor, because rainfall records collected by NPS at

Phantom Ranch cannot be considered valid for other areas of the canyon.

Recording climatic data in the river corridor is only one step, however, in

resolving the complex geomorphic processes that affect the condition of aeolian

deposits, and, by extension, many archaeological sites. The information gathered

and presented in this and our other reports could form the basis for a wide array

of detailed modeling studies in the future. It is becoming increasingly clear that

aeolian transport and deposition of sediment can and do affect the condition of

archaeological sites in the river corridor, and that the availability of sediment from

open-sand areas on fluvial sandbars is an integral component in these

processes. Although transport modeling formed a component of this study

(Appendix 1), three further, related, research initiatives are suggested to address

these issues in more detail:

1) A detailed aerodynamic modeling study of one or more locations that

would involve numerical representation of air flow, including dynamic

interactions with obstacles, slope, and irregular spatial boundaries.

2) Construction of a new sand-trap design, currently under development at

the USGS in Santa Cruz, CA, that would allow unprecedented temporal

resolution of sediment-transport response to specific, short-duration wind

conditions. Future research in these two linked directions would
undoubtedly lead to a better understanding of the aeolian-transport

systems that affect natural and cultural resources in Grand Canyon.

3) Because there is an approximately exponential inverse relationship

between aeolian sand transport and vegetation cover (Bressolier and

Thomas, 1977; Buckley, 1987), studying the response of aeolian transport

systems to localized removal of riparian vegetation would be worthwhile.

Areas such as the sandbar downstream of the 202.9-mile study site, or

along the east bank of the river near Comanche, are possible locations

where vegetation removal could promote additional aeolian sand transport

given that aeolian dune fields are located nearby and that vegetation

cover is known to have increased locally in post-dam time (Figs. 11, 13).

To promote wind-blown sand transport effectively, near-complete removal

of vegetation would be necessary; wind velocities are negligible near the

bed on surfaces with ~40 percent vegetation cover (Buckley, 1987), and in

many riparian areas in Grand Canyon the vegetative cover is much denser

than that. Nevertheless, it is recommended that if vegetation-removal

projects are conducted in the future, wind and aeolian sand transport

should be measured at the work sites for several months, ideally spanning

a windy season, before and after vegetation removal to assess its effects.
CONCLUSIONS

Aeolian sediment serves as a substrate on which many archaeological

sites in the Colorado River corridor were formed (in many locations also

associated with fluvial and slope-wash sediment) and, more commonly, as a

surficial deposit covering cultural material. Without sufficient aeolian sand,

artifacts are exposed by wind deflation of the surrounding sediment, forming a

lag deposit on the land surface. Many aeolian dune fields in Grand Canyon show

evidence of reduced sediment supply, in the form of cryptogamic crust and

vegetation covering relict dune forms. The presence of well developed

microbiotic soil crusts and colonization by grasses indicates a lack of sediment

deposition and decreased dune mobility, respectively, compared to times in the

past when supply and transport of aeolian sand were abundant enough to

generate dunes. Dune fields in Grand Canyon with well developed cryptogamic

crust and vegetation typically experience an order of magnitude lower aeolian

sand-transport rates than those without. Although vegetation and cryptogamic

crust limit mobility (migration) of dunes compared to their prior state of sediment

abundance, this does not imply that dunes and associated archaeological sites

are safe from erosion. Dunes with substantially reduced sediment supply (and

with vegetation and soil crust) have insufficient sediment transport and deposition

to counteract gully incision, and in these vegetated, cryptogamic-soil-rich dunes

large gullies and arroyos cause major erosion and degradation of dunes and

archaeological sites.

Reduction in the source areas that provide wind-blown sand to aeolian

deposits is evident from comparison of aerial photographs taken in the 1960s

with modern images; at the sites studied for this project, many fluvial sandbars

are smaller, and riparian vegetation more abundant, now than in 1965. This is

consistent with findings of previous studies in many other areas of the river

corridor (Turner and Karpiscak, 1980; Beus and others, 1985; Schmidt and Graf,

1987; Kearsley and others, 1994; Leschin and Schmidt, 1995; Hazel and others,

1999; Schmidt and others, 2004).
Exposure at the land surface leaves cultural material vulnerable to erosion

by gully incision. We view the problem of archaeological-site preservation as

essentially a competition between two processes: precipitation runoff, which

creates gullies that erode into sediment deposits, and aeolian sand transport,

which fills in small gullies inhibiting their growth (gullies act as natural traps for

wind-blown sand). The ability of aeolian sand transport to counteract gully

formation and to heal small gullies (< 1 m wide and deep) is a major factor

controlling geomorphic evolution of Grand Canyon sediment deposits. Areas that

had heavy rainfall but high sand-transport rates during this study can be absent

of gullies (which form episodically but are quickly filled by wind-blown sand)

whereas areas that had low precipitation and little aeolian sand transport during

this study have been eroded substantially by gullies and arroyos, destabilizing

archaeological sites. Although many factors (long-term climatic fluctuations,

drainage-basin geometry, substrate composition, and effective base level)

contribute to gully formation, we infer that the arroyo incision would not have

progressed as far as it has in some areas if aeolian sand-transport rates were

significantly greater. For example, several kilometers upstream of the deeply

incised Palisades terrace, gullies are typically absent from the Malgosa dune

field—a site that, during this study, had twice the rainfall but ten times the aeolian

sand-transport rates of Palisades.

Two types of aeolian deposits occur in the Grand Canyon river corridor,

defined by relative locations of aeolian deposits, nearby fluvial sandbars, and the

locally dominant sediment-transport direction. Modern Fluvial Sourced (MFS)

deposits formed as the wind transported sand inland from a river-level sandbar,

creating an aeolian dune field downwind; Relict Fluvial Sourced (RFS) deposits

formed by in situ wind reworking of pre-dam flood deposits. Aeolian deposits of

both types are known to contain archaeological material. Both MFS and RFS

aeolian deposits are sensitive to dam operations, but for different reasons.

Changes in open, dry fluvial sandbar area and dimensions (a function of dam

operations) affect the transport of aeolian sand to MFS dune fields downwind; we

have defined criteria that can be used to judge the sensitivity of particular
archaeological sites within MFS deposits to dam operations. RFS deposits

comprise the uppermost, wind-reworked part of pre-dam terraces that are largely

alluvial in composition, containing sediment deposited by flows up to and

exceeding 5,660 m3 s-1 (200,000 ft3 s-1). Because sediment-rich floods of that

magnitude do not occur in this regulated river, fluvial deposits from which RFS

aeolian dune fields formed no longer constitute an active sediment source; no

substantial new deposition is likely to occur on RFS dune fields as long as the

lack of large, sediment-rich floods continues.

Management of Glen Canyon Dam operations affects aeolian sand

transport in the Colorado River corridor through Grand Canyon. The size and

number of fluvial sandbars from which wind remobilizes sand can be increased

using controlled, sediment-rich flows on the order of 1,270 m3 s-1 (45,000 ft3 s-1),

with the result that wind-blown sand transport can significantly increase in the

vicinity of new flood deposits and that new flood sand can be transported by wind

to elevations above the flood stage, replenishing sediment in MFS aeolian

deposits. All of those processes were documented during this study following the

November 2004 1,160 m3 s-1 (41,000 ft3 s-1) high-flow experiment. It is important,

however, that if enhancing wind-blown sand transport to aeolian dune fields

(thereby increasing the protective cover on associated archaeological sites) is

one of the management goals of controlled high flows, post-flood dam operations

must be used to maximize the size and number of fluvial sandbars during the

spring windy season, between April and early June, when aeolian sand-transport

rates are ~5–15 times greater than in other seasons. Following the November

2004 high flow, newly deposited sandbars were eroded substantially by daily flow

fluctuations of 142–566 m3 s-1 (5,000–20,000 ft3 s-1) between January and March

2005, before the spring windy season began. Sandbar erosion by high daily flow

fluctuations thus removed much of the sand that was potentially available for

aeolian redistribution, limiting the benefits of the 2004 flood for archaeological

resources. Because high fluctuating flows export approximately twice as much

fluvial sand as under Modified Low Fluctuating Flow (MLFF) operations of Glen

Canyon Dam, more aeolian sand would be available to be blown inland (to MFS
dune fields and the archaeological sites within them) if sediment-rich high flows

were followed by low fluctuating flows rather than by high daily fluctuations.

