U.S. Department of the Interior

U.S. Geological Survey

Professional Paper 1825

Conditions and Processes Affecting Sand Resources at 

Archeological Sites in the Colorado River Corridor Below 

Glen Canyon Dam, Arizona
Cover: Active source-bordering dune, approximately 1 m high, near RM 70 along the Colorado River, Grand Canyon, 

Arizona. Photograph taken in July 2008, several months after the upwind sandbar had enlarged substantially as a result of 

the March 2008 controlled flood.
Conditions and Processes Affecting Sand 

Resources at Archeological Sites in the 

Colorado River Corridor Below Glen Canyon 

Dam, Arizona

By Amy E. East, Brian D. Collins, Joel B. Sankey, Skye C. Corbett, Helen C. Fairley, 

and Joshua Caster

Professional Paper 1825

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Director

U.S. Geological Survey, Reston, Virginia: 2016

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Suggested citation:

East, A.E., Collins, B.D., Sankey, J.B., Corbett, S.C., Fairley, H.C., and Caster, J., 2016, Conditions and processes 

affecting sand resources at archeological sites in the Colorado River corridor below Glen Canyon Dam, Arizona: U.S. 

Geological Survey Professional Paper 1825, 104 p., http://dx.doi.org/10.3133/pp1825.

ISSN 2330-7102 (online)
iii

Contents

Abstract...........................................................................................................................................................1

Introduction and Background......................................................................................................................2

Landscape Context of Cultural Resources in the Colorado River Corridor.................................2

Aeolian-Fluvial-Hillslope Connectivity: Background and Previous Studies ...............................8

The Role of Glen Canyon Dam Operations ............................................................................10

Project Objectives........................................................................................................................................14

Section I - Potential Aeolian Sand Supply to River-Corridor Archeological Sites in Grand 

Canyon National Park....................................................................................................................14

Background..........................................................................................................................................14

Research Question .............................................................................................................................14

Classification System and Methods ................................................................................................15

Results ..................................................................................................................................................19

Discussion............................................................................................................................................29

Section II - Gullies and Aeolian Sand Activity in the Geomorphic Context of the Colorado River 

Corridor............................................................................................................................................30

Background..........................................................................................................................................30

Research Questions ...........................................................................................................................31

Methods................................................................................................................................................31

Results ..................................................................................................................................................33

Discussion............................................................................................................................................37

Spatial Variations in Aeolian Sand Activity ...........................................................................37

Gully Annealing by Aeolian Sand............................................................................................38

Section III - Landscape Change at Archeological Sites Receiving Sand Supply After Controlled 

Floods, Grand Canyon National Park ..........................................................................................40

Background..........................................................................................................................................40

Research Questions ...........................................................................................................................42

Methods................................................................................................................................................42

Topographic Change Detection...............................................................................................42

Meteorological Monitoring ......................................................................................................43

Time-Series Photography.........................................................................................................43

Results ..................................................................................................................................................43

AZ:C:05:0031................................................................................................................................43

Site Description.................................................................................................................43

Topographic Change (2010–13).......................................................................................45

Topographic Change (2013–14).......................................................................................45

AZ:C:13:0321................................................................................................................................48

Site Description.................................................................................................................48

Topographic Change (2010–13).......................................................................................50

Topographic Change (2013–14).......................................................................................51

AZ:B:10:0225................................................................................................................................53

Site Description.................................................................................................................53

Topographic Change (2010–13).......................................................................................55

Topographic Change (2013–14).......................................................................................55
iv

AZ:G:03:0072................................................................................................................................59

Site Description.................................................................................................................59

Topographic Change (2010–13).......................................................................................61

Topographic Change (2013–14).......................................................................................61

Discussion: Landscape Change and Response to Controlled Floods........................................64

Section IV - Landscape Change at Archeological Sites in a Sediment-Starved Reach: Glen 

Canyon..............................................................................................................................................66

Background..........................................................................................................................................66

Research Questions ...........................................................................................................................67

Methods................................................................................................................................................67

Topographic Change Detection...............................................................................................67

Geomorphologic Analysis ........................................................................................................67

Meteorological Monitoring ......................................................................................................67

Time-Series Photography.........................................................................................................68

Topographic Change Detection........................................................................................................68

AZ:C:02:0077................................................................................................................................68

Site Description.................................................................................................................68

Topographic Change (2012–13).......................................................................................70

AZ:C:02:0075................................................................................................................................72

Site Description.................................................................................................................72

Topographic Change (2012–13).......................................................................................73

AZ:C:02:0032................................................................................................................................75

Site Description.................................................................................................................75

Topographic Change (2012–13).......................................................................................76

AZ:C:02:0035................................................................................................................................78

Site Description.................................................................................................................78

Topographic Change (2012–13).......................................................................................80

Landscape Change in Glen Canyon.................................................................................................82

Climatologic Comparison of Glen and Marble–Grand Canyons .................................................84

Section V - Synthesis and Conclusions ...................................................................................................88

References Cited..........................................................................................................................................89

Appendix........................................................................................................................................................98
v

Figures

1. Map of the Colorado River corridor through Glen and Marble–Grand Canyon, Arizona........... 3

2. Photographs of fluvial sandbars, showing growth after controlled flooding.....................4

3. Photographs of aeolian dunes in the Colorado River corridor .............................................5

4. Photographs showing gully erosion at archeological sites in river-derived sand 

deposits ..........................................................................................................................................6

5. Aerial images showing geomorphic context of aeolian dune fields....................................9

6. Photographs showing gullies and tributary washes as aeolian sand traps.....................11

7. Photographs showing active versus inactive aeolian sand deposits ...............................13

8. Photographs showing geomorphic features used to infer wind direction .......................17

9. Schematic diagrams illustrating archeological-site classification system......................18

10. Results of archeological-site classification in four time intervals.....................................19

11. Map showing prevailing wind directions in the Colorado River corridor.........................20

12. Maps showing example of a site that transitioned from type 1 to type 3 .........................21

13. Maps showing example of a site transitioning from type 1 to type 2a ..............................22

14. Maps showing example of a site that transitioned from type 1 to type 2a to type 3.......23

15. Maps showing example of a site that transitioned from type 3 to type 2 .........................24

16. Graph of archeological-site types by river mile, in 1973......................................................25

17. Graph of archeological-site types by river mile, in 1984–85................................................26

18. Graph of archeological-site types by river mile, in 1996......................................................27

19. Graph of archeological-site types by river mile, in 2012–14................................................28

20. Gully-detection algorithm example..........................................................................................32

21. Graph of results of gully analyses by reach...........................................................................33

22. Graph of distribution and count of gully widths.....................................................................34

23. Graph of potential gully area and active sand ratio..............................................................34

24. Graph of potential gully area per mapped sand unit ............................................................34

25. Graph of gully termination count per unit area......................................................................35

26. Graph of gully annealing in historical photo record .............................................................36

27. Photographs showing gully annealing, 1984–2009................................................................36

28. Graph of grain-size analyses from Glen and Grand Canyons .............................................39

29. Map showing weather-station and archealogical-site locations ......................................41

30. Photograph of terrestrial lidar scanner..................................................................................42

31. Map showing survey-area at site AZ:C:05:0031.....................................................................44

32. Photograph showing geomorphic setting of site AZ:C:05:0031...........................................45

33. Maps showing topographic change at site AZ:C:05:0031....................................................46

34. Map showing survey area at site AZ:C:13:0321.....................................................................49

35. Photograph showing geomorphic setting of site AZ:C:13:0321...........................................50

36. Maps showing topographic change at site AZ:C:13:0321....................................................51

37. Map showing survey area at site AZ:B:10:0225.....................................................................54

38. Photograph showing geomorphic setting of site AZ:B:10:0225 ..........................................55

39. Maps and graph showing topographic change at site AZ:B:10:0225 ................................56

40. Map showing survey area at site AZ:G:03:0072.....................................................................60

41. Photograph showing geomorphic setting of site AZ:G:03:0072 ..........................................61

42. Maps showing topographic change at site AZ:G:03:0072US ..............................................62

43. Summary of landscape change at four sites in Marble–Grand Canyon ...........................65
vi

44. Map showing survey area at site AZ:C:02:0077.....................................................................69

45. Photograph showing geomorphic setting of site AZ:C:02:0077...........................................70

46. Map showing topographic change at site AZ:C:02:0077......................................................71

47. Map showing survey area at site AZ:C:02:0075.....................................................................72

48. Photograph showing geomorphic setting of site AZ:C:02:0075...........................................73

49. Map showing topographic change at site AZ:C:02:0075......................................................74

50. Map showing survey area at site AZ:C:02:0032.....................................................................75

51. Photograph showing geomorphic setting of site AZ:C:02:0032...........................................76

52. Map showing topographic change at site AZ:C:02:0032......................................................77

53. Map showing survey area at site AZ:C:02:0035.....................................................................79

54. Photograph showing geomorphic setting of site AZ:C:02:0035...........................................80

55. Map showing topographic change at site AZ:C:02:0035......................................................81

56. Summary of landscape change at four sites in Glen Canyon .............................................82

57. Graphs showing site slope and watershed-area comparison............................................83

58. Graph of spatial distribution of rainfall, 2007–2010 ...............................................................84

59. Graphs of temporal summaries of rainfall, 1952–2012 and 2013–2014...............................85

60. Graphs of rain intensity, 1958–2012 .........................................................................................86

61. Graphs showing wind-speed measurements........................................................................87

Tables

1. Time intervals used in archeological-site-classification analysis and Colorado River 

high flows preceding each interval .........................................................................................15

2. Summary of topographic change at site AZ:C:05:0031.........................................................48

3. Summary of topographic change at site AZ:C:13:0321.........................................................53

4. Summary of topographic change at site AZ:B:10:0225.........................................................58

5. Summary of topographic change at site AZ:G:03:0072US ...................................................64

6. Summary of topographic change at site AZ:C:02:0077.........................................................70

7. Summary of topographic change at site AZ:C:02:0075.........................................................74

8. Summary of topographic change at site AZ:C:02:0032.........................................................78

9. Summary of topographic change at site AZ:C:02:0035.........................................................81
vii

Conversion Factors

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

meter (m) 1.094 yard (yd) 

Area

square meter (m2

) 0.0002471 acre 

square kilometer (km2

) 247.1 Acre

square meter (m2

) 10.76 square foot (ft2) 

square kilometer (km2

) 0.3861 square mile (mi2)

Volume

liter (L) 33.82 ounce, fluid (fl. oz)

liter (L) 2.113 pint (pt)

liter (L) 1.057 quart (qt)

liter (L) 0.2642 gallon (gal)

cubic meter (m3

) 264.2 gallon (gal) 

cubic meter (m3

) 0.0002642 million gallons (Mgal) 

cubic centimeter (cm3

) 0.06102 cubic inch (in3) 

cubic decimeter (dm3

) 61.02 cubic inch (in3) 

liter (L) 61.02 cubic inch (in3

) 

cubic meter (m3

) 35.31 cubic foot (ft3)

cubic meter (m3

) 1.308 cubic yard (yd3) 

cubic meter (m3

) 0.0008107 acre-foot (acre-ft) 

Flow rate

meter per second (m/s) 3.281 foot per second (ft/s) 

meter per minute (m/min) 3.281 foot per minute (ft/min) 

meter per hour (m/hr) 3.281 foot per hour (ft/hr)

meter per day (m/d) 3.281 foot per day (ft/d)

meter per year (m/yr) 3.281 foot per year ft/yr) 

cubic meter per second (m3

/s) 35.31 cubic foot per second (ft3/s)

 Acronyms

DEM digital elevation model

EmLCR Eminence–Little Colorado River reach

FF Furnace Flats reach

GCDAMP Glen Canyon Dam Adaptive Management Program

GCMRC Grand Canyon Monitoring and Research Center

GIS geographic information system

GLCA Glen Canyon reach

GP Granite Park reach

RM river mile (measured as distance downstream from Lees Ferry)

SCA Stevens–Conquistador Aisle reach

TIN triangulated irregular network

UGG Upper Granite Gorge reach
viii

Acknowledgments

This study was supported by funding from the Bureau of Reclamation through the U.S. 

Geological Survey (USGS) Grand Canyon Monitoring and Research Center, and was conducted 

in collaboration with the National Park Service (NPS; Grand Canyon National Park and Glen 

Canyon National Recreation Area). We thank Jennifer Dierker and Thann Baker (NPS), David 

Bedford (USGS), and Mary Barger (Bureau of Reclamation) for valuable discussions regarding 

landscape processes and archeological sites. The ideas of David Rubin, Jack Schmidt, Ted 

Melis, Ivo Lucchitta, Richard Hereford, and Bob Webb were instrumental to our conceptual 

understanding of landscape processes discussed herein. Aaron Borling, Joe Hazel, Keith Kohl, 

Mark Mastin, and Paul Rauss provided survey support. Carol Fritzinger coordinated field 

logistics; boatmen Carolyn Alvord, Don Bacco, Kirk Burnett, Seth Felder, Dave Foster, Mark 

Perkins, and Dom Zanzucchi contributed in numerous ways to the field operations and data 

collection. Jeanne DiLeo drafted two of the figures in this report. We thank Jack Schmidt and 

Paul Grams for their feedback and discussions of our work. Reviewers Kathleen Nicoll and 

Joel Pederson provided valuable comments that improved the manuscript. We thank Katherine 

Jacques and Jacqueline Olson for their conscientious editorial work.
ix

David R. Bedford

1974–2016

Dave Bedford, USGS research geologist, worked on this project for many years. He contributed 

scientific direction and consistently provided new insights into the processes that shape the landscape 

and archeological record of Grand Canyon. His enthusiasm for scientific inquiry, his thoughtfulness to 

assist others, and his warmhearted friendship will always be remembered.

Conditions and Processes Affecting Sand Resources at 

Archeological Sites in the Colorado River Corridor Below 

Glen Canyon Dam, Arizona

By Amy E. East, Brian D. Collins, Joel B. Sankey, Skye C. Corbett, Helen C. Fairley, and Joshua Caster 

Abstract

This study examined links among fluvial, aeolian, and 

hillslope geomorphic processes that affect archeological sites 

and surrounding landscapes in the Colorado River corridor 

downstream from Glen Canyon Dam, Arizona. We assessed the 

potential for Colorado River sediment to enhance the preservation 

of river-corridor archeological resources through aeolian sand 

deposition or mitigation of gully erosion. By identifying locally 

prevailing wind directions, locations of modern sandbars, and 

likely aeolian-transport barriers, we determined that relatively 

few archeological sites are now ideally situated to receive aeolian 

sand supply from sandbars deposited by recent controlled floods. 

Whereas three-fourths of the 358 river-corridor archeological 

sites we examined include Colorado River sediment as an integral 

component of their geomorphic context, only 32 sites currently 

appear to have a high degree of connectivity (coupled interactions) 

between modern fluvial sandbars and sand-dominated landscapes 

downwind. This represents a substantial decrease from past 

decades, as determined by aerial-photograph analysis. Thus, 

we infer that recent controlled floods have had a limited, and 

declining, influence on archeological-site preservation.

Within the study area, overland-flow (gully) erosion is 

less severe in sand landscapes with active aeolian sand than in 

landscapes that lack aeolian transport; gullies terminate more 

commonly in active sand (sand that is mobile by wind rather than 

stabilized by biologic soil crust). We infer that these characteristics 

largely result from aeolian sand transport being an effective gullylimiting and gully-annealing mechanism. Aeolian sand activity 

in the river corridor varies substantially as a function of reach 

morphology and dominant wind direction relative to the rivercorridor orientation, factors that control accommodation space for 

river-derived sand and the modern sand supply to aeolian dunes. 

These attributes, together with an inverse correlation between 

aeolian sand activity and gully occurrence, define varying degrees 

of net long-term gully-erosion risk for sediment deposits and 

associated archeological sites in different regions of the river 

corridor. Over most of the river corridor, including some of the 

archeologically richest regions, sand is too inactive with respect 

to aeolian transport to anneal gullies effectively. At eight selected 

archeological sites that we studied with high-resolution terrestrial 

lidar scans for more than a year, sand loss by overland flow (gully 

erosion) and aeolian deflation generally exceeded deposition, such 

that erosion dominated over most monitoring intervals—even at 

four sites with strong connectivity to modern sand supply.

The Glen Canyon reach of the river corridor appears 

especially vulnerable to gully erosion. Among the sites that we 

monitored in detail, erosion generally dominated over deposition 

to a greater degree at four Glen Canyon sites with no modern 

sand supply than at four Marble–Grand Canyon sites with aeolian 

sand supply from controlled-flood sandbars. Although gross 

annual-scale erosion rates were similar among the Glen Canyon 

sites and among the Marble–Grand Canyon sites, a relative lack 

of depositional processes led to greater net erosion at the Glen 

Canyon sites. Having found no differences in weather patterns 

to suggest greater erosive forcing in Glen Canyon, and no 

conclusively influential differences in the slope or watershed area 

contributing to gully formation, we attribute the greater erosion at 

the Glen Canyon sites to a combination of inherent geomorphic 

context (high terraces that do not receive modern sediment supply) 

and pronounced effects of postdam sediment-supply limitation.

We conclude that most of the river-corridor archeological 

sites are at elevated risk of net erosion under present dam 

operations. In the present flow regime, controlled floods do not 

simulate the magnitude or frequency of natural floods, and are 

not large enough to deposit sand at elevations that were flooded at 

annual to decadal intervals in predam time. For archeological sites 

that depend upon river-derived sand, we infer elevated erosion 

risk owing to a combination of reduced sand supply (both fluvial 

and aeolian) through (1) the lower-than-natural flood magnitude, 

frequency, and sediment supply of the controlled-flooding 

protocol; (2) reduction of open, dry sand area available for wind 

redistribution under current normal (nonflood) dam operations, 

which do not include flows as low as natural seasonal low flows 

and do include substantial daily flow fluctuations; and (3) impeded 

aeolian sand entrainment and transport owing to increased riparian 

vegetation growth in the absence of larger, more-frequent floods. 