Operating under low fluctuating flows or steady low flow between the end of a

controlled high flow and the start of spring high winds is predicted to retain much

more of the post-flood sandbar area until the next spring windy season when

wind can transport sand into aeolian deposits above the flood-stage elevation.

Although preservation of artifacts in the Colorado River corridor on

geologic time scales is unrealistic due to continual downcutting and backwasting

of this bedrock canyon, tens to hundreds of archaeological sites in Grand

Canyon could be better preserved on decadal to century time scales by restoring

fluvial and aeolian sedimentation to resemble more closely those processes that

occurred before the river was regulated by upstream dams, including sedimentrich floods that left fluvial deposits from which sediment was remobilized by wind.
ACKNOWLEDGEMENTS

This study has been supported by funding from the U.S. Bureau of

Reclamation, through the Grand Canyon Monitoring and Research Center

(GCMRC) and is a cooperative effort with the National Park Service (NPS). The

following people are thanked for their collaboration at many stages of this project:

J. Balsom, M. Barger, K. Burnett, S. Davis, B. Dierker, J. Dierker, J. Draut, H.

Fairley, C. Fritzinger, A. Fuller, S. Goeking, guides and students from Grand

Canyon Youth, R. Griffiths, J. Grissom, D. Harris, G. Hatcher, J. Hazel, S.

Hueftle, R. Hunter, L. Jackson, S. Jones, M. Kaplinski, K. Killoy, J. Logan, T.

Melis, F. Nials, M. Piller, S. Reeder, M. Rubin, J. Running, T. Sabol, E. Todd, D.

Topping, R. Tusso, N. Voichick, S. Wright, S. Wyse Mietz, M. Yeatts. D.

Sherman (Texas A&M University) provided shear-velocity values for two of the

aeolian transport models listed in Appendix 1. J. Johnson and R. McDonald are

thanked for their valuable reviews of this manuscript. R. Hereford and R. Hunter

are also thanked for their comments and discussion of this report.
APPENDIX 1: Evaluation of Aeolian Sediment-Transport Models:

Can Supply Limitation be Discerned?

An outstanding question remains regarding the evolution of aeolian

sediment deposits in the Colorado River corridor of Grand Canyon: to what

extent has the sediment supply to individual sand deposits been limited by Glen

Canyon Dam operations? Aeolian sand supply has always been somewhat

restricted by confinement within this bedrock canyon—dune fields can become

only so large before they encounter canyon walls, the river, substantial

accumulations of talus, and other spatial limitations. It is apparent from this study,

however, that aeolian sediment supply is now more limited than in pre-dam time

because wind-blown sediment transport to at least some aeolian deposits

(particularly of the MFS variety) increases or decreases in response to changes

in fluvial sandbar area upwind, and dam operations are known to have reduced

the size and number of fluvial sandbars overall.

We wanted to investigate whether it is possible to use aeolian sedimenttransport models (established empirically through field and wind-tunnel

experiments) to discern sediment-supply limitation from field measurements of

wind and sand transport. If the modeled transport under given wind conditions

closely matches measured transport during those conditions in a field setting

where sediment supply is not limited by sand availability, soil crusts, vegetation,

or bed roughness, it may be possible to infer sediment-supply limitation in other

field settings where the same models consistently over-predict measured sand

transport. In that case, the inferred sediment-supply limitation could be caused by
one or more factors (lack of sand availability, soil crusts, vegetation, or bed

roughness) and further work would be necessary to back out which of the

possible causes were relevant. The experiments discussed in this Appendix were

conducted as part of the Grand Canyon aeolian sedimentation study but involved

field work in the Dumont Dunes, southern Death Valley, California, as well as in

Grand Canyon.

Eight models of aeolian sediment transport were tested through a field

experiment in a desert environment in the absence of factors such as vegetation,

limited sand area, and soil moisture, which may affect comparable studies

conducted in coastal sand dunes. Comparison of measured sand flux with

transport predicted by the tested models indicates that although models may

perform better in this field setting where sources of uncertainty have been

minimized, in general the models did not yield results accurate enough to

suggest their potential use in determination of sediment-supply limitation. We

believe this to be in part a function of the limited range of wind velocities that

occurred during the field experiment. Subsequent high-resolution tests on the

Malgosa dune field (river-mile 57.9, Grand Canyon) provided a greater range of

wind velocities, but there the physically more complex setting (canyon walls,

vegetation, steeply sloping dune face) were such that the results cannot be

considered to have been obtained under simple, supply-unlimited conditions, as

was the case at the Dumont Dunes site. This remains a topic that could benefit

from further study; multiple avenues of environmental management could benefit

from deterministic transport models through which the effect of a limited sediment

supply could be quantified.

INTRODUCTION

Quantification and prediction of sediment transport by wind is a primary

focus of many geological and agricultural studies. An accurate understanding of
the movement of sand and dust by aeolian processes has widespread practical

implications for predicting landform evolution, erosion of agricultural areas, and

guiding environmental management of coastal and desert regions. Numerous

studies, beginning with Bagnold (1941), have devised empirical relations

between wind conditions and resulting sediment motion; this and other transport

equations have been developed and tested in wind-tunnel and field settings with

the goal of being utilized as predictive models.

Many comparative studies have indicated that the application of commonly

cited aeolian transport models to field settings is problematic, however, and that

measured sediment flux may vary substantially from that predicted by models

(Hunter and others, 1983; Berg, 1983; Sarre, 1988, 1989; Bauer and others,

1990, 1996; Goldsmith and others, 1990; Sherman and Hotta, 1990; Sherman

and others, 1998; Craig, 2000). Assessment of discrepancies between predicted

and measured aeolian transport rates prompted Chapman (1990) and Bauer and

others (1996) to suggest that field-based aeolian sediment-transport

investigations suffer from inherent problems of indeterminacy, due to the great

number of physical factors affecting such complex systems relative to the

number of variables represented in current transport equations. Studies of sand

transport on coastal dunes have identified complications of transport prediction

such as vegetative cover, soil moisture, topographic slope, variable sediment

composition and grain size, presence of salt crystals in sand, and variations in

sand-source area due to tidal fluctuations and runnel development.

One approach to field studies in a discipline prone to complications of

indeterminacy, as proposed by Bauer and others (1996), is to minimize

uncertainty by focusing on systems that have been simplified to the greatest

degree possible. By eliminating or minimizing variables that are known to

exacerbate uncertainties in such transport investigations, the predictive capability

of standard transport equations can be most accurately assessed. Recognizing

that it may never be possible to entirely eliminate sources of uncertainty, we
chose to test eight aeolian transport models in a desert sand-sheet environment

where the complications mentioned above are absent or minimal. In doing so, we

evaluate whether the models used in this study show sufficient predictive value to

be utilized in future assessment of sediment-supply limitation.

Sediment-Supply Limitation:

Existing transport equations were developed under the assumption that

sand transport will be limited by wind conditions (velocity, and therefore bed

shear stress necessary to mobilize sediment). Sediment transport and erosion

processes are commonly affected by limitations in sediment supply, however, a

factor not incorporated into current models (Leys, 1999). Three implications arise

from this: (1) the predictive capability of current aeolian transport models will

decrease as a given sediment source diminishes by erosion over time; (2)

discrepancies between measured and predicted sediment flux may in some

cases be attributable to limited availability of sediment for transport; (3) it may be

possible to identify and quantify limitation in the amount of available sediment

from such discrepancies. This investigation was motivated by (2) and (3) above.

By first evaluating the predictive power of existing transport models in a supplyunlimited desert setting unaffected by variables that can influence transport rates

measured elsewhere, we evaluate the potential use of these models for

determining the degree to which sediment supply is limited.

Aeolian Transport Models Tested:

Most commonly used sediment-transport models incorporate a shear

velocity parameter, u*, which is defined as a function of shear stress (u*

2 = τ/ρ,

where τ is boundary shear stress and ρ the fluid density). Wind-velocity profiles

are typically used to calculate shear velocity using the Karman-Prandtl equation,

or ‘law of the wall’:
€

u(z) = u*

κ

ln z

z0











 (1)

in which u(z) is the wind velocity at elevation z, κ is the von Karman constant

(taken to be 0.4) and z0 is the roughness length of the sand surface. Shear

velocity is considered relative to a critical threshold of motion represented by the

shear velocity u*t, given by Bagnold (1941) as:

€

u*t = At

ρs − ρ

ρ

gd (2)

in which At is a dimensionless constant equal to 0.1 for the initiation of saltation

(Bagnold’s fluid threshold), ρs and ρ are the densities of quartz sediment and air

respectively (2650 kg m-3 and 1.2 kg m-3), g is gravitational acceleration (9.8 m s2

) and d is the sediment grain diameter in meters.