If dam operations were to increase the supply of sand available 

for windblown transport—for example, through larger floods, 

sediment augmentation, or increased fluvial sandbar exposure by 

low flows—and also decrease riparian vegetation, the prevalence 

of active aeolian sand could increase over time, and the propensity 

for unmitigated gully erosion could decrease. Although the 

evolution of river-corridor landscapes and archeological sites has 
2 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

been altered fundamentally by the lack of large, sediment-rich 

floods (flows on the order of 5,000 m3/s), some combination of 

sediment-rich flows above 1,270 m3/s, seasonal flows below 

226 m3/s, and riparian-vegetation removal might increase the 

preservation potential for sand-dependent archeological resources 

in the Colorado River corridor.

Introduction and Background 

Landscape Context of Cultural Resources in the 

Colorado River Corridor

The Colorado River corridor through Glen, Marble, and 

Grand Canyons, Arizona (fig. 1), is an iconic landscape that 

has drawn human interest for more than 10,000 years. Set 

within the large bedrock canyon incised into the surrounding 

Colorado Plateau, the river corridor preserves a complex 

geomorphic and human history (Fairley, 2003). Climatic 

fluctuations within the canyon region and in the Colorado 

River headwaters varied the flow and sediment supply in 

the river and controlled other landscape processes in the 

canyon, leading to formation of a stratigraphic and cultural 

record that spans thousands of years (Fairley and others, 

1994; O’Connor and others, 1994; Hereford and others, 1993, 

1996; Fairley and Hereford, 2002; Anderson and Neff, 2011; 

Pederson and O’Brien, 2014). Cultural resources along the 

river corridor consist of the physical evidence of past human 

activities, as well as places and natural resources of traditional 

importance to Native American cultures (Fairley, 2005). Many 

remnants of past human activity, including archeological 

sites and historical structures, are situated within or on top 

of fine-grained sediment deposits derived from the Colorado 

River1

. These historically and culturally significant places 

are susceptible to degradation and erosive damage over time, 

owing to natural weathering and erosion and to modern 

anthropogenic impacts (Balsom and others, 2005; Fairley, 

2005). The latter can occur as a direct effect of visitor use and, 

as we investigate in this report, also can include changes to 

the river-corridor landscape as a result of Glen Canyon Dam 

operations.

The Colorado River basin drains 637,000 km2

 in an arid 

to semi-arid (dryland) region where large dams provide water 

storage. Glen Canyon Dam, 216 m tall, impounds Lake Powell, 

the second-largest reservoir in the United States (fig. 1). After 

the completion of Glen Canyon Dam in 1963, the hydrology and 

sediment supply downstream from the dam changed substantially 

(Topping and others, 2000, 2003; Rubin and others, 2002; 

Wright and others, 2005), as is common in dam-controlled rivers 

1

Although fine-grained sediment includes sand, silt, and clay particle sizes, the 

fluvial and aeolian deposits relevant to this study are dominated by sand. Therefore, 

this report generally uses terms such as sand supply and sand deposits except when 

referring specifically to sediment with abundant silt and clay.

(Williams and Wolman, 1984; Chien, 1985; Collier and others, 

1996; Schmidt and Wilcock, 2008). Glen Canyon Dam has 

reduced the fluvial sediment supply to upper Marble Canyon by 

more than 90 percent, with all sediment inputs now supplied by 

tributaries downstream from the dam (Topping and others, 2000; 

Wright and others, 2005).

Dam-released flows generally exceed lower seasonal 

predam flows that previously induced sand accumulation in 

the river channel, and dam releases do not include the natural 

floods that occurred regularly before dam closure (Topping 

and others, 2003; Magirl and others, 2008; Wright and others, 

2008). Owing to the loss of sediment supply, the reduced 

magnitude and frequency of floods, and to riparian vegetation 

growth in the absence of large floods, there has been a 

systemwide decrease in the size and number of subaerially 

exposed fluvial sand deposits since the 1960s (Turner and 

Karpiscak, 1980; Beus and others, 1985; Schmidt and Graf, 

1987, 1990; Johnson, 1991; Kearsley and others, 1994; Hazel 

and others, 2010). Sandbar decline has been punctuated by 

episodic aggradation of some bars during occasional higher 

flows such as those that occurred in 1983–1985, in controlled 

floods released in 1996, 2004, 2008, 2012, 2013, and 2014, 

and by sediment input from tributary floods (Lucchitta and 

Leopold, 1999; Hazel and others, 2010; Topping and others, 

2010). Additional, lower-magnitude experimental floods in 

2000 and a natural Little Colorado River flood in winter 1993 

resulted in some sandbar aggradation as well (table 1; Melis 

and others, 2011).

Controlled floods, involving dam releases of approximately 

1,200 m3

/s over several days, can successfully increase sandbar 

area and volume in the Colorado River corridor (fig. 2), although 

sandbar response in any particular location is a complex result 

of sediment concentration and grain size in each flood (Webb 

and others, 1999; Topping and others, 2006, 2010; Wright and 

Kaplinski, 2011; Draut and Rubin, 2013; Grams and others, 

2013, 2015). However, the typical controlled-flood magnitude 

(maximum 1,270 m3/s) is approximately half of the predam mean 

annual flood peak (2,400 m3/s) and only one-fifth as large as the 

maximum historic predam flood (5,940 m3/s, in 1884; Topping 

and others, 2003). Paleoflood-deposit elevations suggest that the 

largest predam floods may have exceeded 8,400 m3

/s (O’Connor 

and others 1994; Topping and others, 2003; Greenbaum and 

others, 2014). Therefore, controlled floods released from Glen 

Canyon Dam do not simulate the magnitude or frequency 

of natural floods, and are not large enough to deposit sand at 

elevations that were flooded at annual to decadal intervals in 

predam time.

Wind reworks the sand in fluvial sandbars to form aeolian 

(windblown) sediment deposits in the Colorado River corridor, 

as in other dryland environments (for example, Gilbert, 1899; 

Lancaster, 1995; Han and others, 2007). Aeolian deposits are 

a common feature of the landscape (fig. 3), composed of sand 

supplied from predam and postdam fluvial deposits. As a result of 

reduced fluvial sand supply, windblown sand supply in the river 

corridor downstream from Glen Canyon Dam has been reduced, 

and, in many locations, eliminated (Draut and Rubin, 2008). The 
Introduction and Background 3

Lake Powell

Lake Mead

Colorado River

Havasu Creek

Diamond

Cr

Tapeats

Cr

Shinumo

Cr

Bright Angel Cr

Little Colorado River

Paria R

Kanab Creek

UTAH

NEVADA

ARIZONA

GRAND CANYONPARASHANT NM GRAND

CANYON

NP

ESCALANTE NM

KAIBAB

HAVASUPAI

NAVAJO 

HOPI HOPI

WUPATKI

NM

GLEN

CANYON

NRA

HUALAPAI

Glen Canyon Dam

Lees Ferry

LAKE MEAD

NRA

EXPLANATION

National Park land

Other NPS land

Tribal lands

River mile

Study reach and 

identifier

USGS stream gaging

station and No.

Note:

NP=National Park

NM=National Monument

NRA=National Recreational Area

RM

150

GP

GLCA

EmLCR

FF

UGG

SCA

GP

09402500

(RM 87)

09402500

RM

50

RM

100

RM

0

RM

150

RM

200

RM

250

B

San Juan R

Lake

Powell

Lake

Mead

Colorado River

Dirty Devil R

Escalante

Pa

R

ria R

L Colorado R

Green R

Phoenix

Las

Vegas

Cataract 

Canyon

Gila R

B

A

UTAH

NEVADA

ARIZONA

NEW

MEXICO

COLORADO

WYOMING IDAHO

CALIFORNIA

114° 113°30' 113° 112°30' 112° 111°30' 111°

115° 110° 105°

114°30'

37°

40°

35°

36°

30'

36°

35°

30'

Base map modified from National Atlas, National Hydrographic Dataset 

and other digital data, various scales. Coordinate reference system: 

NAD 1983 NSRS2007 StatePlane Arizona Central FIPS 0202. Horizontal 

datum is North American Datum of 1983. 

0 10 20 30 40 MILES

0 20 40 60 80 KILOMETERS

0 50 MILES

0 100 KILOMETERS

Figure 1. (A) The Colorado River watershed (gray shaded region). Major dams are indicated with black bars. Box outlines the area 

shown in (B). (B) The Colorado River corridor through Grand Canyon National Park, Ariz., and adjacent areas. River miles (RM) begin with 

RM 0 at Lees Ferry, 15 miles downstream from Glen Canyon Dam, and increase downstream. Glen Canyon includes the canyon region 

from Lake Powell downstream to Lees Ferry (RM 0). Marble Canyon extends from RM 0 to 61, and Grand Canyon from RM 61 to 277. 

Gray regions indicate river reaches studied in detail during this study: Glen Canyon (GLCA), Eminence to Little Colorado River (Em-LCR), 

Furnace Flats (FF), Upper Granite Gorge (UGG), Stevens-Conquistador Aisles (SCA), and Granite Park (GP).
4 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

Figure 3A

A B

C

E F

D

Figure 2. Growth of fluvial sandbars as a result of controlled floods. (A) Sandbar at RM 70 at a flow of approximately 226 m3/s, 11 

days before the March 2008 controlled flood of 1,250 m3

/s. (B) Same sandbar as in (A), one month after the March 2008 controlled 

flood, shown at a flow of approximately 226 m3

/s. Person standing on sandbar for scale in (B). Arrow indicates position and 

orientation of photograph in figure 3A. (C) Sandbar at RM 66 shown two days before the November 2004 controlled flood of 1,190 

m3

/s, shown at a flow of 226 m3/s. (D) Same sandbar as in (C), three weeks after the November 2004 controlled flood, at a flow of 

283 m3

/s. (E) Sandbar at RM 58 shown three days before the November 2004 flood, at a flow of 226 m3/s. (F) Same sandbar as in 

(E), three weeks after the November 2004 flood, at a flow of approximately 226 m3

/s. Person standing on far end of sandbar shows 

scale. For reference, arrows indicate rocks in common to photographs (E) and (F).
Introduction and Background 5

A B

C

E F

D

Figure 3. Examples of aeolian sedimentary deposits in the Colorado River corridor. (A) Active source-bordering dune approximately 

1 m high, migrating inland toward photographer’s right, that formed at the landward edge of the sandbar shown in fig. 2B. Photograph 

taken in July 2008, several months after the upwind sandbar had enlarged substantially as a result of the March 2008 controlled flood. (B) 

Active source-bordering dune at RM 190, with dune crest (dashed line) indicating dune migration toward the photographer’s right. This 

location is approximately 30 m downwind (inland) from the river margin. (C) Aeolian deposit (sand ramp) of light-colored Colorado River 

sand plastered against talus slopes well above the historic high-flow river stage. Prevailing wind direction at this location (measured at a 

weather station) is toward the photographer’s left, approximately perpendicular to the trend of the river corridor. The main sand source for 

these aeolian deposits is fluvial sandbars at right, just out of view. (D) Dunes with mature mesquite trees as part of the vegetation cover. 

Color of sand in dunes indicates wind-derivation from Colorado River fluvial deposits (in this case, predam flood deposits, as there is no 

modern fluvial sandbar upwind), whereas red sediment in interdune swale (foreground) indicates slopewash runoff derived from local 

bedrock sources. (E) Large, partly vegetated dune field at RM 70.6. (F) Aeolian dunes (sunlit) atop a small debris fan. Lightest-colored areas 

are active aeolian sand with little to no biologic crust cover.
6 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

loss of sand supply has altered the connectivity of aeolian, fluvial, 

and hillslope processes—that is, the interactions among wind, 

river, and hillslope sedimentary and ecosystem processes—with 

complex geomorphic and ecological effects (Draut, 2012).

The cultural resources of Glen, Marble, and Grand Canyons 

are commonly situated on and within fluvial and aeolian sand 

deposits derived from the Colorado River (fig. 4; Hereford and 

others, 1993, 1996, 2000a; Fairley and others, 1994; Fairley and 

Hereford, 2002; Draut and others, 2008; O’Brien and Pederson, 

2009a; Anderson and Neff, 2011). The stability of those sand 

deposits through time, and the condition of archeological material 

within them, largely depends upon the alternately erosive and 

depositional capacities of wind and water. Fluvial and aeolian 

sand deposits naturally erode over time as wind and rainfall runoff 

remove some sediment. Particularly as overland flow channelizes 

into gullies, rainfall runoff can erode sand and archeological sites 

substantially (fig. 4; Pederson and others, 2006; O’Brien and 

Pederson, 2009a,b; Collins and others, 2009, 2012; Pederson and 

O’Brien, 2014).

In a natural system, the erosive action of wind and water 

would be counteracted to some degree as river floods and 

subsequent wind action supplied new sand to the landscape. 

In an unregulated Colorado River, sediment-rich floods would 

regularly bring new sand into Glen, Marble, and Grand Canyons 

and deposit it on fluvial terraces, in eddies, and along the channel 

margins; wind would then redistribute much of that sand to 

Cultural

feature

Cultural

feature

Check

dams

Gully

wall

Bedrock

10 CM

30 CM 30 CM

10 CM

A B

C D

Figure 4. Gully erosion, resulting from overland flow, at river-corridor archeological sites. All photographs show gullies that incise sediment 

deposits composed of interbedded fluvial, colluvial, and debris-fan material, with fluvial terraces locally dominating the morphology. (A) Gully 

erosion exposes a cultural feature in Glen Canyon. (B) Gully incision through inactive aeolian sand dunes composed of river-derived sediment 

covered with dark biologic soil crust, in the Furnace Flats reach of eastern Grand Canyon. White line indicates gully flow path. Foreground shows 

red-colored sediment derived from local bedrock, presumably deposited by overland flow. (C) Gully incision through an archeological site in riverderived sand covered with biologic soil crust, Furnace Flats reach, May 2004. Rock checkdams (arrows) were built in the 1990s to mitigate gully 

erosion. (D) The same gully as in (C) in May 2013, with checkdams breached.
Introduction and Background 7

upland landscapes, replenishing sand supply to aeolian dunes (for 

example, Draut, 2012)2. Thus, wind and water both deposit and 

erode sand. The balance between supply (deposition) and erosion 

determines landscape evolution and cultural-site condition in a 

dryland river corridor.

The loss of sand supply to fluvial and aeolian deposits 

owing to the construction and subsequent operations of Glen 

Canyon Dam, and thus the reduced potential to counteract 

erosion, has consequences for the preservation of cultural 

sites. That dam operations may affect archeological-site 

preservation is of concern to the Glen Canyon Dam Adaptive 

Management Program (GCDAMP) stakeholders, Native 

American tribes, and Federal and State agencies that manage 

the river corridor and dam operations.

Beginning in the 1990s, concern arose that the predam sand 

that forms the setting of many cultural sites might have eroded 

more rapidly in the late 20th century than previously (Hereford 

and others, 1993), although little quantitative information was 

available to assess that possibility. Several factors were proposed 

to explain an apparent increase in erosion rates—increased rainfall 

intensity and magnitude during the late 1970s and early 1980s, 

reduced sand supply owing to Glen Canyon Dam operations, and 

impacts from visitation and recreational use of the river corridor 

(Hereford and others, 1993; Thompson and Potochnik, 2000; 

Fairley, 2005). Regarding the likely role of reduced sand supply, 

Hereford and others (1993) suggested that lack of sediment supply 

at the elevations of large predam floods had caused gully erosion 

to incise more deeply into predam flood deposits and to migrate 

farther headward (upslope), increasing the net material lost 

from rainfall runoff. Hereford and others (1993) postulated that 

whereas the predam river corridor contained many ephemerally 

flowing gullies graded to terrace surfaces formed and maintained 

by Colorado River floods, the postdam landscape included more 

gullies graded to the elevation of the regulated river, several 

meters lower than the predam flood terraces. In predam time, large 

sediment-rich floods filled large gullies with sand (McKee, 1938), 

as still occurs in the Colorado River upstream from Lake Powell 

(Thompson and Potochnik, 2000). Hereford and others (1993) 

proposed that because gully channels in postdam Marble–Grand 

Canyon are no longer infilled by sand during large Colorado River 

floods, and because gullies can incise to a lower base level now, 

gully erosion of predam sand deposits and associated cultural sites 

has increased in postdam time.

Based on qualitative field observations, researchers 

proposed that the reduced fluvial discharge and sand supply, 

as well as expansion of riparian vegetation in the regulated 

river had, in turn, reduced the aeolian sand supply to upland 

landscapes and archeological sites, further exacerbating their 

potential for erosion (Lucchitta, 1991; Hereford and others, 

1993; Neal and others, 2000; Thompson and Potochnik, 2000). 

The ability of aeolian sand to infill gullies and aggrade upland 

2

 We use upland to refer to areas above the recently active channel or flood 

zone. Presently, this implies regions above the 1,270 m3/s controlled-flood stage, 

but three decades ago the active channel was the area below the largest historical 

dam release stage of 2,740 m3/s, and in predam time what we now call upland 

areas were part of the active channel when inundated by much larger floods.

sediment deposits, thus counteracting erosion processes to 

some degree, was known from local documentation of gullies 

terminating in and filled by aeolian sand (Hereford and others, 

1993; Draut and Rubin, 2008) and from measurements of 

aeolian inflation (aggradation) at some cultural sites (Yeatts, 

1996; Collins and others, 2009). However, aeolian infilling 

processes had not been evaluated quantitatively over large 

areas. Draut (2012) found that characteristics of aeolian deposits 

differed significantly with the availability of modern upwind 

sand supply—differences that suggested varying susceptibility 

to gully development. The suite of observations on fluvial and 

aeolian upland landscapes from the early 1990s through 2012, 

together with direct measurements of landscape change at 

selected sites (discussed below), formed the basis for a more 

quantitative, larger-scale assessment within this study.

The links among fluvial and aeolian sand movement, 

hillslope runoff through upland landscapes, and dam 

operations that control sand abundance and transport are 

complex and difficult to quantify, especially in a remote 

setting. Despite such challenges, a better understanding 

of these interconnected processes is essential not only to 

understanding fundamental principles of dryland regions, 

but also to optimizing the GCDAMP management goal of 

preserving cultural resources. In accordance with the National 

Historic Preservation Act (Public Law 102-575, Sections 

106, 110) and the Grand Canyon Protection Act of 1992, the 

GCDAMP aims to mitigate the effects of dam operations 

on cultural resources, with the highest priority being to 

preserve those resources in place. Achieving this goal 

requires optimizing conditions that preserve the stability of 

upland sand deposits. One goal of dam operations, including 

controlled flooding, has been to increase the sandbar area 

that can supply windblown sand to upland aeolian deposits, 

potentially increasing the protective cover over archeological 

sites and reducing the net effects of erosional processes 

(Draut and others, 2010; Melis, 2011). To determine the 

effectiveness of such actions, it is necessary to determine 

more quantitatively the extent to which dam operations 

affect archeological sites, and to assess how effectively dam 

operations, without large floods, can supply sand to upland 

areas. Thus, we evaluated one aspect of a complex scientific 

and management problem—how to optimize naturaland cultural-resource conditions in the Colorado River 

ecosystem while meeting national needs for water supply and 

hydropower generation at Glen Canyon Dam.