Table 4 lists the eight aeolian transport models evaluated during this

study. The empirical models of Bagnold (1941) and Zingg (1953), derived from

wind-tunnel studies, are of the same form but differ in the constant and exponent.

Kawamura (1951), a model also based on wind-tunnel experiments, first

incorporated the threshold shear velocity u*t into an aeolian transport model. The

Lettau and Lettau (1977) and White (1979) models, and the Sørensen (1991)

equation, the derivation of which incorporated sand-grain trajectories and grainbed collision processes, also use threshold shear-velocity terms. With the

exception of Lettau and Lettau (1977), equations of this form do not require a

separate term for grain size because u*t is itself a function of particle diameter.

Hsu (1971; see also Hsu, 1973) used a modified Froude number (of the form

u/(gd)n) to express sediment transport rate. Among these eight models, the

Williams (1964) model alone describes q (sediment flux) as a linear function of u*.
Reference Expression for sand transport rate u* range 

(m s-1)

Bagnold (1941)

€

q =1.8 d

D

ρ

g

u*

3 0.6–1.8

Kawamura (1951)

€

q = 2.78 ρ

g (u* − u*t)(u* + u*t)

2

N/A

Zingg (1953)

€

q = 0.83 d

D





 





0.75

ρ

g

u*

3 0.35–1.06

Williams (1964)

€

q = 4.066 ρ

g

u* 0.4–1.35

Hsu (1971)

€

q =10−4e4.79d −0.47 u*

3

(gd)

1.5 0.05–0.8

Lettau and Lettau (1977)

€

q = 4.2 d

D

ρ

g

u* − u ( *t)u*

2 N/A

White (1979)

€

q = 2.61u*

3 1− u*t

u*











 1+

u*t

2

u*

2













ρ

g

0.9–1.7

Sørensen (1991)

€

q =10−4 ρu* u* − u ( *t) u* + 7.6u ( *t + 205) 0.2–0.6

TABLE 4. Aeolian sand-transport models tested in Dumont Dunes (Death Valley,

CA) and Malgosa (Grand Canyon); third column lists shear velocities for which

the models were developed. In the equations, d is grain diameter in meters, D is

a reference grain diameter for 250 μ sand, in meters, u* is shear velocity, u*t is

threshold shear velocity for motion of the grain size considered, ρs and ρ are the

densities of sediment and air respectively (taken to be 2650 kg m-3 and 1.2 kg m3

), g is gravitational acceleration (9.8 m s-2). In the Williams (1964) formula, the

constant is represented as the multiple of two constants obtained for 330 μ sand,

slightly coarser than the 306 μ in this study. Sand flux, q, is expressed in all

models as a mass of sand passing through unit width in unit time (kg m-1 s-1). For

comparison to field data, q values were adjusted to reflect the 10 s averaging

interval of wind data, and the 0.02 m width of the BSNE sand traps used. The

Bagnold (1941) model was derived from wind-tunnel data and tested in desert

dunes. Kawamura (1951), Zingg (1953), Williams (1964), and White (1979)

models were derived from wind-tunnel data. The Hsu (1971) and Lettau and

Lettau (1977) models are based on field studies on beaches. Sørensen (1991)

gives an analytically derived model tested in wind-tunnel experiments.
Several models exist that do not depend upon shear velocity (for example, Kadib

[1965], which uses a normally distributed transport intensity function, and O’Brien

and Rindlaub [1936] which does not include a grain-size term but depends solely

on the cube of wind velocity measured at 1.52 m). We restricted this analysis to

those in Table 4 because the Kadib (1965) model has been found less reliable

than equations that rely on u*, particularly at high wind speeds (Sarre, 1988;

Sherman and others, 1998) and because the shear-velocity term represents flow

characteristics (shear affecting the bed, which determines sediment motion)

independent of the height of measurement.

METHODS

An experiment was conducted in the Dumont dune field at the southern

end of Death Valley in the Mojave Desert, California. The dune field occupies

~20 km2 and contains central star dunes up to 120 m tall, with lower barchan and

linear dunes at its margins (McDonald, 1970; Nielson and Kocurek, 1987). A

sand sheet at its northern edge provides an ideal location to collect sandtransport data in an area unaffected by vegetation or other bed-roughness

elements (other than wind ripples, footprints, and vehicle tracks all < 10 cm high).

Instruments were located in a relatively flat area to minimize the effect of surface

slope on wind velocity. Soil moisture was not detected in the surface sediment

and is usually negligible in this arid environment.

A test was conducted at Dumont Dunes in late February–early March

2004 during a 16-hour interval. Wind velocity was measured using three

spinning-cup anemometers mounted on an instrument tripod at heights of 2.0,

1.0, and 0.3 m and connected to a data logger (Fig. 15). Mean wind velocity,

maximum gust velocity, and mean wind direction were measured with a 1 Hz

sampling interval and recorded as 10-second averages. This sampling strategy

used the shortest averaging interval over which statistical reliability of shear
FIGURE 15. Photograph showing the location of the experiment at Dumont

Dunes, Death Valley, CA. At left is a tripod on which three anemometers (at

heights of 2.0 m, 1.0 m, and 0.3 m) and a data logger were mounted; on the

right are four sand traps at heights 1.0 m, 0.7 m, 0.4 m, and 0.1 m. The surrounding sand sheet is free of vegetation and nonerodible obstacles such as

rocks, and has very little slope for tens of meters in any direction.
velocity measurements can be retained (Namikas and others, 2003; see also

Anderson and others, 1991).

An array of sand traps was set up ~5 m from the anemometer tripod to

collect wind-blown sand (see Zobeck and others, 2003). Four BSNE sand traps

(Fryrear, 1986) were mounted on a vertical pole; these traps are equipped with

vanes to turn them into the wind. Sand trap heights were set at 1.0, 0.7, 0.4, and

0.1 m (Fig. 15). Air temperature was measured at 2.0 and 0.3 m to evaluate local

atmospheric instabilities caused by insolation heating, which may affect velocity

profiles (Rasmussen and others, 1985; Lancaster and others, 1991; Frank and

Kocurek, 1994). The measured temperature gradient was minor as heavy cloud

cover limited direct solar heating during the study. Because vertical motion of air

due to such instabilities is negligible relative to lateral motion at wind speeds high

enough to generate sediment transport (G. Kocurek, pers. comm., 2004; N.

Lancaster, pers. comm., 2004) this factor is not considered further here.

The total sand mass collected from each trap during the experiment was

weighed. Mass-transport rates were calculated by integration of a curve fit to the

mass-vs.-height data. The curve-fitting procedure uses a five-parameter

combined power-law and exponential function, as this has been shown to model

vertical aeolian mass flux more accurately than either power or exponential fits

alone (Sterk and Raats, 1996). Grain size analysis of surface sand at the Dumont

Dunes instrument site by Coulter counter yielded a d50 value of 305.8 μ (an

average of six analyses), which (from Equation 2) gives u*t equal to 0.257. Sand

in the upper part of the Malgosa dune field was similar, yielding a d50 value of

312.9 μ (an average of four analyses) and thus u*t is equal to 0.260. Using these

threshold shear velocities, cumulative values of sand flux were predicted from

wind measurements using the eight models outlined in Table 4 and were then

compared with the measured sand flux.

In May 2004, a similar experiment was conducted during two days of high

winds at the Malgosa dune field, river-mile 57.9, Grand Canyon. Equipment at
one of the weather stations in use at that site (Station Mal U; Draut and Rubin,

2005, 2006) was refitted to match the configuration used in the Dumont Dunes

experiment, with anemometers at heights of 2.0, 1.0, and 0.3 m, BSNE sand

traps at 1.0, 0.7, 0.4, and 0.1 m, and the data logger recording with a 1 Hz

sampling interval and 10-second averaging interval (as opposed to 4-minute

averaging during normal operation of that weather station). Two trials were

conducted at Malgosa on May 11 and 12, respectively; sand traps were emptied

at the end of each of those tests.

RESULTS

Table 5 compares predicted and measured sediment flux for each of the

eight models evaluated in the trial at Dumont Dunes and both trials at Malgosa.