This publication presents the findings of a two-year study 

under the GCDAMP biennial work plan (fiscal years 2013–14, 

project J) intended to refine understanding of the role of dam 

operations in affecting archeological-site conditions. The 

scale of our research ranged from high-resolution monitoring 

of selected individual archeological sites to landscape-wide 

field and remote-sensing analyses. Building on the studies 

mentioned above, this work addresses whether cultural sites 

in Glen, Marble, and Grand Canyon are eroding or changing 

faster or in a significantly different manner than they would if 

the dam were operated differently.
8 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

Aeolian-Fluvial-Hillslope Connectivity: 

Background and Previous Studies

To provide sufficient context for the findings of this study, we 

review some geomorphic processes that govern upland landscape 

evolution in the Colorado River corridor and summarize studies 

that led directly into the current work. Comprehensive treatment of 

recent geomorphic processes in the river corridor can be found in 

works such as Howard and Dolan (1981), Schmidt (1990), Melis 

and others (1995), Webb and others (2003), Magirl and others 

(2005), Yanites and others (2006), Grams and others (2007), Melis 

(2011), and Pederson (2012); we focus here on the fluvial, aeolian, 

and hillslope-runoff processes most relevant to sand transport 

around archeological sites.

Landscape evolution in dryland regions depends strongly on 

connectivity among fluvial, aeolian, and hillslope systems (Loope 

and others, 1995; Kocurek, 1998; Field and others, 2009; Telfer 

and others, 2014), although wind- and water-borne sediment 

transport are rarely studied together (Bullard and Livingstone, 

2002; Bullard and McTainsh, 2003; Belnap and others, 2011). 

In many settings, fluvial–aeolian sedimentary and geomorphic 

interactions affect not only the ways in which human communities 

occupy a landscape, but also the preservation of their archeological 

record (Holliday and others, 2007; Gibling and others, 2008; 

Martínez and Martínez, 2011; Roskin and others, 2014). To 

understand the suite of factors that control sediment supply to, 

and loss from, river-corridor landscapes and cultural sites of Glen, 

Marble, and Grand Canyons requires linking the erosive and sandreplenishing roles of wind and water.

Predam floods were the dominant source of sand in the 

upland (sand) deposits that contain archeological sites; these 

floods were larger than any in the postdam era. The importance 

of large Colorado River terrace deposits as the major substrate 

in upland landscapes is readily apparently within river-corridor 

stratigraphy (Hereford, 1996; Hereford and others, 1996, 1998, 

2000a; Burke and others, 2003; Draut and others, 2005, 2008; 

Anderson and Neff, 2011; Pederson and others, 2011; Neff 

and Anderson, 2012). Locally, flood-deposited sediment is 

interbedded with aeolian, debris-flow, and slopewash strata; 

flood deposits dominate the sedimentary record in major 

terraces that are relevant to many archeological-site locations 

(Hereford, 1996; Hereford and others, 2000a; Burke and others, 

2003; Draut and others, 2005). For clarity, we refer here to 

Colorado River flows and sediment as fluvial and to localized 

slope-runoff flows and sediment as hillslope or slopewash. 

Several river reaches contain well developed predam fluvial 

terraces meters thick and tens of meters wide that include 

dense concentrations of cultural sites (Fairley and others, 1994; 

Anderson and Neff, 2011; Pederson and O’Brien, 2014), for 

example much of Glen Canyon (River Mile [RM] -14 to -6), 

the so-called Furnace Flats reach of eastern Grand Canyon (RM 

66–72), and the Granite Park area (RM 208–210; fig. 1). The 

fluvial terraces presumably were formed by predam floods, but 

the paleomagnitude or recurrence interval of those floods is not 

known with certainty. The formation of those deposits probably 

was affected to some degree by changes in channel gradient, 

upstream sediment supply, and ponding upstream from recently 

active debris fans (Harvey and Pederson, 2011). In most areas 

that contain spatially extensive fluvial deposits and numerous 

archeological sites, the topography is such that today those 

terraces could only be overtopped by floods of 4,800 m3

/s and 

greater, as shown by discharge-elevation models (Magirl and 

others, 2008). A discharge of 4,800 m3/s was last attained in 

1921, and had an estimated return interval of 40 years in the 

predam Colorado River hydrology (Topping and others, 2003). 

Thus, over the past few centuries, the river-derived deposits 

forming the substrate and cover for many cultural sites resulted 

from large, relatively rare (decadal-scale) flood events combined 

with some influence of downstream debris-fan activity that 

altered the local hydraulic control.

Fluvial terraces are commonly overlain by, and interbedded 

with, aeolian sand that indicates wind reworking of surficial flood 

sediment between flood events (Hereford and others, 1993; Burke 

and others, 2003; Draut and others, 2008). The wind mobilizes 

fluvial sand and redeposits it adjacent to the river channel, 

forming dunes that are known as source-bordering dunes, in the 

terminology of Bullard and McTainsh (2003; see also Draut and 

others, 2010; Draut, 2012). Many such aeolian dune fields occur 

in the Colorado River corridor (fig. 3). Maximum elevations 

of aeolian sand vary according to local topography and wind 

conditions, and can be tens of meters above the base of any given 

aeolian deposit. Wind can reshape flood deposits substantially, 

turning their surfaces from near-flat terraces into aeolian dune 

fields (fig. 3). In some places, aeolian sand forms a thick mantle 

on talus slopes at the bases of bedrock canyon walls (sand ramps; 

fig. 3C). Aeolian sediment in this river corridor is derived almost 

entirely from Colorado River deposits of various ages, judging 

from the similar color and composition of aeolian and fluvial 

sand there; any contribution of local bedrock-derived sediment to 

the aeolian deposits is evidently overwhelmed by a much greater 

supply of buff-colored, mature, quartz-rich sand from Colorado 

River deposits (for example, fig. 3D; Hereford and others, 1998, 

2000a; Draut and others, 2005).

In addition to the large dune fields (103

–104 m2) that develop 

atop wide, predam flood terraces (fig. 5A), many smaller aeolian 

dune complexes occur in Marble and Grand Canyons overlying 

tributary debris fans (fig. 5B). These smaller aeolian deposits 

(commonly 101

–103 m2) formed as the wind reworked river flood 

sediment in predam, and in some places also postdam, separation 

and reattachment sandbars in eddies at the downstream side of 

debris fans (the fan-eddy complex of Schmidt, 1990; Schmidt and 

Rubin, 1995). Large, predam floods covered small debris fans 

almost entirely, often leaving deposits that blanketed large parts of 

the fans rather than merely forming separation and reattachment 

bars (Magirl and others, 2008), whereas the controlled floods 

today are not large enough to submerge most tributary debris fans. 

An estimated 1,000 or more fan-eddy complexes occur throughout 

Marble and Grand Canyons (based on approximately 350 having 

been identified in Marble Canyon alone; Hazel and others, 2006); 

most consist of some combination of fluvial and reworked aeolian 

sand overlying the debris fan. Archeological sites commonly occur 

in and on river-derived sand in those settings, though at lower site 
Introduction and Background 9

Colorado River

Colorado River

Wind

Wind

Aeolian dunes

Debris 

fan/colluvium Fluvial terrace

Fluvial terrace

Slopewash A

B

Debris-fan sediment

Colluvial slope

Aeolian dunes

 (well developed)

Aeolian sediment veneer 

on colluvial slope

Aeolian dunes (thin, low)

Fluvial

sandbar

0 250 FEET

0 100 METERS

0 250 FEET

0 100 METERS

Figure 5. Aerial images

showing geomorphic 

context of aeolian dune 

fields. (A) Example of 

aeolian dunes that 

developed on the surface 

of a predam fluvial terrace. 

Red slopewash sediment 

demarcates overland-flow 

channels at upper right. (B) 

Example of aeolian dunes 

atop a debris fan. The 

dunes, which receive sand 

supply from a sandbar 

that forms at the upwind 

side of the debris fan, are 

especially well developed 

in terms of dune height and 

continuous surface cover 

on that side of the fan, as 

is common in Marble–

Grand Canyon debris fans. 

Aeolian deposits on the 

downwind side of the fan 

contain less sand volume. 

A thin veneer of lightcolored aeolian sand also 

mantles the colluvial slope 

in the inland part of the 

debris fan.
10 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

densities than in the wider predam terraces (Fairley and others, 

1994; Pederson and O’Brien, 2014).

Wind action continually modifies aeolian sand areas, whether 

in large dune fields atop major flood terraces or smaller deposits 

overlying debris fans. In many places aeolian sand entrainment 

and transport are impeded by vegetation or biologic soil crust 

(for example, Buckley, 1987; Leys and Eldridge, 1998; Okin and 

others, 2006; Dong and others, 2011), but where the sand is open 

and bare, aeolian deposition aggrades (inflates) and aeolian erosion 

degrades (deflates) the land surface, and aeolian dunes migrate 

downwind over time. Spatial patterns of inflation and deflation, 

and the rates and orientations of dune migration, depend upon 

local wind dynamics and the availability of upwind sediment 

supply. Thus, land surfaces in aeolian dune fields undergo frequent 

natural changes in elevation and local sand-cover thickness, 

processes that can affect the stability of archeological sites (Draut 

and Rubin, 2008). The greatest potential for aeolian landscape 

modification occurs during spring, when dry, windy weather is 

common and sand-transport rates generally are greater than at 

other times of year (Draut and Rubin, 2005, 2006, 2008; Draut and 

others, 2009a,b; Dealy and others, 2014); however, aeolian sand 

transport can occur during any time of year. Severe sandstorms 

were mentioned frequently in the accounts of early Colorado River 

runners, including some in conjunction with incoming summer 

monsoonal storms. For example, an autumn 1938 journal entry 

by river-runner Buzz Holmstrom noted, “sky so full of sand sun 

blotted out” (Boyer and Webb, 2007).

The locations of aeolian dune fields relative to fluvial 

sandbars, pre- and postdam flood elevations, and wind patterns 

indicate whether substantial modern, postdam sand supply reaches 

them. Some aeolian dunes form directly inland from modern 

sandbars that were deposited by controlled floods; after floods 

recede, in places where the dominant wind direction brings sand 

inland from the river, new aeolian dunes form above the flood 

stage (for example, fig. 3A). Other aeolian landscapes in the 

canyons have no modern, controlled-flood sandbar upwind, and 

thus receive little to no aeolian supply from modern sandbars, but 

are relict features composed of wind-reworked predam sand. Draut 

(2012) termed these two groups modern-fluvial-sourced (MFS) 

and relict-fluvial-sourced (RFS) aeolian deposits, respectively. 

Glen Canyon Dam operations reduce sediment sources 

to aeolian landscapes not only by eliminating floods, but also 

by eliminating low flows (<100 m3

/s, 4 percent of the predam 

mean annual flood) that would have exposed additional fluvial 

sediment to wind, and creating conditions conducive to vegetation 

encroachment on channel-margin and sandbar deposits. Dam 

operations also involve daily flow fluctuations great enough that 

the lowest-elevation fluvial sand exposed typically does not have 

time to dry and be mobilized by wind before being submerged 

again by rising flows. However, the reduced aeolian sand supply 

resulting from lack of sustained low flows and from larger daily 

flow fluctuations likely affects upland sedimentary systems less 

than the loss of large floods and increased vegetation cover. The 

relatively greater importance of the loss of large floods is evident 

from the upland sedimentary record around archeological sites, 

where flood deposits form a more ubiquitous and fundamental 

geomorphic context than do aeolian deposits (Hereford and others, 

2000a; Draut and others, 2008; Anderson and Neff, 2011).

Upland sand deposits of the Colorado River corridor are 

modified not only by wind, but also by rainfall runoff. As in other 

dryland regions worldwide, sediment deposits erode as rainfall 

runoff focuses into overland flow that is channelized in gullies. 

Flow through gullies also can include contributions from sapping 

or subsurface flow (for example, Faulkner and others, 2004; 

Pederson and others, 2006; Tebebu and others, 2010; Svoray 

and others, 2012). Gully development is a complex function of 

rainfall intensity, infiltration capacity of the sedimentary substrate, 

and upslope catchment area. In sand deposits of the Colorado 

River corridor, gullies commonly form and evolve in response 

to overland flow from upslope areas mantled by relatively 

impermeable colluvium and bedrock, even if the rainfall intensity 

and infiltration capacity of the sand otherwise would not have 

induced gully formation (Collins and others, 2016). Gully erosion 

has damaged numerous archeological sites in Glen, Marble, and 

Grand Canyons (fig. 4; Hereford and others, 1993; Pederson and 

others, 2003, 2006; Balsom and others, 2005; Hazel and others, 

2008; O’Brien and Pederson, 2009b; Collins and others, 2009, 

2014, 2016). Although overland flow is the dominant process 

of archeological-site degradation in this setting, more gradual 

hillslope processes such as creep also cause substantial erosion 

over time (Pederson and O’Brien, 2014).

Many previous studies have addressed gully formation and 

enlargement, but relatively little is known about mechanisms 

that may anneal gullies—that is, processes that impede or 

counteract gully growth (Antevs, 1952; Patton and Schumm, 

1975; Ghimire and others, 2006; Le Roux and Sumner, 2012). 

In the Colorado River corridor, windblown sand has been 

observed to fill gullies on the order of 10 cm wide and 10 cm 

deep (fig. 6A, B). Topographic depressions capture aeolian 

sand—not only gullies, but also larger tributary channels 

oriented transverse to the dominant wind direction (fig. 6). 

Spatially limited stratigraphic examinations have not revealed 

clear examples of paleogullies filled with windblown sand 

(Hereford and others, 2000a; Draut and others, 2005; Anderson 

and Neff, 2011). However, the aeolian-sand-trapping tendency 

of channels, and examples in the literature of aeolian sand filling 

paleogullies (McIntosh and others, 2004; Xu and others, 2006; 

Mazaeva and others, 2011), suggest that aeolian sand may have 

annealed gullies in the Colorado River corridor in the past, and 

potentially could today, given sufficient sand supply.

The Role of Glen Canyon Dam Operations

Given the complex interactions among fluvial and 

aeolian sand transport and the erosive potential of wind 

and rain, it is not straightforward to identify how dam 

operations affect these processes or damage cultural sites. 

To assess the effects of dam-induced sand-supply limitation 

on upland landscapes, the most rigorous approach would be 

to quantify differences between predam and postdam upland 

landscapes—comparing site elevations, dune topography, 

ground cover, and degree of gully development in sand 
Introduction and Background 11

Wind

Wind

Wind

Cultural

feature

A B

C

E

D

30 CM

30 CM

Figure 6. Aeolian sand filling topographic depressions. (A) A recently formed gully (foreground) terminating in aeolian sand. (B) Aeolian sand, 

lighter-colored than its surroundings, partly fills a gully incised through biocrusted sand. Gully flow path was downslope toward the photographer. 

(C) Aeolian dune migrating into a gully, covering a cultural feature that had been exposed several years earlier by gully erosion. Dune migration 

direction is toward photographer’s right. (D) Aeolian dune migrating into and partly filling a tributary channel. Dune migration direction is toward 

photographer’s right. Dunes commonly migrate into a tributary wash but do not cross it entirely, presumably because tributary flow episodically 

removes dune sand. (E) Landscape-scale view of similar phenomenon as in (D), with aeolian dunes having migrated into a tributary channel. Note 

well developed dune complex on upwind side of tributary (right side of photograph) compared to little to no dune morphology on downwind (left) 

side of tributary; same location as in figure 5B.
12 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

deposits in the early 1960s and today. However, few of the 

necessary records exist for predam time periods. The first 

detailed geomorphic maps of the river corridor were made in 

the 1990s, for only a few select areas (Leschin and Schmidt, 

1995; Hereford, 1996; Hereford and others, 1993, 1998, 

2000a,b). Historical photographs provide some information 

about the size and location of fluvial sandbars, and the 

presence of upland gullies, in the 1960s and earlier (for 

example, Schmidt and Graf, 1990), but problems of exposure 

and resolution limit their utility for quantifying upland 

ground cover, dune-migration rates, or gully dimensions. 

However, even with few representations of predam upland 

geomorphic conditions, we can identify some links between 

dam operations and upland sand resources by analyzing 

upland conditions in the modern river corridor at landscape 

scale, and by measuring rates and processes of landscape 

change at site scale.

Statistically significant differences in ground cover 

(biologic soil crust and vegetation), open sand area, and 

aeolian sand-transport rates between upland sand deposits 

with and without modern aeolian sand supply (MFS and 

RFS landscapes) indicate that availability of modern sand 

supply plays a critical role in the geomorphic and ecosystem 

evolution of upland landscapes (Draut, 2012). Because 

upland areas no longer receive fluvial sand replenishment 

from floods—their primary sediment source in predam 

time—and because vegetation now covers most channelmargin and predam sandbar deposits, the only remaining 

upland sediment supply comes from aeolian sand transport 

from controlled-flood sandbars. Analyzing 27 upland sand 

areas of Marble–Grand Canyon, Draut (2012) found that 

landscapes with modern aeolian sand supply from controlledflood sandbars have less biologic soil crust, less vegetation, 

and much greater sand-transport activity than do upland 

areas without modern aeolian sand supply.

With fewer, smaller modern sources of aeolian 

(originally fluvial) sand under dam operations, many upland 

areas now apparently contain less active aeolian sand 

than is natural. Active sand is that which shows evidence 

for contemporary aeolian sand transport, including wind 

ripples and, locally, slipfaces at the angle of repose (fig. 7; 

Lancaster, 1994); in the Colorado River ecosystem, active 

aeolian sand typically contains less than 20 percent biologic 

crust cover (Draut, 2012). The proportion of active aeolian 

river-derived sand in lower Marble Canyon (RM 44–61) is 

substantially less than in the geomorphically and climatically 

similar Cataract Canyon reach of the Colorado River 

upstream from Lake Powell, where sediment-rich floods 

occur annually; this difference is inferred to result from 

dam-imposed flow and sand-supply limitation (Draut, 2012). 