In the 16-hour Dumont Dunes experiment, mean sustained winds recorded at 2.0

m (averaged over 10-second intervals) were 3.6 ± 1.6 m s-1 with gusts up to 10.8

m s-1. The actual mass flux is considered to span an efficiency range of

105–115% for the BSNE traps with the grain size at Dumont Dunes and the wind

speeds measured (Goossens and others, 2000). In the first trial at Malgosa,

which lasted seven hours, mean sustained winds recorded at 2.0 m were 5.8 ±

3.2 m s-1 with gusts up to 20.8 m s-1. In the second trial at Malgosa, which lasted

14.5 hours, mean sustained winds at 2.0 m were 3.1 ± 1.9 m s-1 with gusts up to

13.7 m s-1. Mass flux during both high-resolution experiments at Malgosa was

considered to span an efficiency range of 95–115% for the BSNE traps with the

available sand size and the wind speeds measured during the two trials

(Goossens and others, 2000)2. Figure 16 shows histograms of u* values

calculated for the entire duration of each experiment.

 2 Sand traps would ideally be isokinetic (100% efficiency), meaning that the trap would not disrupt

air flow around it and would thus capture 100% of the sediment locally in transport per unit width.

No perfectly isokinetic sand traps have yet been designed. If a trap is found to operate at <100%

efficiency, air is being diverted around (away from) the trap orifice and the amount of sediment

captured by the trap thus under-represents the amount of sediment locally in transport. A trap
The Kawamura (1951) model predicts cumulative sediment flux that most

closely approximates the measured transport during the test at Dumont Dunes.

Three models were found to overpredict sediment transport in that trial: Bagnold

(1941) by a factor of ~1.5 to 1.7 (given the sand trap efficiency range), the Hsu

(1971) model that uses a modified Froude number (by a factor of more than two)

and the Williams (1964) model (by more than one order of magnitude). The

remaining four models underpredicted the observed sediment flux. Zingg (1953)

gives predicted transport that is ~75 to 82% of the measured amount. The Lettau

and Lettau (1977) and White (1979) equations performed similarly, indicating

43% to 50% of the measured transport. The Sørensen (1991) model predicted

only 25 to 28% of the sand flux measured at Dumont Dunes.

In the first trial at Malgosa, in which the absolute sand transport over the

experiment duration was two orders of magnitude greater than in the Dumont

Dunes trial, sand-flux predictions made by the models performed generally with

similar accuracy as in the experiment at Dumont Dunes though with a greater

tendency toward over-prediction (Table 5). The model that most closely

approximated measured transport in Test 1 at Malgosa was that of Zingg (1953),

which yielded ~84–101% of the measured sand transport, given a sand-trap

efficiency of 95–115%. The Kawamura (1951) model, which had best predicted

the Dumont Dunes data, over-predicted sand transport in the first Malgosa test

by a factor of 2.5–3. In the second Malgosa trial, during which total sand

transport was intermediate between the amounts measured at Dumont Dunes

and in the first Malgosa experiment (order 100 g), all models greatly overpredicted aeolian sand transport.

 

operating at >100% efficiency is causing diversion of air flow into the trap, thus over-representing

the amount of sediment locally in transport per unit width.
TABLE 5. Comparison of modeled sand flux with sand flux measured using sand

traps during high-resolution experiments at Dumont Dunes and Malgosa.

Measured sand transported is given for each test as an integrated value for

height 0–1 m, along with modeled flux predicted from wind data measured at 10-

second intervals. Modeled fluxes in kg m-1 s-1 have been adjusted to account for

the 10-second logging interval and the 0.02-m width of sand trap orifices. Values

are indicated for the possible range of measured sand flux given the efficiency

range of BSNE sand traps (105–115% for the grain size and the wind velocity

range during the study in Dumont Dunes, and 95–115% for the Malgosa trials;

Goossens and others, 2000).

Dumont Dunes Measured 105% Trap

Efficiency

115% Trap

Efficiency

Measured transported sand (g) 61.8 58.9 53.7

Ratio of Predicted: Measured

Sediment Flux

Bagnold (1941) 1.47 1.54 1.69

Kawamura (1951) 0.856 0.898 0.985

Zingg (1953) 0.712 0.747 0.819

Williams (1964) 34.9 36.6 40.2

Hsu (1971) 2.29 2.41 2.64

Lettau and Lettau (1977) 0.430 0.452 0.495

White (1979) 0.406 0.426 0.467

Sørensen (1991) 0.241 0.253 0.277

Malgosa, Test 1 Measured 95% Trap

Efficiency

115% Trap

Efficiency

Measured transported sand (g) 2770 2910 2400

Ratio of Predicted: Measured

Sediment Flux

Bagnold (1941) 1.82 1.73 2.09

Kawamura (1951) 2.58 2.45 2.96

Zingg (1953) 0.880 0.840 1.01

Williams (1964) 17.3 16.4 19.8

Hsu (1971) 2.85 2.70 3.27

Lettau and Lettau (1977) 1.99 1.89 2.29

White (1979) 1.38 1.31 1.59

Sørensen (1991) 0.620 0.590 0.710
Malgosa, Test 2 Measured 95% Trap

Efficiency

115% Trap

Efficiency

Measured transported sand (g) 183 193 160.0

Ratio of Predicted: Measured

Sediment Flux

Bagnold (1941) 48.6 46.2 55.9

Kawamura (1951) 63.0 59.8 72.4

Zingg (1953) 23.6 22.4 87.5

Williams (1964) 603 574 694

Hsu (1971) 76.1 72.3 87.5

Lettau and Lettau (1977) 42.6 40.5 49.0

White (1979) 32.0 30.4 36.8

Sørensen (1991) 16.3 15.5 18.7
0 0.2 0.4 0.6 0.8 1 1.2 1.4 0

500

1000

1500

2000

2500

3000

0 0.2 0.4 0.6 0.8 1 1.2 1.4 0

200

400

600

800

1000

1200

1400

1600

Frequency

0 0.2 0.4 0.6 0.8 1 1.2 1.4 0

200

400

600

800

1000

1200

1400

1600

Frequency

u*

Malgosa

Trial 1

May 11, 2004

Malgosa

Trial 2

May 12, 2004

u*

u*

Frequency

Dumont Dunes

FIGURE 16. Histograms of shear velocity (u*) measured during high-resolution experiments at 

Dumont Dunes and Malgosa. A. Shear velocity during the 16-hour Dumont Dunes experiment on 

February 29 to March 1, 2004, in which mean sustained winds recorded at 2.0 m (averaged over 

10-second measurement intervals) were 3.6 ± 1.6 m s-1 (one standard deviation) with gusts up to 

10.8 m s-1. Shear velocity was calculated using Equation (1). B. Shear velocity during the first 

high-resolution experiment at Malgosa, which lasted seven hours on May 11, 2004, with mean 

sustained winds at 2.0 m of 5.8 ± 3.2 m s-1 with gusts up to 20.8 m s-1. C. Shear velocity during 

the second high-resolution experiment at Malgosa, which lasted 14.5 hours on May 12, 2004, 

with mean sustained winds at 2.0 m of 3.1 ± 1.9 m s-1 (one standard deviation) with gusts up to 

13.7 m s-1.
DISCUSSION

Potential Sources of Error:

Calibration of sand-trap efficiency plays a clear role in evaluating

measured sand transport, and can be a substantial source of error in field studies

if not well-established (McEwan and Willets, 1993; Sherman and others, 1998).

The BSNE trap design used in this study (Fryrear, 1986; Stout and Fryrear,

1989) is one of the most widely used in agricultural and sedimentological studies

(Zobeck and others, 2003). Its efficiency has been determined by multiple windtunnel and field studies using a range of grain sizes for sand and dust, and over

a wide range of wind velocities (Fryrear, 1986; Shao and others, 1993; Goossens

and Offer, 2000; Goossens and others, 2000, 2001). The widespread use of this

trap also allows comparison between studies. The BSNE was chosen for the

present study because existing calibrations suggests that the efficiency range of

95–115% for the grain size and velocities in this experiment can be reasonably

assumed.

Accuracy of shear-velocity estimates is affected by the number of

anemometers used to generate wind profiles. In the case of this experiment, u*

values for each 10-second data point were generated from three measurements

of wind velocity. Although the use of more anemometers would decrease the

uncertainty associated with calculations of u*, the accuracy of the results would

not necessarily increase (Namikas and others, 2003; see comparison of four- vs.

eight-anemometer u* estimates by Namikas, 1999).

Bed-roughness elements are not believed to have affected data collection

significantly during the Dumont Dunes experiment. Location of the instrument

tripod at Dumont Dunes was selected based on the absence of vegetation, large

bedforms, or pebbles on the sand surface. Values of roughness length, z0,

obtained from wind velocity profiles tend to be within an order of magnitude of

twice the Nikuradse roughness for the grain size at the instrument site (2d50/30),
and their departure from this value is attributed to the influence of saltation and/or

ripple development. The low apparent roughness values obtained in Dumont

Dunes indicate that form drag on the bed was negligible during this test. With

unobstructed sand on an approximately flat surface for distances of at least 20 m

surrounding the instrument tripod and sand traps in all directions, we are

reasonably confident that aeolian sand transport measured at Dumont Dunes

reached equilibrium conditions (Anderson and Haff, 1988; Anderson and others,

1991; Raupach and others, 1993; King and others, 2005; R.S. Anderson, written

comm., 2006).

In contrast, bed-roughness elements in the Malgosa dune field are

problematic. Even though this site has among the sparsest vegetation of any

known Grand Canyon aeolian deposit, vegetation and possibly also a nearby

talus slope generated measurably more roughness than in Dumont Dunes; the

Malgosa wind data yielded z0 values five to six orders of magnitude above

Nikuradse roughness. Sand transport measured at Malgosa was probably also

affected by variations in wind velocity caused by the ~20-degree slope of the

stoss side of the dune on which the weather station was located (cf. Lancaster,

1985; Mulligan, 1988). Previous studies have shown that wind-velocity profiles on

the stoss sides of dunes are not log-linear as they are on flat surfaces such as

the sand sheet in our Dumont dunes test, complicating the use of the KarmanPrandtl equation (1) to obtain shear velocity (Frank and Kocurek, 1996;

Lancaster and others, 1996). The height of the lowermost anemometer used in

our study (0.3 m) was also likely not low enough to measure velocity in the part

of the boundary layer where much of the sediment transport occurred (Lancaster

and others, 1996).

Using spinning-cup anemometers in conditions of abundant wind-blown

sand introduces an additional source of error as sand can clog the instruments

and interfere with the movement of the cups. Lower-elevation anemometers in a

vertical array are preferentially affected by this problem, causing the wind speeds
recorded by the lower anemometers to be erroneously low. If the lower

anemometers are preferentially slowed by wind-blown sand, velocity profiles

appear artificially steep and the u* values calculated from them will then lead to

artificially high modeled sand flux q. We believe that this is a potential source of

significant error in experiments of this type where anemometers are used for an

extended time, and was probably the reason why all models greatly overpredicted sediment transport in the second test at Malgosa (Table 5), which was

conducted after anemometers had been subjected to sustained high winds and

abundant sand transport for several days. Although anemometers were cleaned

between the two high-resolution tests at Malgosa (and during every maintenance

visit throughout the two-year Grand Canyon study) wind-blown sand continued to

slow the function of the lower-elevation anemometer there (and also at the

Forster study site, where the lower anemometer was eventually removed). It is

likely that this same problem affected the first high-resolution trial at Malgosa

especially toward the end of the seven-hour test, which may explain why the

transport models showed a greater tendency toward over-prediction in the first

Malgosa test compared to the experiment at Dumont Dunes (Table 5). The

anemometers used at Dumont Dunes had never been deployed in the field

before, suggesting that sand clogging was only a minimal factor there, whereas

those at Malgosa had been in use (with regular cleaning and maintenance) for

the six months before the May 2004 high-resolution tests.

Performance of Tested Models:

Figure 17 shows modeled aeolian sand flux plotted against a range of

shear velocity (u*) values up to 5 m s-1. Differences in the behavior of the eight

models become much more apparent at higher shear velocity (higher wind

speed) than was measured in any of the three experiments conducted at Dumont

Dunes or Malgosa. Values of u* represented in our data sets (Fig. 16) were < 0.5
0 0.5 1 1.5

0

0.5

1

1.5

u*

q (kg m-1 s-1)

1

2

3

4

5

6

7

8

1=Lettau and Lettau (1977)

2=Hsu (1971)

3=Kawamura (1951)

4=White (1979)

5=Bagnold (1941)

6=Zingg (1953)

7=Williams (1964)

8=Sorensen (1991)

FIGURE 17. Sediment flux (q) predicted for various shear velocities (u*) by eight aeolian 

sediment-transport models. A and B show identical curves plotted at different scales to 

highlight different behavior of the models at higher (A) and lower (B) shear velocities. The 

greatest divergence in modeled sediment flux occurs when u*

 > 1.0, such that shear velocities are substantially greater than were calculated from wind data measured during the 

three experiments in this study.

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

10-4

10-3

10-2

10-1

100

u*

q (kg m-1 s-1)

A

B

1 2

3 4

5

6

7

8

8

1

2 5

5

6

6

4

4

3

3
m s-1 at Dumont Dunes (clustering around 0.1 m s-1) and < 1 m s-1 at Malgosa

(clustering around 0.3 m s-1). Available wind velocities measured during these

experiments were therefore low enough that the models tested would not

necessarily show significant differences in the amount of sand transport that they

predict, with the exception of the linear Williams (1964) model, because most of

the divergence in their predicted fluxes occurs at u* greater than 1 m s-1 (Fig. 17).

Several of the models tested in this study showed somewhat greater

predictive capability in the very simplified desert setting of Dumont Dunes than

indicated by previous studies conducted in coastal dune environments. In coastal

settings, aeolian transport rates are often over-predicted and in other cases

under-predicted. Bauer and others (1996) found that predicted rates exceeded

measured rates by factors of 1.9 to 5.6 on a sparsely vegetated foredune zone

beach site, using the Lettau and Lettau (1977) model. Hunter and others (1983)

found that the Bagnold (1941) model over-predicted transport rates measured

from bedform migration of coastal dunes by a factor of 3.6. Over-prediction

factors of five and ~50 were obtained by Goldsmith and others (1990) and Bauer

and others (1990), respectively, applying the Bagnold (1941) equation to wind

and sand transport data on coastal beach areas. Sherman and others (1998)

found overprediction of sand transport on a beach by factors of two and greater

using the Bagnold (1941), Kawamura (1951), and Lettau and Lettau (1977)

models, while Zingg (1953) underpredicted transport. Coastal studies of Sarre

(1988) showed that the predictive power of most tested equations can vary with

shear velocity, with Bagnold (1941) overpredicting transport at shear velocities

comparable to those in this study (u* <0.5 m s-1). Sarre (1989) found that the

White (1979) model substantially overpredicted sand transport on foredunes

studied in an intertidal zone.

The improved predictive value of transport equations in a highly simplified

field setting such as Dumont Dunes would be expected (for example, Sherman

and Hotta, 1990). Aeolian transport in coastal or other more complex sand
systems can be influenced by multiple factors that did not affect the Dumont site:

vegetation (Olson, 1958; Bressolier and Thomas, 1977; Ash and Wasson, 1983;

Wasson and Nanninga, 1986; Buckley, 1987; Bauer and others, 1996; King and

others, 2005) soil moisture (e.g., Svasek and Terwindt, 1974; Sarre, 1988, 1989;

McKenna Neuman and Nickling, 1989; Namikas and Sherman, 1995; Selah and

Fryrear, 1995; Wiggs and others, 2004), salt encrustation on sediment (Nickling,

1984; Sarre, 1989), fetch limitation due to nearby dune crests (Bauer and others,

1990) and tidal fluctuations (Sarre, 1988; Nordstrom and Jackson, 1992),

trapping of sand by runnels and tidal channels (Bauer and others, 1996),

nonerodible surface objects such as rocks (Gillette and Stockton, 1989; Raupach

and others, 1993; King and others, 2005), cryptogamic soil crusts (Leys and

Eldridge, 1998; Belnap, 2003; Goossens, 2004) and perturbation of the wind

velocity profile by waves breaking near shore (Bauer and others, 1996). The

absence of factors that could effectively inhibit sediment transport (such as

vegetation, runnels, moisture, cryptogamic soil crust, or salt deposits) allows

evaluation of the performance of these transport formulae in a field setting that

can be considered, for practical purposes, unlimited with respect to sediment

supply.