The findings of Draut (2012) indicated that further work 

to quantify gully extent in active and inactive aeolian sand 

areas could indicate whether the unnaturally low proportion 

of active sand area downstream from Glen Canyon Dam 

promoted extensive gully development. The initial, spatially 

limited identification of areas with and without modern 

sand supply (MFS and RFS landscapes; Draut, 2012) also 

suggested that a more refined process was needed to classify 

archeological sites according to their relative potential to 

receive aeolian sand after controlled floods.

In view of the potential for rain, wind, and the river 

to alter landscapes associated with cultural sites, multiple 

research and monitoring efforts have attempted to quantify 

landscape change and its causes at selected archeological 

sites. The largest-scale erosion assessment to date has 

been that of Pederson and O’Brien (2014), who developed 

and analyzed a geospatial database of 227 river-corridor 

sites throughout Marble–Grand Canyon selected for their 

relevance to management concerns. Pederson and O’Brien 

(2014) found that gully erosion was the dominant cause 

of severe site degradation, with 184 study sites displaying 

evidence for overland flow and nearly half (112 sites) 

having overland flow as the primary erosive process. Creep, 

rainsplash, and bioturbation also degraded archeological 

sites over time, displacing artifacts downslope or otherwise 

altering their context. Diffusive and creep processes were 

evident at 165 sites, and were the primary degradation 

process at 68 sites (Pederson and O’Brien, 2014). Modern 

aeolian erosion (deflation or scour) and visitation impacts 

also affected sites, but to a lesser degree—aeolian processes 

had some geomorphic effect at 89 of the 227 sites—and 

sediment deposition by wind or water was rare, occurring at 

only 15 sites (Pederson and O’Brien, 2014).

Researchers have documented landscape change at 

high spatial resolution over seasonal to annual time scales 

at approximately 20 archeological sites, using topographic 

surveys, photogrammetry, and terrestrial-lidar measurements 

(Pederson and others, 2003, 2006; Collins and others, 2008, 

2009, 2012, 2014). These analyses complemented monitoring 

by the National Park Service (NPS) that has documented 

changes to cultural sites using direct observation, repeat 

oblique photography, and occasional topographic surveys 

of select gully segments—lower-resolution methods applied 

over greater spatial and temporal scales, encompassing 

several hundred sites over more than two decades. Together, 

these efforts have shown evidence for episodic erosion by 

rainfall-runoff gullying as well as wind deflation, and local 

occurrences of aeolian inflation, with erosion dominating 

(Collins and others, 2016).

Understanding the frequency and magnitude of 

landscape-altering weather events is an important aspect 

of analyzing the potential for dam operations to counteract 

erosive processes and slow the degradation of cultural 

resources. Measurements of landscape change, made 

in conjunction with high-resolution rainfall and wind 

measurements at 15 locations for intervals of 2–6 years each 

since 2003 (Draut and Rubin, 2005, 2006; Draut and others, 

2009a,b; Caster and others, 2014; Dealy and others, 2014), 

have provided valuable information about the magnitude and 

frequency of weather events that affect archeological sites, 

and on threshold conditions for landscape change (Collins 

and others, 2016).
Introduction and Background 13

A B

C

E F

D

Cultural site

Figure 7. Active and inactive aeolian sand deposits. (A) and (B) show examples of active aeolian sand, displaying wind ripples that indicate 

recent sand transport, and in (A), sharply defined dune crests that indicate active migration. Note abundant bare, open sand with sparse 

vegetation and little to no biologic soil crust. (C) and (D) show aeolian dune forms that are inactive, with well developed biologic crust cover 

indicating lack of recent aeolian sand movement. An archeological feature is visible in the foreground of (D). (E) Inactive aeolian dunes with 

surface cover that includes biologic soil crust and abundant vegetation (mostly exotic brome grass, in this case). (F) Landscape view common in 

Colorado River-corridor dune fields, in which there is both active and inactive (vegetated and biocrusted) dune area.
14 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

Project Objectives

The objectives of our study (project J under the 

GCDAMP biennial work plan) were to better define and 

quantify the effects, and potential effects, of Glen Canyon 

Dam operations on upland archeological sites and their 

surrounding river-corridor landscape. Building on the findings 

described above, this phase of research was based upon the 

question:

• Are archeological sites in the Colorado River corridor 

eroding or changing faster or in a significantly different 

manner than they would if Glen Canyon Dam were 

operated differently than it has been?

Within the context of addressing that broad question, we 

investigated landscape processes that included aeolian 

sand transport and overland-flow erosion. Given that some 

archeological sites are known to receive aeolian sand supply 

after controlled flood flows that enlarge fluvial sandbars 

directly upwind (Draut and others, 2010), in Section I we 

addressed the question:

• What number, and what proportion, of cultural sites in 

the Colorado River corridor potentially receive aeolian 

sand supply from controlled flood flows?

One important focus was to define the extent of gully 

formation and annealing processes in the river-corridor 

landscape. To investigate the effectiveness of aeolian 

sand activity as a gully-annealing process, we pursued the 

following additional questions in Section II:

• How does the relative abundance of active and inactive 

aeolian sand vary in different segments of the Colorado 

River corridor?

• How does the degree of gully incision differ in sand 

deposits that are active versus inactive with respect to 

aeolian sand transport?

• To what extent does aeolian sand transport counteract 

gully erosion in Marble–Grand Canyons?

We also evaluated site-scale rates and patterns of landscape 

change at several upland sites known to receive aeolian sand 

from controlled-flood sandbars. In Section III of this report, 

we asked:

• Does aeolian sand supply from controlled-flood sandbars to archeological sites cause enough deposition to 

offset erosion, and thereby protect the archeological 

resources?

• In areas with modern aeolian sand supply, and with 

land surfaces undergoing both gully erosion and active 

aeolian sand transport, is there net sediment loss and 

topographic lowering such that cultural resources are 

affected?

Finally, sites in Glen Canyon have been considered potentially 

more susceptible to erosion linked directly to dam operations 

and, indirectly, to the lack of modern sandbars providing 

sources of windblown sand. Therefore, in Section IV we 

addressed the questions:

• What rates and processes of landscape change occur 

at Glen Canyon sites, and how do they contrast with 

those measured in Marble–Grand Canyon, where the 

modern sand supply is greater?

• Are sites in Glen Canyon more vulnerable to long-term 

erosion than are sites in sediment-richer Marble–Grand 

Canyon?

Section I - Potential Aeolian Sand 

Supply to River-Corridor Archeological 

Sites in Grand Canyon National Park

Background

For decades, researchers have recognized that many 

archeological sites depend on Colorado River-derived sand 

as a substrate, as a cover, or both—either as fluvial deposits 

or as aeolian reworked sand that originated in fluvial deposits 

(hereafter we use river-derived sand to signify both). Prior to our 

study, there had been no systematic assessment of the modern 

potential for wind to supply sand to each site. We evaluated the 

potential for wind to supply sand from recently active fluvial sand 

deposits to river-corridor archeological sites in Marble–Grand 

Canyon (Grand Canyon National Park), between Lees Ferry and 

Separation Canyon (RM 0 to 239.8). Our study reach ended at 

the confluence of Separation Canyon and the Colorado River 

because downstream from that confluence, sedimentary deposits 

of the Lake Mead reservoir delta complicate interpretation of 

sediment-supply effects from Glen Canyon Dam operations. We 

evaluated the geomorphic context of archeological sites, local 

wind directions, the presence of upwind fluvial sandbars, and 

local topography and vegetation; we used those data to assess 

the potential for aeolian sand to reach each of 358 river-corridor 

cultural sites. Our evaluations were facilitated by collaboration 

with the river-corridor archaeology program at Grand Canyon 

National Park, which provided site-monitoring data and facilitated 

many site visits; this study complemented the O’Brien and 

Pederson (2009a,b) landscape-process study of 227 river-corridor 

archeological sites. A similar analysis for approximately 50 rivercorridor sites between Glen Canyon Dam and Lees Ferry, in Glen 

Canyon National Recreation Area, was completed in 2016.

Research Question

• What number and what proportion of archeological 

sites potentially receive aeolian sand supply from 

controlled-flood deposits?
Section I - Potential Aeolian Sand Supply to River-Corridor Archeological Sites in Grand Canyon National Park 15

Classification System and Methods

We evaluated the potential for wind to supply sand 

to cultural sites by using direct field observations and by 

examining historical aerial photographs. This analysis focused 

on four different time intervals (table 1): (1) 2012–14, during 

which time three controlled floods occurred that lasted 3 days 

each with discharge above 1,000 m3/s; (2) April 1996, several 

weeks after a 7-day controlled flood of 1,300 m3/s; (3) 1984–85, 

following a postdam flood with an instantaneous peak discharge 

of 2,740 m3/s in 1983 (maximum daily average discharge was 

2,610 m3/s on June 29, 1983), and flows above 1,000 m3/s 

continuously for several months each in 1983, 1984, and 1985 

(table 1); and (4) June 1973. High flows preceding the June 

1973 imagery included several years in which daily average 

discharge exceeded 1,000 m3/s for weeks to months, and several 

much larger predam peaks (table 1; Topping and others, 2003; 

U.S. Geological Survey, 2015). Thus, for the 2012–14 interval 

and for April 1996, we evaluated potential aeolian sand supply 

resulting from sandbar growth from specific controlled floods 

of similar magnitude, whereas for the two earlier time periods 

(that is, 1984–85 and 1973) we interpreted fluvial sand sources 

that formed from a variety of flows that exceeded the range of 

normal dam operations, rather than from an isolated controlledflood event.

For the 2012–14 (hereafter, modern) time period, we 

assessed the geomorphic effects of controlled floods by direct 

field observation shortly after the floods receded. Particularly 

for the November 2012 high flow, we evaluated geomorphic 

effects, including deposition of fluvial sandbars upwind from 

specific archeological sites, by using oblique photographs 

taken on river trips during flows of 226 m3

/s during the two 

weeks immediately after that flood, as well as from field 

observations made in May 2013 during two additional river 

trips on flows between 226 and 375 m3/s. Similarly, effects of 

the November 2013 controlled flood were evaluated by field 

observations of recent sand deposits in May 2014 during a 

river trip on flows between 181 and 308 m3/s.

We analyzed the effects of the 1996 controlled flood 

by inspecting archival photographs that were taken at a flow 

of 226 m3

/s approximately three weeks after the flood. We 

compared the presence of fluvial deposits in April 1996 after 

the flood to those visible in aerial photographs taken in March 

1996, just before the flood. Although our analyses of the 

effects of the 1996 and modern controlled floods were based 

on different data resources (archival aerial photographs versus 

oblique photography and field visits), the accuracy of both 

assessment methods is comparable, as both analyses primarily 

relied on photographic evidence taken within two weeks 

following each flood.

The 1983 flood (2,740 m3

/s) was the largest postdam flow 

release. Effects of this peak flow, and the months-long high 

flows in 1983 and 1984 (table 1), were evaluated by examining 

two sets of aerial photographs—taken in October 1984 and 

June 1985—for the presence of fluvial sandbars upwind from 

each archeological site. The October 1984 aerial photographs 

were taken at a discharge of approximately 140 m3

/s, and 

we used these photographs whenever possible. To analyze 

20 archeological sites for which the 1984 aerial images were 

missing from photographic archives (including where the 

original flight path missed a short section between RM 209.3 

and RM 209.9, and everywhere downstream from RM 214), we 

used aerial photographs taken in June 1985 during discharge 

of 850–1,020 m3/s. We did not distinguish among fluvial 

deposits formed during the peak discharge of the flood of 1983, 

deposits formed by the high flows of 1984 whose magnitude 

was nearly the same of later controlled floods, and other flows 

of approximately 850 m3/s, although Schmidt and others (2004) 

were able to distinguish among those deposits in some reaches. 

Rather than trying to specify which 1980s flows had contributed 

more substantially to sandbar presence or sand abundance, we 

focused on ascertaining whether upwind fluvial sand resources 

existed for each archeological site at that time. Similarly, for 

the 1973 time interval (for which we used photographs taken in 

June 1973 at discharge ranging from 170 to 368 m3/s), at most 

locations we did not attempt to identify which of the specific 

high-flow events in the preceding two decades had supplied 

fluvial sand—except in some cases where bright, apparently 

open sand deposits seemed to match the 3,540-m3/s shoreline 

modeled by Magirl and others (2008) that represented the 1958 

flood peak. Instead we simply assessed whether fluvial sand 

deposits existed in 1973 that could have supplied aeolian sand 

to archeological sites downwind. For each time interval, we 

assumed that a subaerially exposed, open, unvegetated fluvial 

sandbar potentially could serve as a source from which wind 

could transport sand.

Table 1. Time intervals used in archeological-site-classification 

analysis and Colorado River high flows preceding each interval. 

Data indicate daily average discharge measured at U.S. 

Geological Survey gaging station 09402500 (fig. 1).

Time interval 

analyzed

Recent preceding high flows

Discharge (m3

/s) Dates

2012–2014 1,080 November 12–14, 2014

1,060 November 13–15, 2013

1,270 November 20–22, 2012

1,250 March 6–8, 2008

1,190 November 22–24, 2004

910 September 6–8, 2000

900 May 4–6, 2000

1996 1,300 March 27–April 2, 1996

760–860 January and February 1993

1984–1985 1,000–1,320 May 18–June 28, 1985

1,000–1,260 May 7–July 18, 1984

1,000–2,610 June 4–August 11, 1983

1973 1,000–1,580 April 24–June 26, 1965

1,000–2,400 April 19–July 11, 1962*

1,000–1,300 June 6–26, 1960*

1,000–3,030 April 21–June 24, 1958*

1,000–3,510 May 8–August 11, 1957*
16 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

We classified each of the 358 river-corridor archeological 

sites for each of the four time intervals (2012–14 or modern, 

1996, 1984–85, and 1973) based on the degree to which 

each site potentially received windblown sand from adjacent, 

upwind active fluvial sand deposits. Our evaluation of each 

site included documenting (1) geomorphic and sedimentary 

context (whether fluvial, aeolian, or other, as assessed in this 

analysis and by any previous research at each site), (2) site 

elevation relative to inundation potential for historic flood 

flows (based on flood stages modeled by Magirl and others, 

2008), (3) dominant wind direction, (4) the presence of a 

recent fluvial sand deposit upwind of the site, assessed for 

each time step independently, and (5) the presence of any 

vegetation or topographic barriers between the fluvial deposit 

and downwind archeological site. These metrics were used to 

define five categories, or types, of archeological sites. Types 

1–4 are those whose geomorphic context includes riverderived sand as an integral component—fluvial, aeolian, or 

both. Type 5 sites are those at which river-derived sand is 

absent or, if present, is merely incidental to site context. The 

site-type definitions are as follows:

• Type 1: Sites with an adjacent, upwind, recent subaerial 

fluvial sand deposit, and where there are no substantial barriers to impede aeolian sand transport from the 

flood deposit toward the archeological site. 

• Type 2: Sites with an adjacent, upwind, recent subaerial 

fluvial sand deposit, but with a barrier separating the 

flood deposit from the archeological site. Barriers were 

interpreted to limit potential aeolian sand transport 

from the fluvial deposit toward the archeological 

site, but may not eliminate sand movement entirely 

from sandbar to archeological site. We defined three 

subtypes:

• Type 2a: Vegetation barrier present (may be riparian 

vegetation or higher-elevation, nonriparian upland 

vegetation).

• Type 2b: Topographic barrier present (most commonly a tributary channel, but in several cases a 

steep bedrock cliff or large boulder deposit).

• Type 2c: Both vegetation and topographic barriers 

present.

• Type 3: Sites at which an upwind shoreline exists for 

a recent high flow, but where the recent high flow 

resulted in no open, unvegetated sandbar along the 

river margin.

• Type 4: Sites at which there is no upwind shoreline corresponding to a recent high flow, but whose geomorphic context does involve river-derived sand.

• Type 5: Sites in the river corridor at which Colorado 

River-derived sand is absent or is only incidental to 

site context, such as sites entirely on bedrock or talus.

Where possible, we assessed locally dominant wind 

directions from direct measurements at weather stations 

operated in the river corridor by GCMRC (Draut and Rubin, 

2005, 2006, 2008; Draut and others, 2009a,b, 2010; Caster 

and others, 2014; Dealy and others, 2014). If no weather 

station operated near a particular area of interest, we inferred 

the locally prevailing wind direction from orientations of 

aeolian dune crests and slip faces, sand shadows in the lee 

of rocks and vegetation, and wind ripples (fig. 8). These 

features, if clearly visible in the field, reliably indicate the 

dominant directions of recent sand-transporting winds. Field 

observations of geomorphic wind indicators were made 

during the 2013 and 2014 river trips; we assumed that the 

locally dominant wind directions inferred in 2013–14 were 

the same in the three earlier time intervals we considered. 

This assumption is reasonable because dune crests, slipfaces, 

and large sand shadows are persistent features that tend to 

form over time scales longer than one or several wind events 

or seasons. Our observations at many of these sites over 

many years indicate that dune and sand-shadow orientations 

commonly persist over timeframes of at least a decade. 

Wind ripples, being much smaller features than dunes or 

sand shadows (fig. 8), are more transient and may be less 

representative of the long-term locally dominant wind 

direction. For this reason, we inferred wind direction from 

wind ripples only if neither well-defined dunes nor sand 

shadows were available. Similarly, if sand shadows of various 

sizes were present, we inferred wind direction from the 

largest available sand shadows, based on the assumption that 

those would reflect the dominant wind direction. However, 

at no sites did we observe large and small sand shadows that 

differed markedly in their orientations. This suggests that 

variable competing wind directions are not a major factor 

affecting river-corridor geomorphology in Marble–Grand 

Canyon, an assumption generally corroborated by weatherstation data.

Thus, we defined type 1 archeological sites as having 

the greatest potential to receive windblown sand supply from 

recently deposited fluvial sandbars, with the potential for 

aeolian sand supply decreasing in order from types 2–4. The 

presence of vegetation or topographic barriers at type 2 sites 

does not preclude aeolian sand from reaching a downwind 

site, but is interpreted to impede aeolian sand transport at 

least in part. We infer that vegetation or topographic barriers 

limit aeolian sand movement primarily by reducing the sand 

flux that moves by saltation or reptation (creep), transport 

mechanisms that rely on near-bed interactions among sand 

grains (for example, Nickling and McKenna Neuman, 2009, 

and references therein). Saltation and reptation can move 

substantial amounts of sand along a continuous sand bed 

(especially the coarser fraction of sand grains; Lancaster, 

1995; Cheng and others, 2015), but where the ground surface 

contains, for example, cobble-bedded tributary channels or 

dense vegetation, sand entrainment and transport by saltation 

and reptation will be more limited. Alternatively, aeolian sand 

can travel in suspension higher above the bed, potentially 
Section I - Potential Aeolian Sand Supply to River-Corridor Archeological Sites in Grand Canyon National Park 17

reaching sites that are downwind of vegetation or topographic 

barriers. Archeological sites are known to receive aeolian sand 

deposition in at least some cases from suspension transport. 

Aeolian sand deposition (inflation) on a type 2b site, separated 

by a tributary channel from the nearest upwind fluvial sandbar, 

has been documented previously during high-resolution lidar 

surveys in Marble Canyon (Collins and others, 2009). During 

strong wind events, we have also observed sand traveling 

in suspension from sandbars on one side of the river to the 

other (several hundred meters). Although aeolian transport 

in suspension across the river is a possible means of sand 

delivery to archeological sites, for the sake of assessing the 

most readily available sand sources we considered only 

sandbars directly adjacent to sites (on the same side of the 

river) and on the upwind side of the site with respect to the 

locally dominant wind direction.

Type 3 sites, which have upwind shorelines where no open 

sandbar formed during recent flooding, include sites that formerly 

may have been supplied by aeolian sand derived from eddy 

sandbars or channel-margin sand deposits. These former sand 

sources are now commonly covered and stabilized by postdam 

riparian vegetation (Sankey and others, 2015). Some new sand 

deposition may occur within the vegetation, but vegetation cover 

may not allow new deposition to be detected using aerial imagery. 

Places where riparian vegetation extends all the way to the river 

margin would be classified as type 3, whereas riparian vegetation 

Dune crest

A B

C D

Figure 8. Examples of geomorphic features used to infer wind direction. Black arrows show inferred local prevailing wind 

directions. (A) and (B) Sand shadows formed as sand accumulated on the downwind, lee side of an obstacle such as a rock or 

vegetation. Sand shadows point downwind. Transverse wind ripples (which form perpendicular to wind direction) corroborate the 

wind direction inferred from orientations of the large sand shadows in the lee of shrubs. (C) Dune crest and slipface orientation 

indicate dune migration from right to left; this dune is a source-bordering dune formed from a 2012 controlled-flood sandbar. (D) A 

more complex situation in which two sets of wind ripples are superimposed on the stoss (upwind) side of an active dune; compass

for scale. The superimposed bedforms indicate recent wind oriented slightly oblique to the dominant prevailing wind direction; the 

dominant direction was inferred from the orientation of the largest bedform present (the dune crest).
18 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

forming a border at the landward side of an open sand area, but 

with open sand remaining between vegetation and the river, would 

constitute a type 2a situation (fig. 9). Vegetation can alter fluvial 

depositional processes by increasing flow resistance, reducing 

water velocities and erosive power, increasing bank cohesion, and 

thus affecting fluvial sediment mobility, transport and deposition 

(Thorne, 1990; Nepf, 1999; Tal and Paola, 2010). The role that 

vegetation may play in impeding or enhancing future deposition is 

currently under study by others (Ralston and others, 2014; Mueller 

and others, 2015), and is not discussed further in this report. 

However, changes in vegetation conditions along shorelines that 

currently do not show evidence of having recent sand deposits 

could result in a reclassification of some type 3 sites in the future.

Type 4 sites do not receive sand transported by wind from 

recent flood deposits given the respective positions of flood 

deposits and locally dominant wind directions. Type 4 sites 

received river-derived sand (fluvial and aeolian) either from 

predam river flows (see Introduction) or from other, lowerelevation former source areas that are now covered by vegetation 

or are otherwise no longer evident in the modern postdam 

landscape. Many locations in the river corridor, including those 

with type 4 characteristics, would have received sand on or 

upwind of them only as a result of floods of 4,810 m3/s and 

larger (Magirl and others, 2008). This interpretation is consistent 

with predam flood deposits having been identified at type 4 sites 

for which detailed stratigraphic analyses exist (for example, 

O’Connor and others, 1994; Hereford and others, 2000a; Draut 

and others, 2008; Anderson and Neff, 2011; Pederson and others, 

2011; Museum of Northern Arizona, 2012); most type 4 sites 

also contain aeolian dunes. Therefore, although type 4 sites have 

River flow

TYPE 1

TYPE 2c

TYPE 2a

TYPE 2b

TYPE 3 TYPE 4 TYPE 5

Extent of 

pre-dam 

river-derived 

sand 

Archeological 

site

Recent 

sandbar

Tributary channel

Vegetation

barrier

Vegetation and 

topographic 

barriers

Topographic 

barrier 

(tributary 

channel)

No 

recent 

sandbar

Wind direction 

not from river 

toward site Site context 

unrelated to 

river-derived 

sand

Wind 

Figure 9. Schematic diagram showing 

plan view of each archeological-site type 

in the classification system; see text for full 

description. Type 1, sites with an adjacent, 

upwind, recent subaerial fluvial sand deposit 

and no substantial barriers to impede 

aeolian sand transport. Type 2, sites with an 

adjacent, upwind, recent subaerial fluvial 

sand deposit, but with a barrier separating 

the flood deposit from the archeological site. 

Type 2 sites are divided into three subtypes: 

Type 2a, vegetation barrier present; Type 2b, 

topographic barrier present; Type 2c, both 

vegetation and topographic barriers present. 

Type 3, sites at which an upwind shoreline 

exists for a recent high flow, but where there 

is no open, unvegetated sand along the 

upwind river margin. Type 4, sites at which 

there is no upwind shoreline corresponding 

to a recent high flow, but whose geomorphic 

context does involve river-derived sand. Type 

5, sites in the river corridor at which Colorado 

River-derived sand is absent or is only 

incidental to site context.
Section I - Potential Aeolian Sand Supply to River-Corridor Archeological Sites in Grand Canyon National Park 19

river-derived sand as an integral part of their context, they would 

not receive substantial aeolian sand from 1,200-m3/s controlledflood flows or, in many cases, even from deposits of larger 

postdam flows. Several type 4 sites do not appear to contain flood 

strata either, but appear to contain river-derived aeolian sediment 

as a cover despite not being downwind from any apparent flood 

shoreline nor near any substantial terrace deposit from which 

aeolian sand could have originated. In those cases (approximately 

3–5 of the 358 sites we analyzed), aeolian sand may have reached 

those sites during occasional wind events whose direction deviated 

from the usual prevailing wind orientation.

Type 5 sites are unaffected by aeolian sand supply. They are 

included in this report for the sake of inventory completeness, in 

order to determine what proportion of river-corridor archeological 

sites potentially received aeolian sand supply. 

Results

Fluvial sandbars and vegetation change substantially over 

time in Marble–Grand Canyon, thus the number and proportion of 

archeological sites in most of our defined categories changed over 

the intervals considered as well (fig. 10, appendix). The changing 

site classifications over time resulted from (1) changes in sandbar 

occurrence (whether fluvial sand deposits were present upwind of 

each site), (2) changes in vegetation—almost always growth, but 

in rare cases vegetation loss—and, less commonly, (3) tributary 

activity having changed local topography such that sandbar 

formation or aeolian sand movement (or both) was affected.

We determined that 266 of the 358 sites considered (74 

percent) have Colorado River sand as an integral part of their 

geomorphic context. Among the sand-dependent sites, 232 fall into 

8 6

4

6

63

8

7

7

7

1

2

4

1

2

2

9 1

2

2

2

1

1

12 1

12

18

18

22

23

20

2

1

5

5

1

3

1

1

1

1

6

6

3

12

3

9

4

4

16

10

10

10

13

1973 1984–85 1996 2012–14

+2 N/A +1 N/A

+1 N/A

+1 N/A

Number of archeological sites

50

100

150

200

250

300

350

Type 1

(32)

Type 2a

(44)

Type 2b

(35)

Type 2c

(29)

Type 3

(92)

Type 2a

(41)

Type 2b

(42)

Type 3

(85)

Type 2c

(18)

Type 1

(46)

Type 2a

(56)

Type 2b

(53)

Type 3

(25)

Type 2c

(16)

Type 1

(83)

Type 2c

(17)

Type 1

(99)

Type 2b

(54)

Type 3

(27)

Type 2a

(41)

Type 4

(34)

Type 5

(92)

Type 4

(34)

Type 5

(92)

Type 4

(32)

Type 5

(92)

Type 4

(26)

Type 5

(92)

1

Types 1-4: river-derived sand integral to site context (fluvial or aeolian)

EXPLANATION

1 = Flood sediment deposition adjacent and upwind

2a = Vegetation barrier

2b = Topographic barrier

2c = Both vegetation and topographic barriers

3 = Flood shoreline upwind, but no deposition

4 = No flood shoreline upwind

5 = River-derived sand absent or only incidental to site context

Number of archeological sites

Figure 10. Results of archeological-site classification for

each of four time intervals: 1973, the mid-1980s (1984 or 1985, 

depending on the availability of photographs), April 1996, and 

2012–2014. Colored columns indicate number of sites, out of 

358 sites analyzed, in each of the 7 categories defined by this 

study. Types 1–4 refer to sites in which river-derived sand 

forms an integral part of the geomorphic context. At Type 

5 sites, river-derived sand is either absent or, if present, is 

merely incidental to site context. Arrows indicate sites that 

transitioned from one category to another between time steps; 

arrow width is proportional to number of sites that underwent 

transition. N/A, sites potentially inundated by recent flood 

flows, but with aerial view of possible deposition zone not 

visible due to overhanging bedrock. See text for details.
20 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

types 1–3 (65 percent of the sites, or 87 percent of sand-dependent 

sites)—those that are downwind of controlled-flood shorelines (of 

1,200 m3/s flows). The remaining 34 sand-dependent sites met the 

criteria for type 4, having locally dominant wind directions that 

would not supply sand to them from river shorelines reached by 

modern dam operations, even shorelines of 1,200-m3/s controlled 

floods. Our documentation of spatial variations in local wind 

directions showed that although many regions of the canyon have 

persistent wind directions (most commonly toward upstream), 

wind direction can vary greatly around the mouths of large 

tributaries (fig. 11), with associated spatial variations in likely 

sand-transport paths.

The number and proportion of type 1 sites—those with 

the greatest potential to receive aeolian sand supply—decreased 

substantially over each time step (fig. 10). Whereas 98 cultural 

sites met the criteria for type 1 conditions in 1973, in the modern 

(2012–14) time step there were only 32 type 1 sites. Therefore, 

the proportion of type 1 river-corridor sites has decreased from 

A

B

C

D

0

GLCA

Em-LCR

50

FF

UGG

SCA 100

150

GP

200

250

37°

36°

500 M

114° 113° 112°

200 M

200 M

EXPLANATION

0 10 20 MILES

0 10 20 KILOMETERS

Study reach and name SCA

Aeolian-bedform observations

Weather-station data

150 River mile

Figure 11. (A) Dominant wind directions inferred in Glen, Marble, and Grand Canyons from weather-station measurements (gray arrows) or 

geomorphic features (pink arrows). River miles (RM), measured downstream from Lees Ferry, in white text. Throughout most of the canyon a 

wind direction oriented toward upstream prevails, although with local variations around tributary confluences, particularly the large tributaries 

such as (B) Nankoweap and (C) Unkar, and also at (D) tight bends in canyon orientation. For clarity, additional observations that duplicated 

those shown in (A) were omitted, including approximately 40 locations between RM 65 and 71 and approximately 15 locations between RM 

170 and 225. This graphic represents the current, most comprehensive understanding of dominant wind directions through the canyon and 

although local variations are illustrated, additional local variations also occur on smaller scales. Other situations of local variability probably 

exist that have not been observed to date. Six study reaches are indicated where aeolian sand activity and gully prevalence were mapped in 

detail (Section II): Glen Canyon (GLCA), Eminence to Little Colorado River (Em-LCR), Furnace Flats (FF), Upper Granite Gorge (UGG), StevensConquistador Aisles (SCA), and Granite Park (GP).
Section I - Potential Aeolian Sand Supply to River-Corridor Archeological Sites in Grand Canyon National Park 21

27 percent in 1973 (37 percent of the 266 sites that depend on 

river-derived sand) to less than 9 percent today (12 percent of the 

sand-dependent sites). Moreover, the incidence of sites moving 

from some other category into type 1 decreased over each time 

step, from 9 sites that entered type 1 between 1973 and 1984–85 to 

only one site becoming type 1 in 1996, and none becoming type 1 

between 1996 and 2012–14.

The decrease in type 1 sites since 1973 occurred partly by 

shoreline positional changes, such that open fluvial sand deposits 

occurred farther inland from the river margin in the 1973 and 

1980s imagery, left by flows higher than those in the two later 

time steps (fig. 12). Positional changes of recent flood shorelines 

accounted for five sites having transitioned from type 1 to type 

4 between 1973 and 1984—as of 1973, bright, apparently open 

fluvial sand deposits were visible in and upwind of five sites near 

each other that likely were deposits of the 3,540-m3/s flood of 

1957 (they correspond well with the modeled shoreline; Magirl 

and others, 2008). By 1984, those sites were judged to be type 

4—given the local shoreline morphology and prevailing wind 

direction, the most recent high flows (including the 2,740-m3/s 

flood peak of 1983) had no shoreline upwind of those sites. 

Shoreline positional differences between the 1980s high water and 

the 1996 controlled flood also accounted for 12 type 1 sites having 

become type 2b sites between 1984–85 and 1996 (fig. 10), and for 

2 of the 8 sites that transitioned from type 1 to type 2a between 

1984–85 and 1996.

Vegetation growth caused many sites to transition from 

type 1 to type 2, or from type 2b to type 2c (fig. 10). Even at 

some sites still apparently able to receive aeolian sand supply 

from fluvial sandbars today, vegetation encroachment appears 

to have reduced the sand supply or appears likely to necessitate 

a change in classification soon (fig. 13). In the most recent time 

step, 1996 to 2012–14, when the similar magnitude of recent 

controlled floods meant that there were no positional differences 

in shoreline extent, the loss of type 1 sites was equally attributable 

to vegetation growth (six type 1 sites becoming type 2a) and to 

sandbar nondeposition (becoming type 3). Other sites transitioned 

from type 1 to 2a to 3, as first vegetation growth overtook 

previously open fluvial sandbars and then either shoreline erosion 

removed the vegetation-covered sand (fig. 14) or, if the shoreline 

position did not change, vegetation overgrew the remaining part 

of the sandbar (Sankey and others, 2015). We observed vegetation 

loss in several places between 1973 and 1984–85, evidently from 

removal or overprinting of vegetation by flood deposits from the 

2,740-m3

/s flow of 1983.

Wind

Flow

A

50 M

Aeolian dunes

B

C

Figure 12. Example of a site that transitioned from type 1 to type 3 (see text for site-type descriptions). (A) Aerial photograph from 

1984, showing fluvial sand deposits (asterisks) directly adjacent to and upwind of an aeolian dune field; river flow is approximately 140 

m3

/s. White line shows approximate limit of the 1983 flood (Magirl and others, 2008; table 1). An archeological site in the aeolian dune 

field was ranked as type 1 because there were open fluvial sand deposits adjacent to the dune field on its upwind side. (B) April 1996, 

at a flow of 226 m3

/s. Black line shows approximate extent of the shoreline from the March 1996 controlled flood (Magirl and others, 

2008; table 1). Although there was a small sandbar at the downstream end of the eddy, there were no recent fluvial sand deposits 

directly adjacent to and upwind of the dune field, indicating a type 3 ranking for 1996. (C) May 2013, at a flow of 226 m3

/s. Black line 

shows approximate extent of the recent controlled-flood shoreline; the lack of adjacent, upwind fluvial sand deposits indicated type 3 

classification. Note abundant riparian vegetation growth on the sand deposits of the 1980s.
22 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

Wind

A

Flow

Aeolian dunes

B

C

50 M

Figure 13. Example of a site transitioning from type 1 to type 2a because of riparian vegetation growth. (A) In June 1985 open, unvegetated 

fluvial sand deposits (blue asterisk) were directly upwind of, and adjacent to, a dune field, indicating type 1 conditions. River flow in (A) was 

between 850 and 1,020 m3

/s. (B) In June 1996, although fluvial sandbar area was less than in (A), the site(s) in the dune field still met criteria for 

type 1 classification; flow was 226 m3

/s. (C) As of May 2013, after substantial riparian vegetation growth in the preceding decade, the site barely 

qualified as type 1, with only a small expanse of fluvial sand (enlarged by the recent controlled flood) having nearly vegetation-free connectivity 

with the dune field; river flow in (C) was 226 m3

/s. This dune field contains archeological site AZ:G:03:0072, discussed in Section III.
Section I - Potential Aeolian Sand Supply to River-Corridor Archeological Sites in Grand Canyon National Park 23

Wind

Driftwood Flow

50 M

Aeolian dunes

A B

C D

C

Figure 14. Example of a site that transitioned from type 1 to type 2a to type 3. (A) In October 1984 several open, unvegetated fluvial 

sand deposits (asterisks) were present upwind of this dune field (which is bisected by a tributary channel), presumably left by high 

flows in 1983 and 1984, indicating type 1 conditions. The 1983 flood (table 1) probably also deposited the large driftwood pile given 

that its peak stage was directly above that (Magirl and others, 2008). River flow in (A) was approximately 140 m3

/s. (B) In April 1996, 

just after a controlled flood, a sandbar filled most of the eddy. Riparian vegetation (and the driftwood pile) separated the sandbar 

from downwind aeolian dunes, indicating type 2a conditions. Flow was 226 m3

/s. (C) By May 2002, vegetation had covered most of the 

area that was open sand in 1996; flow was 226 m3

/s. (D) By May 2013 (shown at a flow of 226 m3/s), the river had eroded the vegetated 

sandbar substantially. We observed no sandbar in that eddy after the 2012 or 2013 controlled floods, indicating type 3 conditions for the 

modern time step. 
24 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

In general, over the latter two time steps we found a substantial 

increase in the number and proportion of type 3 sites—those with 

no fluvial sand deposit upwind. Although some sites that were 

nondepositional (type 3) in 1996 did have upwind sandbars after 

more recent controlled floods (15 sites that had been type 3 in 1996 

became type 2 in 2012–14; fig. 10), there was still a net gain of 

type 3 sites over that time step. Type 3 sites included more than one 

fourth of all river-corridor archeological sites in the most recent time 

step (92 of 358), and more than one third of all sand-dependent sites 

(92 of 266 sites in types 1–4).