The Kawamura (1951) equation, one of the four in Table 4 that

incorporates threshold shear velocity into its expression for transport rate, most

closely represented the measured sand flux at Dumont Dunes. Although the

actual transport was slightly under-predicted, the range of values indicates that

the Kawamura (1951) model may have predicted up to 98.5% of the actual

transport if the BSNE traps performed at the upper limit of their known efficiency

range for this grain size. The efficiency range for which Kawamura (1951) most

closely approximates the measured sand flux occurs for BSNE traps collecting

medium-sized sand at wind velocities below ~7 m/s (Goossens and others,

2000). The Kawamura (1951) model over-predicted sand flux in the first Malgosa

trial by a factor of 2.5–3; however, the performance of models at Dumont Dunes
is considered a more accurate reflection of their predictive capability given that

the second test at Malgosa and possibly also the first were apparently affected

by sand-clogged lower anemometers, leading to erroneously high shear-velocity

estimates.

The other three tested models that use a threshold velocity term resulted

in less accurate predictions of sand transport at Dumont Dunes. The Lettau and

Lettau (1977) and White (1979) equations produced similar results, while

Sørensen (1991) predicted just over one quarter of the actual transport. In a

study of aeolian transport predictions conducted on coastal dunes in an intertidal

zone, Sarre (1988) also found close agreement between measured transport and

that predicted by Kawamura (1951), though the agreement improved with a

constant of 2.4 used in place of the standard 2.78. Sarre (1988) found that, in

general, equations using the cube of u* and a u*t term provided the closest

approximation of measured transport, with White (1979) performing the best.

Svasek and Terwindt (1974) determined that the Bagnold (1941) and Kawamura

(1951) models both performed poorly compared when tested on a beach, which

was attributed to capillary forces associated with moisture in the beach sand.

Sherman and others, (1998), in a study of transport equations on coastal dunes

in Ireland, found that the Kawamura (1951) model overpredicted actual transport

rates by a factor of two to three (after correction for soil moisture and surface

slope). Lettau and Lettau (1977) yielded ratios of predicted to measured transport

that ranged from 1.0 to 1.2 after such corrections when applied to one coastal

system (Sherman and others, 1998) and ratios of 1.9 to nearly 6 in another

coastal area (Bauer and others, 1996). Hsu (1971), a method based on the cube

of u* that is essentially a modified Froude number, overpredicted transport in this

study by a factor greater than two. This behavior is similar to that found by Sarre

(1988), in which Hsu (1971) overpredicted transport at low to moderate u* in a

similar manner to Bagnold (1941).
Arguably the most commonly used aeolian transport equation, that of

Bagnold (1941) that depends on the cube of u*, overpredicted transport rates in

this study by a factor of at least 1.5. This is comparable to the result of Sarre

(1988) that showed over-predictive behavior of Bagnold (1941) for shear

velocities in the range measured during this study (<0.5 m/s, and usually <0.4

m/s). This model yields a close fit to our data when the constant in this equation

is lowered from 1.8 to 1.2. Zingg (1953), whose model is of the same form as that

of Bagnold but differs in its constant and exponent, predicted ~75 to 82% of the

measured sand transport at Dumont Dunes. This is similar to the upper end of

this model’s predictive power determined by Sherman and others (1998) after

corrections were applied to remove effects of soil moisture and slope. Sarre

(1988) also found that the Zingg (1953) equation was a reasonable predictor of

transport in the range of shear velocities considered here, though at higher u*

this model under-predicted actual sand flux at Dumont Dunes

Williams (1964), the only model in this study that considers sediment

transport rate to be a linear function of shear velocity, performed poorly in this

assessment. Transport in the Dumont Dunes trial was over-predicted by more

than an order of magnitude using this method; Willams (1964) also overpredicted sand flux in both Malgosa trials to a much greater degree than did the

other models (Table 5). It is not possible to draw a direct comparison between

this study and the wind-tunnel work that led to the Williams (1964) model,

however, as the constant terms in that equation apply to a specific grain size

(330 μ, coarser than that collected at the Dumont dune field). However, using

sediment with a similar grain size to that required by Williams (1964), Sarre

(1988) obtained transport results that differed substantially from those predicted

by this model.

Relevance for Investigation of Supply-Limited Systems:
Testing of these eight transport models in the Dumont Dunes setting,

where sediment supply is essentially unlimited, indicates that the Kawamura

(1951) model appears most feasible as one that could potentially be used to

discern the effect of supply limitation, bearing in mind the limited range of wind

velocities available in that Dumont Dunes experiment and the more variable

behavior of the models at higher shear velocity (Fig. 17). In order to be useful in

the context of determining the degree of sediment-supply limitation, a transport

model would need to accurately predict sediment transport rates to within 5% (a

reasonable confidence interval) in a supply-unlimited setting. The Kawamura

(1951) model predictions are within this boundary for the upper end of the sandtrap efficiency range. Given that BSNE sand-trap efficiency decreases with

increasing wind speed, this suggests that this model is most applicable at lower

wind speeds where transport rates are low. This implication is consistent with the

result of Sarre (1988), in which Kawamura (1951) predicted transport most

closely at the lowest shear velocities considered (<0.35 m s-1; the Sarre [1988]

field data also agreed well with the Lettau and Lettau [1977] and White [1979]

models). The wide variation in model applicability among different field areas and

at different shear velocities indicates that the use of this or any other model to

assess the effects of sediment-supply limitation warrants caution; for this reason,

we did not pursue similar modeling applications extensively as a means of

identifying sediment-supply limitation in Grand Canyon dune fields, which are

much more complex environments than our highly simplified Dumont Dunes test

site. More extensive testing is warranted to determine the reliability of this model

in supply-unlimited areas with, for instance, different grain size, sorting, and in

higher wind velocities than those represented here.

Although developing a new transport model specifically for supply-limited

systems is beyond the scope of this work, the practical applications of such a

model would be extensive (Leys, 1999). Quantification of, for instance,

agricultural erosion processes would benefit from modeling capabilities that
include adjustment for diminished sediment supply as erosion proceeds.

Environmental management problems that concern sediment availability could be

more fully addressed if it were possible to determine the degree to which a

system undergoing aeolian sediment transport is supply-limited. Examples

include sediment mobilization on subaerial sand bars in dam-controlled river

systems such as Grand Canyon, and the condition of beaches used for

recreation on eroding coastlines. The development of models that can be used

for such purposes is recommended as a future research direction.

CONCLUSIONS

Evaluation of eight aeolian sediment transport models in a desert setting

considered to have unlimited sediment supply indicates that the Kawamura

(1951) model most closely predicted the cumulative sand flux measured during

the 16-hour study at Dumont Dunes, Death Valley, CA. This model appears the

most potentially viable as a method by which sediment-supply limitation might be

discerned, although we recognize that at higher wind velocities than were

measured during this study the predictive capability of the tested models could

vary substantially. Additional testing of this and other models in supply-unlimited

settings is advisable as a future direction of research, as is development of an

aeolian transport model that could quantify expected reduction in sediment

transport as sediment supply becomes progressively more limited. The ability to

quantify the influence of supply limitation on aeolian erosion, transport, and

deposition would be advantageous to many environmental management

initiatives.
APPENDIX 2: Estimation of historical changes in aeolian sand sources

using a GIS database developed at Utah State University

This appendix catalogs an effort to evaluate changes in the availability of

sand involved in aeolian transport systems within the Colorado River corridor

over decadal scales. To do this, we used a Geographic Information System (GIS)

database developed by the Fluvial Geomorphology Laboratory, Department of

Aquatic, Watershed, and Earth Resources, Utah State University that is based on

aerial photographs taken from 1935–1996 between Glen Canyon Dam and the

Unkar area (river-mile 73). The aerial photographs and GIS resources discussed

below were part of the data set used by Schmidt and others (2004) to document

decreased fine sediment in the river corridor between the dam and Bright Angel

Creek (river-mile 88). For the purposes of our evaluation, although site-by-site

changes are apparent, widespread decadal-scale trends in aeolian deposits and

sediment-source areas were not readily apparent. We believe that this is due in

part to great differences in river discharge in the early photographs compared to

those taken in the 1990s, when all aerial photography missions were flown during

steady 226 m3 s-1 (8,000 ft3 s-1) flows. Lack of regional decadal trends is likely

also due to difficulties of interpreting sedimentary features on the land surface

using only aerial photographs without the benefit of supplemental information

such as oblique photographs and field ground-truthing.

Aerial Photographs Incorporated Into GIS at Utah State University:

Aerial photographs of five reaches of the Colorado River corridor between

Glen Canyon Dam and approximately river-mile 73 were studied by members of

the Fluvial Geomorphology Laboratory led by Dr. J. C. Schmidt, Department of

Aquatic, Watershed, and Earth Resources, at Utah State University (USU).
Scientists from the USU research group interpreted geomorphology in multiple

sets of photographs taken between 1935 and 1996, identified geomorphic units

and mapped their boundaries as polygons, and stored the layers of polygons as

shape files in an GIS database.

Using the polygons in those shape files that represent previously

interpreted geomorphic units, the total area of sediment deposits considered

relevant to aeolian sand-transport systems were summed for each reach of the

river in each set of available photographs. Geomorphic assignments (for

example, of debris fans, channel-margin sand deposits, aeolian deposits, and

eddy sandbars) made by USU scientists were not re-interpreted during this

exercise, but representative areas were checked by independently studying the

original photographs from which the features had been mapped. Reaches of the

river for which USU scientists compiled shape files of geomorphic features, and

ages of available aerial photographs used for each, are listed in Table 6. Table 7

lists dates of aerial photographs included in the USU GIS database, and the

available mapped reaches in each.

Name of reach River miles

Glen Canyon (GL) -15.2(dam)–0

Lees Ferry (LF) -0.4–8.3

Redwall Gorge (RW) 29.2–35.3

Point Hansbrough (PH) 42.3–48.9

Little Colorado River (LCR) 60.0–72.8

TABLE 6. Reaches of the Colorado River, and corresponding river miles,

analyzed using a geomorphology GIS database developed by Utah State

University. River miles are those given by the Grand Canyon Monitoring and

Research Center (GCMRC) at http://www.gcmrc.gov/products/ims/ims.htm and

are measured downstream from Lees Ferry (Fig. 1), with negative miles

indicating distance upstream from Lees Ferry. These were the same reaches

used for aerial photographic analyses by Leschin and Schmidt (1995) and

Schmidt and others (2004).
Date Discharge Reaches

mid-1930s Unknown; appears ~ 142–226 m3 s-1

(5,000–8,000 ft3 s-1)

LF, RW, PH, LCR (partial)

1951 Unknown LF

1952 252–380 m3 s-1 (8,800–13,400 ft3 s-1)

on Sep. 24–25, ~175 m3 s-1 (6,200 ft3

s-1) on Oct. 8.

GL (Sep.), LF (Sep. and

Oct.), RW (Sep.), PH (Sep.)

1954 119–156 m3 s-1 (4,200–5,500 ft3 s-1) LCR

1958 186–201 m3 s-1 (6,600–7,100 ft3 s-1) LCR

1965 680–792 m3 s-1 (24,000–28,000 ft3 s-1) LF, RW, PH, LCR

1973 76–396 m3 s-1 (2,700–14,000 ft3 s-1) LF, RW, PH, LCR

1984 144–226 m3 s-1 (5,100–8,000 ft3 s-1) GL, LF, RW, PH, LCR

1990 142 m3 s-1 (5,000 ft3 s-1) GL, LF, RW, PH, LCR

1992 226 m3 s-1 (8,000 ft3 s-1) GL, PH, LCR

1996 (March) 226 m3 s-1 (8,000 ft3 s-1) GL, LF, RW, PH, LCR

1996 (April) ~226 m3 s-1 (8,000 ft3 s-1) GL, LF, RW, PH, LCR

1996 (Sept.) 226 m3 s-1 (8,000 ft3 s-1) LF, RW

1997 (August) 226 m3 s-1 (8,000 ft3 s-1) RW

TABLE 7. Dates and river discharges of aerial photographs included in the USU

GIS database, and the available mapped reaches in each. Abbreviations of reach

names are given in Table 6 above. The March and April 1996 sets of

photographs were flown before and after the first controlled-flood experiment

(Webb and others, 1999b; Schmidt and others, 2001), respectively. Discharge is

based on instantaneous discharge from the Lees Ferry gage at the time of

overflight photography (Topping and others, 2003).
Categories of geomorphic features that together indicate aeolian deposits

were grouped together and their areas summed for each river reach and set of

photographs. Polygons designated as aeolian sand in the GIS database were

those that the USU group determined to be “fine-grained sand deposits, often

with dune-like features visible, commonly found on high terrace deposits and

large, high-relief gravel bars. Reworking of a deposit between times of

photography was considered to be good evidence of aeolian sand if the deposit

was topographically higher than any level reached by river flows during the

intervening years” (description excerpted from USU laboratory procedure notes).

In the case of numerous polygons the exact geomorphic category was not

identified exclusively but was listed under a combined heading, such as “aeolian

sand / channel margin” deposit, or “aeolian sand / gravel”. Pre-dam sediment

deposits formed above the 2,700 m3 s-1 (97,000 ft3 s-1) stage (a level reached by

the post-dam flood of 1983) were assigned categories of ‘upper terrace’ or ‘high

terrace’ deposits. Also summed for this exercise were areas of polygons

representing fluvial sand deposits in eddies: separation bars, reattachment bars,

and, when the distinction between separation and reattachment bars could not

be made, simply eddy bars (Rubin and others, 1990; Schmidt, 1990).

Results and Interpretation:

Figures 18–22 show bar graphs of the summed area of different types of

sediment deposits, by year, for each of the five reaches listed in Table 6. The

total mapped areas occupied by various types of sand deposits varies

substantially between sets of photographs for each reach. This is believed to be

largely an artifact of the variable river flows represented in each set of

photographs until it became customary, in the 1990s, to conduct overflight

photography only during steady 226 m3 s-1 (8,000 ft3 s-1) discharge. Differences

in flow between set of photographs should not have affected the mapped area of

‘upper’ or ‘high terrace’ sand, however, as those deposits lie above the above the

2,700 m3 s-1 (97,000 ft3 s-1) stage.
Schmidt and others (2004) used changes in the eddy sandbars visible in

these aerial photographs, combined with oblique photography and site-specific

field study, to estimate that the area of sand in the pre-dam and post-dam flood

zone was 1–20 percent less in the 1990s than in pre-dam time. The Schmidt and

others (2004) analysis did not distinguish between sand deposits that were

strictly fluvial and those that had undergone (or were available for) wind

reworking. It is apparent from the aerial photographs, particularly in the upstream

(GL and LF) reaches, that eddy sandbar area decreased as a result of the 1996

controlled flood. This is consistent with the more widely documented pattern of

accretion and growth of sandbars farther downstream in Grand Canyon at the

expense of sandbars that had been eroded from Marble Canyon, and was a

primary why the 2004 controlled flood was designed to follow recent substantial

input of sediment from the Paria River (Topping and others, 2006; note that

because the Paria River joins the Colorado at Lees Ferry, sandbars in the Glen

Canyon reach would still have been sediment-starved and eroded during the

2004 flood).

Lack of apparent regional decadal trends in aeolian sand is partly due to

difficulties of interpreting sedimentary features on the land surface using only

aerial photographs without the benefit of supplemental information such as

oblique photographs and field ground-truthing (using sedimentary structures, for

instance, to determine more accurately the depositional history of sediment by

examining it in three dimensions). In particularly the earlier sets of photographs,

poor quality (commonly overexposure), shadows, and large scale also

complicate detailed interpretation. Error is also introduced into the geomorphic

mapping methods by inherent subjective decisions that must be made by the

interpreter—whether, in one set of photographs, a deposit should be considered

to contain aeolian sand while in another set of photographs, the same sediment

deposit may look ambiguous or may appear to have no features suggesting wind

reworking of sediment. These complications would obviously be amplified

substantially the more people are responsible for interpretation and polygon

naming, but this source of error was reduced in the USU laboratory by having the
same personnel interpret most or all of the photograph sets and was determined

to be minimal when the accuracy of interpretations made by different scientists

were compared (Schmidt and others, 2004).
Sum of components containing aeolian sand

2 10 4

4 10 4

6 10 4

8 10 4

1 10 5

1.2 10 5

1952 1984 1990 1992

1 10 5

2 10 5

3 10 5

4 10 5

5 10 5

6 10 5

7 10 5

1952 1984 1990 1992

2 10 4

4 10 4

6 10 4

8 10 4

1 10 5

1.2 10 5

1984 1990 1992 Mar

1996

Sum of 'high-terrace' deposits

Sum of eddy sand deposits

A B

C

Apr

1996

Mar

1996

Apr

1996

Mar

1996

Apr

1996

FIGURE 18. Total areas of various types of sediment deposits mapped

from aerial photographs in the Glen Canyon (GL) reach. A. Total area

of polygons mapped by USU scientists as containing aeolian sand,

whether alone or in combination with another geomorphic category. B.