At several sites, tributary floods or debris flows influenced the 

local morphology enough to effect a transition from one category 

to another. Debris-flow deposition can change the morphology of a 

fan-eddy complex such that the eddy traps fluvial sand more readily; 

the clearest example of this in our dataset is shown in figure 15 (see 

also Yanites and others, 2006). We noted several other sites at which 

Flow

Driftwood

A B

C

50 M

Eddy

Sandbar

Figure 15. Example of a site that transitioned from type 

3 to type 2 because of changes in local topography owing 

to debris-fan activity. (A) In an aerial photograph taken in 

May 2002 (at river flow 226 m3

/s), shortly before a summer 

2002 monsoon storm caused a debris flow that enlarged 

this fan and the rapid associated with it, the eddy on the 

downstream side of this fan trapped little sand. No sandbar 

formed along this eddy shoreline as a result of the 1996 

controlled flood (type 3). (B) In May 2005 the eddy was more 

bowl-shaped than before the 2002 storm and apparently 

functioned as a better trap for fluvial sand. A sandbar 

formed along the eddy shoreline on the downstream side of 

the debris fan as a result of the November 2004 controlled 

flood. Flow in (B) was 226 m3

/s. (C) In May 2013 a controlledflood sandbar was visible along the eddy shoreline; flow 

was 226 m3

/s. Note riparian vegetation growth, especially 

between 2005 and 2013. 
Section I - Potential Aeolian Sand Supply to River-Corridor Archeological Sites in Grand Canyon National Park 25

tributaries incised through formerly continuous fluvial or aeolian 

sand deposits (such as one site that transitioned from type 1 to type 

2c between 1996 and 2012, fig. 10).

Site transitions from one category to another showed 

some noteworthy spatial patterns (figs. 16–19). The incidence 

of type 2a sites (those with vegetation barriers) has increased 

with time in the western canyon, particularly downstream 

from RM 190 (figs. 16–19). The aforementioned increase in 

type 3 occurrence is especially pronounced in the western 

canyon; 20 of the 22 sites that became type 3 in the most 

recent time step (1996 to 2012–14) were downstream from 

RM 175 (figs. 18, 19).

5

Types 1–4: river-derived sand integral to site 

context (fluvial or aeolian)

EXPLANATION

1 = Flood sediment deposition adjacent and upwind

2a = Vegetation barrier

2b = Topographic barrier

2c = Both vegetation and topographic barriers

3 = Flood shoreline upwind, but no deposition

4 = No flood shoreline upwind

5 = River-derived sand absent or only incidental to site context

Streamflow, in cubic meters per second

5,940

1,270

226

River mile

0 50 100 150 200

Channel width, in meters

0

100

200

300

350

500

450

600

Site type

5

4

3

2c

2b

2a

1

Figure 16. Distribution of river-corridor archeological sites by site type as of 1973, with distance along the river 

corridor, in river miles downstream from Lees Ferry (fig. 1). Gray-shaded and black regions indicate width of river flow 

(Magirl and others, 2008) for discharge of 226 m3

/s, 1,270 m3/s (controlled-flood discharge), and the highest historical 

discharge, 5,947 m3

/s (in spring 1884). See text for full description of site types. Although the site-classification analysis 

continued to RM 239.8, the shoreline modeling by Magirl and others (2008) extended only to RM 226.
26 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

men16-3129_fig 17

Types 1–4: river-derived sand integral to site 

context (fluvial or aeolian)

EXPLANATION

1 = Flood sediment deposition adjacent and upwind

2a = Vegetation barrier

2b = Topographic barrier

2c = Both vegetation and topographic barriers

3 = Flood shoreline upwind, but no deposition

4 = No flood shoreline upwind

5 = River-derived sand absent or only incidental to site context

Streamflow, in cubic meters per second

5,940

1,270

226

River mile

0 50 100 150 200

Channel width, in meters

0

100

200

300

350

500

450

600

Site type

5

4

3

2c

2b

2a

1

Figure 17. Distribution of river-corridor archeological sites, by site type in 1984 (in 1985 for sites downstream from river-mile 

214), with distance along the river corridor, in river miles downstream from Lees Ferry (fig. 1). Gray-shaded and black regions 

indicate width of river flow (Magirl and others, 2008) for discharge of 226 m3

/s, 1,270 m3/s (controlled-flood discharge), and the 

highest historical discharge, 5,947 m3

/s (in spring 1884).
Section I - Potential Aeolian Sand Supply to River-Corridor Archeological Sites in Grand Canyon National Park 27

men16-3129_fig 18

Types 1–4: river-derived sand integral to site 

context (fluvial or aeolian)

EXPLANATION

1 = Flood sediment deposition adjacent and upwind

2a = Vegetation barrier

2b = Topographic barrier

2c = Both vegetation and topographic barriers

3 = Flood shoreline upwind, but no deposition

4 = No flood shoreline upwind

5 = River-derived sand absent or only incidental to site context

Streamflow, in cubic meters per second

5,940

1,270

226

River mile

0 50 100 150 200

Channel width, in meters

0

100

200

300

350

500

450

600

Site type

5

4

3

2c

2b

2a

1

Figure 18. Distribution of river-corridor archeological sites, by site type in 1996 (immediately after the spring 1996 

controlled flood), with distance along the river corridor, in river miles downstream from Lees Ferry (fig. 1). Gray-shaded 

and black regions indicate width of river flow (Magirl and others, 2008) for discharge of 226 m3

/s, 1,270 m3/s (controlledflood discharge), and the highest historical discharge, 5,947 m3

/s (in spring 1884).
28 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

Types 1–4: river-derived sand integral to site 

context (fluvial or aeolian)

EXPLANATION

1 = Flood sediment deposition adjacent and upwind

2a = Vegetation barrier

2b = Topographic barrier

2c = Both vegetation and topographic barriers

3 = Flood shoreline upwind, but no deposition

4 = No flood shoreline upwind

5 = River-derived sand absent or only incidental to site context

Streamflow, in cubic meters per second

5,940

1,270

226

River mile

0 50 100 150 200

Channel width, in meters

0

100

200

300

350

500

450

600

Site type

5

4

3

2c

2b

2a

1

Figure 19. Distribution of river-corridor archeological sites, by site type in 2012–14, evaluated from field visits, with 

distance along the river corridor, in river miles downstream from Lees Ferry (fig. 1). Gray-shaded and black regions 

indicate width of river flow (Magirl and others, 2008) for discharge of 226 m3

/s, 1,270 m3/s (controlled-flood discharge), 

and the highest historical discharge, 5,947 m3

/s (in spring 1884).
Section I - Potential Aeolian Sand Supply to River-Corridor Archeological Sites in Grand Canyon National Park 29

Discussion

We have used a classification system based on 

geomorphic and sedimentary context, local prevailing wind 

direction, fluvial sandbar occurrence, local vegetation, and 

topographic features to assess the relative potential for 

Colorado River sand supply to reach archaeological sites 

today and in the recent past. Our analysis shows a substantial 

decrease in sites with the greatest potential to receive sand 

supply, with the number and proportion of type 1 sites 

today being a third of what they were 40 years ago. This 

indicates that controlled flooding from Glen Canyon Dam, 

one goal of which is to increase sand supply to cultural sites, 

has had limited and decreasing influence on river-corridor 

archeological resources. Some of those limitations may be 

counteracted by deliberate vegetation removal in the future, a 

management action that potentially could reverse some of the 

transitions from type 1 to type 2a sites that we noted (or from 

type 2b to type 2c), or perhaps even transitions to type 3 that 

resulted from vegetation having completely overgrown sand 

deposits (Sankey and others, 2015; Mueller and others, 2015). 

The recent increase in type 3 incidence (fig. 19) is 

consistent with other observations of generally decreasing size 

and number of fluvial sandbars in Grand Canyon National 

Park (Kearsley and others, 1994; Hazel and others, 2006, 

2010), coupled with increased riparian vegetation (Turner 

and Karpiscak, 1980; Johnson, 1991; Webb and others, 2007; 

Sankey and others, 2015; Mueller and others, 2015). The 

abundance of type 3 sites may reflect the fact that although 

sand deposits tend to form in the same depositional zones 

repeatedly (most commonly in eddies; Schmidt, 1990; Hazel 

and others, 2006; Wright and Kaplinski, 2011; Grams and 

others, 2013), the exact response of any given eddy can vary 

from one flood to another. An eddy that contains a large 

subaerial sandbar after one controlled flood may have little or 

no subaerial sandbar after another flood of similar magnitude, 

depending on locally available fluvial sediment supply 

during each of the floods. Alternatively, the increase in type 

3 sites may reflect the recent finding that in some locations, 

vegetation encroachment has narrowed the active river channel 

such that densely vegetated surfaces are not even inundated by 

controlled flooding (Mueller and others, 2015).

It is possible that the larger number of type 3 sites 

inferred in the 2012–14 time step is in part an artifact of our 

analysis methods, which may have been affected by slight 

differences in river flow at the time of photography. We 

relied on oblique photographs and field visits to rank sites in 

the 2012–14 time step, instead of using aerial photographs as 

we had in the earlier time steps; aerial photographs following 

the 2012–14 controlled floods were not available at the time 

of our analysis. The oblique photographs and field visits 

occurred at flows ranging from 181 to 375 m3

/s and generally 

greater than 226 m3/s, thus the sandbars (or lack of sandbars) 

we detected using these data sources may have been less 

visible than those that would have appeared in aerial 

photographs, which, since the mid-1990s, typically are taken 

at a steady flow of 226 m3

/s. However, we do not believe that 

artifacts of analysis methods could explain our substantially 

greater inference of type 3 sites downstream from RM 175, 

as the daytime flows when we assessed sandbar occurrence 

in that part of the canyon were similar to those when we 

assessed sandbars farther upstream (by comparison of data 

from USGS stream gages at RM 0, RM 87, and RM 225). 

If, in the future, aerial photographs are taken at 226 m3/s 

immediately following a controlled flood (within 3–4 weeks, 

as was the case for the April 1996 aerial photographs), we 

suggest conducting a similar analysis of sandbar presence 

and archeological-site classification based on postflood 

aerial photographs, to assess whether differences in analysis 

methods contribute to the spatial trends such as those evident 

in figures 18 and 19.

Several important caveats and limitations accompany 

this classification analysis. First, having high potential for 

recent sand supply (a type 1 situation) does not necessarily 

correspond to aeolian landscapes becoming or remaining 

very active (in the sense of Lancaster, 1994; fig. 7A,B). 

Although landscapes with modern sand supply generally 

have more active aeolian sand than do landscapes without 

modern sand sources (Draut, 2012), some type 1 landscapes 

are more active than others with respect to aeolian transport 

and dune migration. Even at some type 1 sites, the area or 

proportion of active aeolian sand may be fairly low, or the 

exposed archeological features may be within relatively 

inactive sand amid an otherwise active aeolian landscape. 

We have not analyzed the abundance of active aeolian sand 

relative to archeological-site occurrence because any such 

exercise would be complicated by a site-detection bias. 

Active dune migration repeatedly covers and exposes certain 

areas, and active aeolian deposition (inflation) can keep 

artifacts covered indefinitely; in inactive settings without 

those processes, archeological sites become preferentially 

exposed. Some archeological sites likely remain undetected 

because they exist in active aeolian sand, whereas sites on 

inactive sand surfaces (fig. 7C–F) or exposed in vertical 

gully cutbanks (more common in inactive sand landscapes; 

Section II) are more likely to be discovered.

Second, the inference that new windblown sand supply 

can reach archeological sites does not necessarily mean that 

sand will be deposited and remain on a site. Our analysis 

inferred the potential for sand transport to the site and its 

surroundings, but actual deposition and volumetric increase 

(inflation of the ground surface) often cannot be determined 

by examining photographs, visiting the sites, and measuring 

wind directions. To ascertain whether aeolian sand has been 

deposited on a site and caused volumetric inflation requires 

either repeated high-resolution topographic surveys or 

detailed examination of subsurface stratigraphy at repeated 

intervals. This study did include high-resolution topographic 

surveys at four type 1 sites (Section III).

Third, we did not attempt to quantify how effective 

sand replenishment might be given the relative source area 

or volume of upwind fluvial sand compared to the sand area 
30 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

or volume in the landscape in and around archeological 

sites. Some of the upland aeolian landscapes have much 

greater areas and volumes than those of the modern 

sandbars upwind, surely limiting the ability of the sandbars 

to replenish those landscapes. Quantifying replenishment 

effectiveness would require detailed, repeated, highresolution topographic mapping of both fluvial deposits and 

downwind landscapes (such as from airborne lidar), which is 

beyond the scope of this assessment.

A related problem concerns variations in potential 

aeolian sand supply as a function of longevity of the flood 

deposits, which is largely determined by (1) how quickly 

the flood deposits erode during normal, postflood dam 

operations, (2) by the relative timing of floods, and (3) the 

timing of the typically dry, windy spring season when most 

sand transport occurs (Draut and Rubin, 2008). All of these 

factors affect the amount and rate of aeolian sand transport 

from flood deposits toward downwind landscapes and 

archeological sites.

As described, these limitations mean it is not possible 

to predict the rate at which sand will accumulate at a specific 

site (if it accumulates at all) or to predict the relative rates 

of accumulation at sites classified as type 1 or 2. Some of 

these outstanding questions will be investigated through the 

2015–17 GCDAMP triennial work plan.

This classification is intended to be an iterative process 

in which the site type for any of the type 1, 2, or 3 sites may 

be assigned differently after future flood flows. Although the 

cultural-site geomorphic context and locally dominant wind 

directions are presumably fairly constant, site classifications 

in any particular time step should not be considered a final 

determination. Type 4 sites would not be expected to change 

categories unless there was either a different evaluation of 

locally dominant wind direction or a substantial change in 

where a flood shoreline reaches topographically. Although 

wind directions are assumed to remain essentially constant 

through time, locally dominant wind directions can vary over 

small spatial scales, and so some sites may be reassigned 

to different classes if more precise information about local 

wind directions becomes available at a later date. A major 

shift in high-flow shorelines in response to topographic 

rearrangement after a tributary debris flow or rockfall could 

cause a type 4 site to convert to type 1, 2, or 3, but that had 

not occurred at any sites in the time intervals we considered. 

Type 5 sites are not expected to change categories.

How much sand supply reaches any of the 232 sanddependent archeological sites in Marble–Grand Canyon 

today depends upon local sandbar response to any particular 

flow regime—whether open, dry, subaerial sandbars exist 

upwind of the sites—and whether vegetation or topographic 

barriers impede aeolian sand transport. The following 

sections will address whether modern aeolian sand activity 

reduces gully erosion in archaeologically rich upland sand 

deposits, and how modern sand supply and its influence 

on landscape evolution affect preservation of selected 

archeological sites.

Section II - Gullies and Aeolian Sand 

Activity in the Geomorphic Context of 

the Colorado River Corridor

Background

The role of gully erosion in landscape evolution is not 

unique to the Colorado River corridor, but is important in arid 

and semi-arid regions worldwide (Wainwright and others, 2011; 

Turnbull and others, 2012; D’Odorico and others, 2013). The 

causes of gully formation and enlargement have long interested 

geomorphologists, both from a theoretical standpoint of drainage 

development and because soil erosion and landscape degradation 

associated with gullies are major dryland-management concerns 

(Bryan, 1925; Antevs, 1952; Patton and Schumm, 1975; Ghimire 

and others, 2006; Zhu, 2012). Although many studies have 

investigated gully initiation and enlargement, which occurs 

largely from rainfall-runoff events and can include subsurface 

flow contribution (Faulkner and others, 2004; Tebebu and 

others, 2010; Svoray and others, 2012), there has been little 

research into mechanisms that limit gully formation or by which 

gullies anneal. In the Colorado River corridor and elsewhere, 

researchers have recognized that aeolian sand can fill gully 

channels (fig. 6) and have suggested that aeolian sand activity 

could counteract landscape erosion and cultural-site degradation. 

Gully development involves a competition between factors 

that cause incision, and those that limit erosion or that promote 

annealing or infilling after erosion has occurred. A more thorough 

understanding of this balance between gully incision and 

annealing today in Glen, Marble, and Grand Canyons is essential 

for evaluating the effects of dam operations on the landscape and 

cultural resources. In this section and in a companion publication 

(Sankey and Draut, 2014), we report on the first systematic, 

landscape-scale assessment of how extensively aeolian sand may 

mitigate gully erosion. Through this field and remote-sensing 

study we assessed the broader landscape context for geomorphic 

processes that we evaluate at the site scale in Sections III and IV.