Total area of polygons containing sediment mapped as ‘upper’ or ‘high

terrace’ deposits (pre-dam deposits formed above the 2,700 m3 s-1

[97,000 ft3 s-1] stage). While not deposited by wind, those categories

indicate sediment available for remobilization by wind. C. Total area of

polygons containing fluvial sand deposits in eddies, including ‘separation bars’, ‘reattachment bars’, and, when separation and reattachment bars were not distinguished as separate categories, ‘eddy bars’.

Fluvial deposits in eddies, when exposed subaerially, indicate source

areas from which sand could be reworked and transported by wind.

The March and April 1996 sets of aerial photographs were taken just

before and just after the first controlled flood release from Glen Canyon

Dam (Schmidt and others, 2001), respectively. Note the decreased

area of eddy sandbars in the Glen Canyon reach (between the dam

and Lees Ferry) over the time of the 1996 flood; sandbars downstream

in Grand Canyon that accreted during the 1996 flood were built by sand

eroded from eddy sand deposits upstream in the Glen Canyon and

adjacent reaches.
Sum of components containing aeolian sand Sum of 'high-terrace' deposits

A B

5000

1 10 4

1.5 10 4

2 10 4

2.5 10 4

3 10 4

3.5 10 4

1935 Sep

1952

Oct 1965 1973 1984 1990

1952

Mar

1996

Apr

1996

Sep

1996

5 10 4

1 10 5

1.5 10 5

2 10 5

2.5 10 5

3 10 5

3.5 10 5

1935 1951 Sep

1952

Oct 1965 1973 1984 1990

1952

Mar

1996

Apr

1996

Sep

1996

Sum of eddy sand deposits

C

5 10 4

1 10 5

1.5 10 5

2 10 5

1935 1951 Sep

1952

Oct 1965 1973 1984 1990

1952

Mar

1996

Apr

1996

Sep

1996

FIGURE 19. Total areas of various types of sediment deposits mapped

from aerial photographs in the Lees Ferry (LF) reach. A. Total area of

polygons mapped by USU scientists as containing aeolian sand,

whether alone or in combination with another geomorphic category. B.

Total area of polygons containing sediment mapped as ‘upper’ or ‘high

terrace’ deposits (pre-dam deposits formed above the 2,700 m3 s-1

[97,000 ft3 s-1] stage). Those categories indicate sediment available for

remobilization by wind. C. Total area of polygons containing fluvial

sand deposits in eddies, including ‘separation bars’, ‘reattachment

bars’, and, when those were not distinguished as separate categories,

‘eddy bars’. Fluvial deposits in eddies, when exposed subaerially, indicate source areas from which sand could be transported by wind. The

March and April 1996 photographs were taken just before and just

after the first controlled flood, respectively. As in the Glen Canyon

reach, eddy sand deposits in the Lees Ferry reach appear to have

decreased slightly in area between March and April 1996 as a result of

the flood, and to have decreased further between April and September

1996. It is possible that the decreased area from March to April 1996

of deposits containing aeolian sand was related to the loss of subaerial

sandbars in eddies; it is not known why, if that were the case, deposits

containing aeolian sand apparently increased between April and September 1996 while eddy sandbar area declined.
Sum of 'high-terrace' deposits Sum of eddy sand deposits

A

B

1935 1951 1965 1973 1984 1990 Mar

1996

Apr

1996

Sep

1996

Aug

1997

5000

1 10 4

1.5 10 4

2 10 4

2.5 10 4

2 10 4

4 10 4

6 10 4

8 10 4

1 10 5

1935 1951 1965 1973 1984 1990 Mar

1996

Apr

1996

Sep

1996

Aug

1997

FIGURE 20. Total areas of various types of sediment deposits mapped from aerial photographs in the Redwall Gorge

(RW) reach. A. Total area of polygons mapped by USU scientists as containing ‘upper’ or ‘high terrace’ sediment deposits

(pre-dam deposits formed above the 2,700 m3 s-1 [97,000 ft3 s-1] stage). While not deposited by wind, those categories

indicate sediment available for remobilization by wind. B. Total area of polygons containing fluvial sand deposits in eddies,

including ‘separation bars’, ‘reattachment bars’, and, when separation and reattachment bars were not distinguished as

separate categories, ‘eddy bars’. Fluvial deposits in eddies, when exposed subaerially, indicate source areas from which

sand could be reworked and transported by wind. Aeolian sediment was not mapped as a separate category for the Redwall Gorge reach; aeolian deposits in this reach are now rare and those that do occur are small relative to those in other

reaches.
Sum of components containing aeolian sand Sum of 'high-terrace' deposits

A B

5000

1 10 4

1.5 10 4

2 10 4

2.5 10 4

1965 1973 1984 1990 1992 Mar

1996

Apr

1996

2 10 4

4 10 4

6 10 4

8 10 4

1 10 5

1.2 10 5

1.4 10 5

1935 1952 1965 1973 1984 1990 1992 Mar

1996

Apr

1996

Sum of eddy sand deposits

C

5 10 4

1 10 5

1.5 10 5

2 10 5

2.5 10 5

1935 1952 1965 1973 1984 1990 1992 Mar

1996

Apr

1996

FIGURE 21. Total areas of various types of sediment deposits mapped from aerial photographs in the Point Hansbrough

(PH) reach. A. Total area of polygons mapped by USU scientists as containing aeolian sand, whether alone or in combination with another geomorphic category. Aeolian deposits

were not mapped as separate categories on the 1935 or

1952 sets of photographs for the PH reach. B. Total area of

polygons containing sediment mapped as ‘upper’ or ‘high

terrace’ deposits (pre-dam deposits formed above the 2,700

m3 s-1 [97,000 ft3 s-1] stage). While not deposited by wind,

those categories indicate sediment available for remobilization by wind. C. Total area of polygons containing fluvial

sand deposits in eddies, including ‘separation bars’, ‘reattachment bars’, and, when separation and reattachment bars

were not distinguished as separate categories, ‘eddy bars’.

Fluvial deposits in eddies, when exposed subaerially, indicate source areas from which sand could be reworked and

transported by wind. The March and April 1996 sets of aerial

photographs were taken just before and just after the first

controlled flood release from Glen Canyon Dam (Schmidt

and others, 2001), respectively. Note the decreased area of

eddy sandbars as a result of the 1996 flood.
Sum of components containing aeolian sand

A

Mar

1996

Apr

1996

1 10 5

2 10 5

3 10 5

4 10 5

5 10 5

6 10 5

7 10 5

8 10 5

1935 1954 1958 1965 1973 1984 1990 1992

Sum of 'high-terrace' deposits

Sum of eddy sand deposits

B

Mar

1996

Apr

1996

2 10 5

4 10 5

6 10 5

8 10 5

1 10 6

1.2 10 6

1.4 10 6

1935 1954 1958 1965 1973 1984 1990 1992

C

5 10 4

1 10 5

1.5 10 5

2 10 5

2.5 10 5

3 10 5

3.5 10 5

1935 1954 1958 1965 1973 1984 1990 1992 Mar

1996

Apr

1996

FIGURE 22. Total areas of various types of sediment deposits

mapped from aerial photographs in the Little Colorado River

(LCR) reach. A. Total area of polygons mapped by USU scientists as containing aeolian sand, whether alone or in combination

with another geomorphic category. B. Total area of polygons

containing sediment mapped as ‘upper’ or ‘high terrace’ deposits

(pre-dam deposits formed above the 2,700 m3 s-1 [97,000 ft3 s-1]

stage). While not deposited by wind, those categories indicate

sediment available for remobilization by wind. C. Total area of

polygons containing fluvial sand deposits in eddies, including

‘separation bars’, ‘reattachment bars’, and, when separation and

reattachment bars were not distinguished as separate categories, ‘eddy bars’. Fluvial deposits in eddies, when exposed subaerially, indicate source areas from which sand could be reworked

and transported by wind. The March and April 1996 sets of aerial

photographs were taken just before and just after the first controlled flood release from Glen Canyon Dam (Schmidt and others, 2001), respectively.
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