Cultural sites in the river corridor, and particularly the 

sand-dependent sites (types 1–4; Section I), are concentrated 

in the widest reaches of the canyon, such as the Furnace Flats 

area (RM 66–73; fig. 19) and in some areas of the western 

canyon. In those widest reaches, the bedrock lithology and 

fault configuration form a particularly broad river corridor 

with ample accommodation space in which to store large 

fluvial terraces (see also Pederson and O’Brien, 2014). Those 

spatially extensive sediment deposits were conducive to 

prehistoric human occupation, providing substrate for farming 

and habitation sites (Fairley, 2003, 2005; Anderson and Neff, 

2011). Multiple access routes there, typically along the faults, 

further facilitated human use of the inner canyon (Fairley and 

others, 1994). The large, relict, predam sand deposits (fluvial 

and reworked aeolian sand) in those wide reaches are now 

largely decoupled from modern aeolian sand supply, with vast 
Section II - Gullies and Aeolian Sand Activity in the Geomorphic Context of the Colorado River Corridor 31

areas not situated downwind from modern, dam-influenced 

sandbars (RFS landscapes; Draut, 2012); much of their area 

is inactive with respect to aeolian transport, and also contains 

large gullies. Pederson and O’Brien (2014) confirmed that the 

Furnace Flats and western canyon areas, with their extensive 

Holocene fluvial terraces, contain a disproportionate number 

of archeological sites with acute gully erosion; their study 

did not include sites in Glen Canyon, where there are also 

extensive fluvial terraces with severely gullied sites. These 

findings—that the reaches with the richest archeological 

record seem most vulnerable to erosion—indicated a need 

to understand more fully the links between aeolian sand 

activity and gully prevalence, and to quantify the spatial 

distribution of both. Thus, we assessed how aeolian sand and 

gully occurrence vary spatially in canyon reaches of different 

widths. We also examined some of the links between aeolian 

and hillslope processes that are evident in most upland sand 

landscapes within the canyon, as gullies and aeolian sand 

activity often occur in spatial proximity. We evaluated whether 

gully prevalence is measurably greater in inactive sand than 

in active aeolian sand, and thus how gully erosion became so 

severe in those reaches of the canyon where the archeological 

record is most concentrated.

Research Questions

• How does the relative abundance of active and inactive 

aeolian sand vary in different segments of the Colorado 

River corridor?

• Hypothesis 1: The proportion of active aeolian sand 

will be inversely related to river-corridor width, 

with a lower proportion of active sand in wide 

reaches (where accommodation space for sediment 

is greater) and a greater proportion in narrower 

reaches.

• How effective is aeolian sand activity as a gullyannealing mechanism—does gully prevalence differ 

measurably in sand deposits that are active versus 

inactive with respect to aeolian transport?

• Hypothesis 2: Gullies are more evident in sand 

deposits that are inactive with respect to aeolian 

transport than in those with active aeolian transport.

• Hypothesis 3: Gullies terminate more commonly in 

active aeolian sand than in inactive sand.

Methods

We combined field and remote-sensing analyses to 

address the science questions and test the hypotheses above. 

This analysis focused on aeolian sand activity and gully 

prevalence in six reaches of the river corridor (fig. 1): Glen 

Canyon (GLCA, RM -13 to -6), Eminence to the Little 

Colorado River (EmLCR, RM 44–61), Furnace Flats (FF, RM 

66–72), Upper Granite Gorge (UGG, RM 87–99), StevensConquistador Aisle (SCA, RM 116–128), and Granite Park 

(GP, RM 207–210).

In each of the six study reaches, we used the area of 

Colorado River-derived sediment above the direct effect 

zone (inundation zone) of contemporary dam operations as a 

means to quantify accommodation space for predam sediment 

storage. This terrestrial area was identified as the area above 

stage-elevation 1,270 m3

/s (the maximum elevation of 

controlled-flood releases, determined by the stage-discharge 

model of Magirl and others, 2008) and below the transition of 

river-derived and slopewash sediment to bedrock or talus. The 

landward boundary of terrestrial area (the transition to bedrock 

or talus) was identified by visual interpretation and delineation 

in a geographic information system (GIS) of 22-cm-resolution 

multispectral imagery acquired in 2009 (Davis, 2012).

For each reach, we evaluated the relative extent of 

modern aeolian sand activity by mapping the surfaces of 

river-derived (fluvial and reworked aeolian) sand deposits in 

the terrestrial (upland) area as being either active or inactive 

with respect to aeolian sand transport, as defined by Lancaster 

(1994; see our fig. 7). Because this analysis was concerned 

only with evidence for contemporary aeolian sand transport at 

the ground surface, we did not differentiate between fluvial, 

aeolian, and mixed fluvial-aeolian origins for the various sand 

deposits. Each deposit was delineated on the 22-cm-resolution 

imagery during field visits in 2011, 2012, or 2013; these data 

were transferred into a GIS as polygons with defined active/

inactive attributes.

Hillslope flowpaths with concave across-slope shape 

with potential to channel overland flow (‘potential gullies’) 

were detected in each study reach using a novel combination 

of overland-flow accumulation and topographic modeling 

procedures commonly available in GIS and remote sensing 

software (ArcGIS, ENVI), and a 1-m-resolution digital 

elevation model (DEM) acquired in 2009 (Sankey and Draut, 

2014). The DEM was derived from a 1-m-resolution digital 

surface model (DSM) derived from automated digital aerial 

photogrammetry with vertical accuracy within 0.30 m (Davis, 

2012) and is the highest-resolution and most accurate digital 

topographic dataset available for all six study reaches. The first 

step of the potential-gully detection procedure entailed running 

an overland-flow accumulation model (ArcHydro Tools in 

ArcGIS) for the DEM to identify thalwegs of possible overland 

flow paths in terrestrial surfaces (Greenlee, 1987; Jenson and 

Domingue, 1988; Tarboton and others, 1991). The second step 

produced a plan convexity product and a root-mean-square 

error (RMSE)-roughness product for the DEM (see Sankey 

and Draut, 2014, for additional details). To detect gullies 

consistently, we applied thresholds to the flow-accumulation 

and plan-convexity raster products using heuristic values that 

we confirmed during field visits. We then used GIS to produce 

an intermediate dataset of individual polygons representing 

sections of hillslope flowpaths with concave across-slope 

shape (that is, potential gullies). We applied threshold values 
32 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

of the RMSE-roughness product to the intermediate dataset 

to reduce the misclassification of topographically smooth 

interdune swales as potential gullies, resulting in a final dataset 

of individual potential-gully polygons (fig. 20). We analyzed 

only those potential-gully polygons that overlapped with the 

previously defined terrestrial area for each reach, thus limiting 

this consideration of potential gullies to those formed in 

river-derived fine sediment rather than through coarse talus. 

The average width of each potential gully polygon in the final 

dataset was measured in GIS to determine the distribution of 

potential gully sizes and estimate detection limits. We estimated 

the relative ability of these methods to detect real gullies by 

using a set of gullies that we identified independently in the 

field and surveyed with stadia rod and total station within three 

archeological sites in the GLCA reach (sites AZ:C:02:0035, 

AZ:C:02:0032, and AZ:C:02:0075); the surveyed gullies 

included segments with sub-meter widths.

We analyzed the resulting data in GIS to test the hypotheses 

above. After determining the terrestrial area and scaling it by 

reach length for each of the six reaches, we identified the ratio 

of potential gully area to terrestrial (sediment) area (the gully 

area ratio) and the ratio of active aeolian sand area to the sum 

of active and inactive sand area (the active sand ratio). We used 

a simple linear regression to examine the relationship between 

gully area ratio and active sand ratio. Using a paired t-test, we 

determined the gully area within active and inactive sand units and 

compared these values among reaches. To assess whether gullies 

terminate more commonly in active sand than in inactive sand, we 

determined the gully count, ratio of count to sand area, and ratio of 

potential gully area to sand area of all potential gullies that entered 

Flow

Wind 

Wind

Flow

Flow

Wind

EXPLANATION

River-derived sand, inactive

Potential gullies

Terrestrial area

Tributary channel

Potential gullies that terminate 

in aeolian sand

River-derived sand, active

A

100 M

100 M

B

100 M

C

100 M

Figure 20. Examples from parts of three study reaches that show the aeolian sand and potential gullies detected with our remotesensing algorithm. (A) a part of the Furnace Flats (FF) reach; (B) a part of the Stevens-Conquistador Aisles (SCA) reach; (C) a part of 

the Granite Park (GP) reach. Reach locations are shown in figure 1. 
Section II - Gullies and Aeolian Sand Activity in the Geomorphic Context of the Colorado River Corridor 33

and terminated within river-derived sand units. Potential gullies 

were considered to terminate within sand units if they entered from 

outside (upslope) and dissipated before passing halfway through 

the unit. The rationale for considering dissipation that occurred 

before an individual gully travelled more than halfway through a 

sand unit was to use a criterion that was common among gullies 

and aeolian sediment units, and to provide spatially unambiguous 

evidence that a gully had entered a sand unit and also terminated 

within it. We compared gully termination in inactive and active 

sand among reaches using paired t-tests.

Finally, we examined the historical aerial photographic 

record for evidence of gullies that have, or have not, annealed 

over time and so are either still evident or are less so today than in 

the past. The entire set of potential gullies that terminate in active 

and inactive sand (detected in the 2009 imagery and DEM) were 

examined visually relative to aerial imagery from 2002 and 1984 

to determine whether any gullies were more pronounced in the 

earlier imagery and (or) exhibited obvious infilling with aeolian 

sand as of 2009. In some cases, vegetation encroachment into 

gullies made it difficult to determine whether aeolian infilling had 

occurred; we recorded these ambiguous situations separately from 

examples of infilling with aeolian sand. Examples of vegetation 

encroachment in conjunction with aeolian infilling were recorded 

separately from examples where infilling with aeolian sand clearly 

was independent of vegetation growth.

Results

Terrestrial area, our proxy for river-corridor width, and 

the proportion of active aeolian sand varied greatly among 

the six study reaches (fig. 21). When scaled by reach length, 

terrestrial area is greatest in the EmLCR, FF, and SCA reaches, 

intermediate in the GLCA and GP reaches, and least in the 

UGG reach (fig. 21A). The active aeolian sand ratio in the 

UGG and SCA reaches (36 and 37 percent, respectively) was 

approximately 2–3 times that in the EmLCR and FF reaches, 

and nearly an order of magnitude greater than in the GLCA and 

GP reaches (fig. 21B). These proportions of active sand reflect 

not only the canyon morphology, but also local prevailing wind 

directions that vary among the six reaches. In places where the 

local wind direction blows at a high angle to the river (including 

much of the SCA reach), readily transporting sand from modern 

fluvial sandbars to inland aeolian dunes on the downwind 

side of the river, the source-bordering dunes contain more 

active aeolian sand than those in other reaches where the wind 

direction is nearly parallel to the river (fig. 11). 

By comparing 14 gullies in the GLCA reach that we 

surveyed with rod and total station, as well as attempted to detect 

with our remote-sensing method, we determined that the algorithm 

was suitable for detecting some parts of real gullies, but not for 

delineating gully sections less than 1 m wide (Sankey and Draut, 

2014). The remote-sensing algorithm identified 90 percent of the 

surveyed gully thalweg lengths in site AZ:C:02:0075, 50 percent 

in site AZ:C:02:0035, and 40 percent in site AZ:C:02:0032. 

The lower detection accuracies at sites AZ:C:02:0035 and 

AZ:C:02:0032 illustrate the relative limitations of 1-m-resolution 

GLCA EmLCR FF UGG SCA GP

50,000

0

100,000

150,000

200,000 A

Reach

0.05

0.10

0.15

0.20

B

C

Active sand area ratio

0.4

0.3

0.2

0.1

0

0

Potential gully area ratio

Terrestrial-area width, in meters

Figure 21. Terrestrial area and gully prevalence, by reach, in order 

from upstream (left) toward downstream (right); reach locations are 

shown in figure 1. Glen Canyon (GLCA), Eminence to Little Colorado 

River (Em-LCR), Furnace Flats (FF), Upper Granite Gorge (UGG), 

Stevens-Conquistador Aisles (SCA), and Granite Park (GP). (A) 

Average terrestrial-area width mapped in each reach of the Colorado 

River-derived sediment between the contemporary active channel of 

stage-elevation 1,270 m3

/s and the upslope transition to bedrock or 

talus. The width shown was calculated by dividing the total terrestrial 

area in each reach by the reach length; thus, terrestrial-area width is 

a proxy for total river-corridor width that excludes the part of the river 

corridor below the 1,270 m3

/s stage. (B) Active aeolian sand area ratio, 

by reach. (C) Potential gully area ratio, by reach. 
34 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

DEM data for delineating the entire thalweg lengths of small 

(narrow, shallow) gullies and gully segments. In total, we detected 

11,010 potential gullies in all six reaches, but the total number 

is probably much greater, in particular for gullies that are a few 

meters wide or narrower. The widest gullies that we detected were 

30 m wide; most gullies were 1–4 m wide (fig. 22).

The inverse relationship of the potential gully area ratio 

(fig. 21C) to active sand area ratio (fig. 21B) was significant 

and strong, indicating lower gully prevalence in reaches 

with a greater proportion of active aeolian sand (p=0.02 in 

a Student’s t-test; fig. 23). We identified significantly more 

potential gully area within inactive sand units than in active 

sand units (paired t-test p=0.03; fig. 24).

We identified 358 potential gullies that terminated 

in mapped (active and inactive) sand units, as opposed to 

potential gullies that travelled through sand units and (or) 

ultimately joined other potential gullies or the mainstem 

river. The number of terminating gullies scaled by sand-unit 

area was significantly greater in active aeolian sand than in 

inactive sand (paired t-test p=0.02; fig. 25). The total area 

of terminating gullies scaled by sand area was moderately 

significantly greater for active aeolian sand than for inactive 

sand (paired t-test p=0.06). It is necessary to scale by sand-unit 

area to make these comparisons of gully prevalence, because 

there is substantially less active sand area than inactive sand 

area in each reach (fig. 21B).

Gully width, in meters

Number of gullies

1,600

1,400

1,200

1,000

800

600

400

200

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 29 30

Figure 22. Distribution of average widths for all potential gullies 

(n=11,010) delineated by the remote-sensing algorithm described 

in the text. 

men16-3129_fig 23

Proportion of active aeolian sand area

(active sand area / active + inactive sand area)

0.25

0.20

0.15

0.10

0.05

0

0 0.1 0.2 0.3 0.4

Gully area ratio (gully area / terrestrial area)

GLCA

GP FF

EmLCR

SCA

UGG

R2 = 0.80

p = 0.02

y = -0.21x + 0.16

Figure 23. Relation of potential gully area ratio and active sand 

area ratio by reach. Glen Canyon (GLCA), Eminence to Little 

Colorado River (Em-LCR), Furnace Flats (FF), Upper Granite Gorge 

(UGG), Stevens-Conquistador Aisles (SCA), and Granite Park (GP).

Gully area, scaled by sand area

0.02

0

0.04

0.06

0.08

0.10

Inactive aeolian sand

Active aeolian sand

EXPLANATION

GLCA EmLCR FF UGG SCA GP

Reach

Figure 24. Potential gully area per mapped sand unit (active or inactive 

sand with respect to aeolian transport), summarized by reach. Vertical 

axis shows the dimensionless ratio obtained by dividing gully area by 

the terrestrial area occupied by active or inactive sand. Comparison 

of potential gully area within active and inactive sand indicated 

significantly more potential gully area within inactive sand units than 

in active sand units (p=0.03). Glen Canyon (GLCA), Eminence to Little 

Colorado River (Em-LCR), Furnace Flats (FF), Upper Granite Gorge (UGG), 

Stevens-Conquistador Aisles (SCA), and Granite Park (GP).
Section II - Gullies and Aeolian Sand Activity in the Geomorphic Context of the Colorado River Corridor 35

Number of gullies

20

40

60

80

100

120

Gully count, scaled by sand area 

(number of gullies per square meter) 0.0002

0.0004

0.0006

0.0008

0.0010

0.01

0.02

0.03

0.04

0.05

Gully area, scaled by sand area

Gullies that terminate in 

inactive aeolian sand

Gullies that terminate in 

active aeolian sand

EXPLANATION

0

0

0

GLCA EmLCR FF UGG SCA GP

A

Reach

B

C

Figure 25. (A) The number of gullies, (B) number of 

gullies divided by sand area, and (C) area divided by sand 

area, for potential gullies that terminated in mapped sand 

units (active and inactive aeolian sand, respectively), 

summarized by reach. A total of 358 potential gullies were 

identified among all reaches that terminated in mapped 

(active and inactive) sand units. Simply comparing counts 

of terminating gullies in inactive versus active aeolian 

sand (A) indicated no significant difference (paired t-test 

p=0.11). However, there was more inactive than active 

aeolian sand in all reaches, and when the counts and 

areas of terminating gullies were normalized by the sand 

area (B and C, respectively), potential gullies evidently 

terminated more commonly in active aeolian sand (paired 

t-test p=0.02 and 0.06, respectively, for tests of results 

presented in B and C). Glen Canyon (GLCA), Eminence to 

Little Colorado River (Em-LCR), Furnace Flats (FF), Upper 

Granite Gorge (UGG), Stevens-Conquistador Aisles (SCA), 

and Granite Park (GP).

Comparison of historical imagery within our six study 

reaches from 1984 and 2002 with that from 2009 indicated 

that, as of 2009, a small proportion (1–3 percent, or as many 

as 11 gullies) of the gullies that terminated in aeolian sand 

showed clear indication of aeolian annealing (figs. 26, 27). 

An additional 1 percent (3 gullies) showed evidence of 

possible aeolian annealing in conjunction with vegetation 

encroachment (fig. 26). Approximately 2–10 percent of gullies 

showed evidence of vegetation encroachment within the 

terminus of the gully, but without clear evidence of aeolian 

infilling (fig. 26). The remaining fraction (approximately 85 

percent of all gullies that occurred in 1984 and 2002) did 

not show evidence of annealing, and were similarly or more 

eroded as of 2009—our qualitative analysis did not have the 

topographic resolution to distinguish with certainty between 

those two outcomes in most cases.
36 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

Year

1984 2002

Fraction of gullies

0.02

0

0.04

0.06

0.08

0.10

0.12

Aeolian annealing as of 2009

Vegetative annealing as of 2009

Aeolian + vegetative annealing as of 2009

EXPLANATION

Figure 26. Fraction of gullies (n=358 among 

all reaches) that terminated in mapped 

aeolian sand units that showed evidence of 

annealing as of 2009 relative to aerial imagery 

from 1984 and 2002.

A

B

10 M

Figure 27. Example from the aerial photographic record of gully annealing by 

apparent aeolian-sand infilling between (A) October 1984 and (B) May 2009.
Section II - Gullies and Aeolian Sand Activity in the Geomorphic Context of the Colorado River Corridor 37

Discussion

Landscape evolution in the Colorado River corridor depends 

strongly on connectivity among fluvial, aeolian, and hillslope 

systems. Understanding factors that enhance or impede gully 

development depends on recognizing not only the roles of 

hillslope runoff, catchment size, and substrate parameters in gully 

formation, but also of aeolian sand transport in counteracting gully 

erosion. Our large-scale mapping work shows that aeolian sand 

can anneal and impede the expansion of gullies as a cumulative 

effect over time where aeolian sand is active; however, our study 

of temporal gully evolution in the recent aerial photographic 

record shows that seeing individual examples of aeolian annealing 

on short time scales is rare. There are large areas of the river 

corridor (including some of the most archeologically rich) where 

sand is too inactive to anneal gullies effectively. The second and 

third hypotheses of this part of our study—that gullies are less 

prevalent, and terminate more commonly, in active aeolian sand—

are supported by the data. The first hypothesis—that aeolian sand 

activity is inversely related to accommodation space as determined 

by river-corridor width—was partly supported, but, as discussed 

below, we found that local wind directions as well as river-corridor 

morphology (width and other factors) play important roles in 

determining accommodation space, aeolian sand activity, and 

gully prevalence.

Spatial Variations in Aeolian Sand Activity

In testing hypothesis 1, which generally was not supported 

by our results, we identified additional factors that complicate 

links between river-corridor width and aeolian sand activity. 

In places where accommodation space—storage space for 

river-derived sediment—is a function of where large, predam 

floods could reach, there is indeed a relationship between width 

(terrestrial area) and the proportion of active aeolian sand, at least 

for the widest and narrowest end-member reaches. The Furnace 

Flats and Eminence–Little Colorado River reaches contain both a 

wide river corridor and a small proportion of active aeolian sand 

(fig. 21A,B). These are not true alluvial reaches (their hydraulic 

control is determined by bedrock and debris fans), but merely are 

wide enough to store large amounts of fluvial sediment, owing to 

bedrock lithology and fault configuration. Because much of that 

terrestrial area is decoupled from modern aeolian sand supply, 

those relict sediment deposits have become overgrown with 

biologic soil crust and have little aeolian sand activity (Draut, 

2012). In some parts of the Furnace Flats (FF) reach, these relict 

sediment deposits contain abundant type 4 archeological sites 

(fig. 19). Sand-activity ratios for the FF reach are skewed by one 

large, low-elevation point bar at RM 69.6 that is situated directly 

downwind of modern sandbars, and so maintains an unusually 

high active sand ratio (82 percent). Without that dune field, the 

active sand area ratio for the FF reach as a whole would decrease 

from 14 percent (fig. 21B) to less than 11 percent. The narrowest 

reach, Upper Granite Gorge, where igneous and metamorphic 

bedrock walls confine the river more tightly than in the Em-LCR 

and FF reaches, stores little relict, predam flood sediment. The 

small amount of river-derived sand in the UGG reach is near the 

river and much of it receives aeolian sand supply from modern 

sandbars, resulting in a relatively large proportion of active aeolian 

sand (36 percent; fig. 21B).

However, spatial variations in wind direction and canyon 

morphology complicate the situation enough that it is not 

possible to generalize relations between aeolian sand activity 

(with potential for gully annealing) and river-corridor width. 

Accommodation space does not depend solely on where large, 

predam floods could deposit sediment. In parts of the canyon 

where the prevailing wind direction deviates from the usual riverparallel orientation, most notably in parts of the SCA reach where 

wind blows transverse to the river (fig. 11; see also Draut and 

Rubin, 2008, and Section III, below), windblown river-derived 

sand extends to a much higher elevation than even the largest 

prehistoric floods reached, forming sand ramps plastered against 

talus slopes (fig. 3C). Thus, in places where the wind direction is 

transverse to the river, accommodation space and sand storage are 

functions of wind conditions more than of river-corridor width 

and predam inundation zones. Transverse winds have brought so 

much river-derived sediment inland in parts of the SCA reach that 

the terrestrial area per unit river length in that reach is virtually 

identical to that of the FF reach (fig. 21A) even though the wallto-wall width is narrower in the SCA reach (90–280 m) than 

in the FF reach (300–450 m). As a result of transverse-directed 

windblown sand transport and a history of eddy sandbars having 

enlarged substantially from controlled floods, the SCA reach has 

numerous active dune fields downwind of modern sandbars and 

a higher active-sand proportion than occurs in any other reach 

(37 percent; fig. 21B). The largest dune field in the SCA reach 

contains 70 percent active sand area; several other SCA locations 

have more than 60 percent active sand, although for the reach as a 

whole most of the sand-ramp area is currently inactive. Therefore, 

unlike the Em-LCR and FF reaches, where sediment storage is 

largely in predam fluvial terraces with little aeolian sand supply 

or activity today, the SCA reach stores large amounts of sand in 

active dune fields and sand ramps, maintaining high connectivity 

to modern fluvial sand supply and precluding a simple inverse 

relationship between river-corridor width and modern sand 

activity.

Results from the Glen Canyon (GLCA) and Granite Park 

(GP) reaches also do not support the first hypothesis, that rivercorridor width would correspond inversely to aeolian sand 

inactivity, but we attribute this to local morphology rather than 

to wind direction. The GP reach, in which prevailing winds 

are oriented generally toward upstream (toward the northeast), 

contains a moderately low terrestrial area scaled by reach length 

(fig. 21A). Although the GP reach does not store a large amount 

of river-derived sand per unit length, much of the sand it does 

store is in two discrete expanses of predam fluvial terraces with 

aeolian-reworked surfaces that are now inactive (4 percent 

active aeolian sand for the reach overall; fig. 21B). One of the 

terraced areas is unusually broad, extending more than 300 m 

inland from the river margin, or 3–4 times the width that the river 

occupies at a nonflood flow of 226 m3

/s. The breadth of the GP 

terraces represents a unique situation in the canyon, and may be 
38 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

related to predam Colorado River floods ponding upstream of 

the large Granite Park debris fan (RM 209.2) after debris flows 

temporarily increased the river-corridor constriction at the fan 

(see also Hereford and others, 2000a). In contrast, other reaches 

with sediment stored largely in predam flood deposits tend to 

have sand deposits that extend inland no more than 1–2 times the 

nonflood river width (although the aforementioned SCA aeolian 

sand ramps do extend inland by 3–4 times the nonflood river 

width). The combination of GP sand deposits extending unusually 

far inland from the river, such that they are exceptionally large 

relative to—and distant from—modern sandbars, along with the 

wind direction transporting sand obliquely rather than directly 

toward much of their area (unlike sand deposits in the SCA 

reach), makes it unusually difficult for modern sandbars to 

replenish the GP sand deposits—hence their low aeolian sand 

activity. In regions of the GP sand deposits that are downwind of 

modern sandbars, dense vegetation and several tributary channels 

also locally inhibit aeolian sand supply, constituting a type 2 

situation for most archeological sites in this reach (fig. 19).

Geomorphic constraints also affect sand storage and aeolian 

sand supply in Glen Canyon, but in different ways than in the 

other study reaches (see also Section IV). Glen Canyon, or at least 

the part of it that is downstream from Glen Canyon Dam and Lake 

Powell (RM -15 to 0), differs from Marble and Grand Canyons in 

terms of bedrock lithology, topographic relief, tributary activity, 

and influence of dam operations. Glen Canyon bedrock is 

mechanically weak but fairly homogenous, and so erodes to form 

a wide river corridor with walls of massive-bedded Mesozoic 

sandstone (Wilson, 1965; Sprinkel and others, 2003; Bursztyn 

and others, 2015) stratigraphically above the Paleozoic sandstone, 

shale, and limestone of Marble and Grand Canyons (having 

been removed by erosion, the Mesozoic sandstones are not part 

of the Marble–Grand Canyon stratigraphy). The 300–400 m of 

topographic relief from rim to river in Glen Canyon is much 

less than in the reaches downstream (1,000–1,900 m in Marble 

Canyon, 1,200–2,000 m in Grand Canyon). The combination of 

uniform, clay-poor sandstone bedrock forming the canyon walls, 

and modest relief, means that there are no large debris fans in the 

Glen Canyon river corridor (in contrast to Marble–Grand Canyon; 

Melis and others, 1994; Webb and others, 2003; Griffiths and 

others, 2004). Without major debris fans, Glen Canyon not only 

lacks the fan-eddy complexes that form rapids and store eddy 

sandbars in Marble–Grand Canyon, but also has no major features 

to segment the hydraulic control on the mainstem river flow; 

the small riffles in Glen Canyon are not large enough to exert 

hydraulic control during flood flows. Whereas sediment-storage 

patterns in the pool-and-drop morphology of Marble–Grand 

Canyon largely depend on debris-fan controls (with secondary 

effects of local wind direction, as in the SCA reach), most 

relict sand in Glen Canyon is stored in thick (as much as10 m), 

longitudinally extensive Holocene terraces atop point bars on the 

inside of meander bends. Some of these terrace deposits extend 

more than 1,200 m continuously along the river, and are located 

near the active river channel. These extensive, thick terrace 

deposits formed during predam high flows, as a function of 

reach-scale hydraulic control and grade changes (Tainer, 2010); 

this terrace morphology remains fairly continuous into uppermost 

Marble Canyon, around RM 4.

Because the GLCA reach is immediately downstream 

from Glen Canyon Dam, the sediment-supply-limiting effects 

of the dam and scouring potential of postdam high flows have 

been especially pronounced there. Postdam channel incision has 

lowered the riverbed in Glen Canyon by 2–4 m along most of the 

reach, and as much as 8 m locally; the water-surface elevation 

for a flow of approximately 150 m3

/s is as much as 2.3 m lower 

than in predam time (Grams and others, 2007). This reach is 

particularly sediment-starved, having essentially no upstream 

supply; the first major sediment-supplying tributary, the Paria 

River, joins the river corridor at the downstream end of Glen 

Canyon (Lees Ferry, RM 0; fig. 1). As a result of pronounced 

sediment deficit and bed winnowing, most fluvial sandbars in the 

contemporary active river channel have become gravel bars in 

postdam time (Grams and others, 2007).

The active sand area ratio in GLCA is notably lower than 

in any other study reach (1.3 percent; fig. 21B), with most 

sand area being heavily biocrusted. The terrestrial area (riverderived-sediment storage) per unit length in the GLCA reach is 

intermediate between that of the FF and GP reaches (fig. 21A). The 

Glen Canyon sediment deposits are gullied to a similar degree as 

those in the FF and GP reaches, with 15 percent of the terrestrial 

area occupied by gullies (fig. 21C). Aeolian dune activity may not 

have been very important in Glen Canyon even in predam time. 

Dune morphology is rare there, even though the predam terraces 

contain ample sediment supply (of similar grain sizes to terraces 

in Grand Canyon that have better-developed dune fields; fig. 28) 

and the wind in Glen Canyon is often strong enough to transport 

sand (Section IV). It is possible that local wind dynamics (zones of 

airflow convergence and divergence) simply were not conducive 

to dune formation, aeolian sand accumulation, or even to sand 

retention in many places; in fact, within our GLCA reach there are 

multiple locations where wind scour has eroded sediment from the 

predam fluvial terraces.

Today, the pronounced Colorado River sediment deficit, 

combined with the lack of fan-eddy complexes that would 

promote storage of any sand that enters the river from small 

tributaries, means that there is very little modern, subaerial, fluvial 

sandbar area to serve as a source for aeolian sand. The dearth of 

active aeolian sand in Glen Canyon today is not surprising in view 

of this lack of modern sandbars to supply aeolian sand, although 

even in the sediment-rich predam era this reach may have 

experienced little gully annealing from aeolian dune activity.

Gully Annealing by Aeolian Sand

Through our field and remote-sensing analyses, we 

determined that gullies are much more prevalent in inactive 

sand than in active aeolian sand (fig. 24). River-corridor reaches 

with a greater proportion of active aeolian sand also have a 

lower proportion of gully area (fig. 23), and gully terminations 

occur more commonly in active aeolian sand than in inactive 

sand deposits (fig. 25). This may be partly attributable to 

aeolian sand having infilled the gullies and annealed them, as 
Section II - Gullies and Aeolian Sand Activity in the Geomorphic Context of the Colorado River Corridor 39

men16-3129_fig 28

Glen 

Canyon 

aeolian 

deposits

(red)

Grand 

Canyon 

aeolian 

deposits

Glen 

Canyon 

fluvial 

deposits

Grand 

Canyon 

fluvial 

deposits

0.01

0

20

40

60

80

100 Cumulative percent finer

0.1 1

Grain size, in millimeters

Figure 28. Grain-size distributions in predam flood 

deposits from Glen Canyon and Grand Canyon. Samples 

were analyzed with a Beckman Coulter LS 100Q laser 

particle-size analyzer at the U.S. Geological Survey Grand 

Canyon Monitoring and Research Center in Flagstaff, Ariz. 

Data from Glen Canyon include 12 fluvial sediment samples 

from predam flood deposits and 3 samples from reworked 

aeolian deposits on the terrace surfaces (this study); data 

from Grand Canyon include 19 fluvial sediment samples 

from predam flood deposits (terraces in the Furnace Flats 

and Granite Park reaches) and 10 samples from aeolian 

deposits (Draut and others, 2008). Values on vertical axis 

indicate the cumulative percent of the sediment sample 

that is finer than the corresponding grain diameter shown 

on the horizontal axis. Grand Canyon analyses were 

curtailed at the lower boundary of coarse silt (0.037 mm).

is commonly observed where topographic depressions act as 

sand traps (fig. 6), and partly attributable to gullies initially 

having formed more readily through inactive sand. Biocrusted, 

inactive sand has lower water-infiltration rates than does 

unconsolidated, uncrusted sand (Chamizo and others, 2012; 

Rodriguez-Caballeroa and others, 2012), promoting overland 

flow that could channelize into erosive gullies once it develops 

sufficient shear stress in a place where the crust is damaged 

or weak enough to erode. It is also possible that gullies, once 

formed, have less potential to continue propagating after they 

enter active sand, owing to the greater infiltration capacity.

Our field observations indicate that these latter processes 

(more difficult gully initiation and growth through active sand) are 

probably less important than aeolian annealing in this field setting 

because the great majority of gullies initiate outside (upslope) of 

the sand deposits rather than in them; overland flow commonly 

utilizes previously established channels across less-permeable 

bedrock or talus and then incises through (active and inactive) 

sand deposits. Our observations during intense rain events confirm 

that gullies through sand commonly form in this way rather than 

by initiating directly from rainfall on sand. Our field and aerialphotographic observations also confirm that aeolian annealing can 

erase gully paths. Thus, we infer that the lower gully prevalence 

in active sand, and the greater likelihood of gully termination 

in active sand, largely result from aeolian sand transport being 

an effective gully-annealing mechanism. Our field observations 

indicate that aeolian infilling occurs in conjunction with some 

backwasting of noncohesive sand in gully walls, which provides 

additional sand to infill gullies.

Our analysis of historical aerial imagery revealed that 

aeolian sand annealing had visibly affected a small but detectable 

proportion (1–3 percent) of the 358 terminating gullies we 

analyzed. Thus, aeolian infilling constituted a relatively 

uncommon fate for gullies in the river corridor, at least between 

1984 and 2009. However, because gullies tend to form in the 

same place repeatedly from overland flow that follows persistent 

flow paths, it is likely that some gullies filled in, were excavated 

again by later overland flow, and then refilled between the relevant 

dates of analysis. We consider this likely, as previous work 

within our study reaches has measured rapid dune migration and 

aeolian inflation in close spatial and temporal proximity to gully 

erosion (Collins and others, 2009, 2012; Draut and others, 2010). 

Our work in Section III confirmed that gullying and aeolian 

sand movement are not rare, but rather are common processes 

measurably changing river-corridor landscapes over time scales of 

months to years.
40 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

Together with previous findings that modern sand supply 

from Colorado River sandbars promotes aeolian sand activity 

in downwind landscapes (Draut, 2012), results of this study 

imply that landscapes receiving aeolian sand supply from 

modern river sandbars are less likely to have advanced gully 

erosion than are biocrusted landscapes without modern sand 

supply or aeolian activity. Although gullies can erode both 

active and inactive sand landscapes—and some parts of the 

landscape will always be more vulnerable to gully formation 

than others are, given that overland flow tends to focus in the 

same places repeatedly—gully erosion evidently becomes 

less prevalent as gullies anneal in active aeolian sand. Thus, 

landscapes and archeological sites that receive modern sand 

supply from fluvial sandbars under the dam-controlled flow 

regime have less risk of developing advanced gully erosion, 

and a greater chance that gullies will anneal or have their 

growth impeded by aeolian sand.

The greatest distinguishing characteristic between active and 

inactive aeolian sand in the Colorado River corridor is biologic 

crust cover (fig. 7). Biologic crust, a community of lichen, mosses, 

fungi, algae, and cyanobacteria widespread on Southwest desert 

soils (Belnap and others, 2001; Belnap and Lange, 2003), tends 

to colonize sand deposits receiving little modern sand supply 

(Draut, 2012). Once a crust is established, it reduces aeolian sand 

entrainment and mobility by armoring the ground surface (Leys 

and Eldridge, 1998; Goossens, 2004; Zhang and others, 2008). 

Vegetation, too, inhibits aeolian sand transport once established 

(for example, Ash and Wasson, 1983; Buckley, 1987; Raupach 

and others, 1993). The growth of vegetation and biologic crust can 

enhance each other, both by virtue of stabilizing the sand surface, 

which facilitates plant recruitment, and because biologic crust fixes 

nitrogen that neighboring organisms use (MacGregor and Johnson, 

1971; Harper and Belnap, 2001). As dam operations have reduced 

aeolian sand supply and sand activity in river-corridor landscapes, 

they have thereby promoted greater-than-natural biologic crust 

abundance (Draut, 2012). This has consequences not only for 

upland ecosystems, but also for how widespread gully erosion 

may become. In one sense, the unnaturally great abundance of 

biologic crust in the regulated Colorado River corridor stabilizes 

relict sand surfaces by preventing sand transport that otherwise 

would cause aeolian dunes to migrate over time. However, instead 

of destabilizing by dune migration (which affects the condition 

of some Marble–Grand Canyon archeological sites), biocrusted 

landscapes can destabilize by localized gully erosion instead, 

once the crust has been disturbed (fig. 24), substantially damaging 

archeological sites. Thus, the common perception that biocrusted 

sand deposits are stable is somewhat misleading in that it does not 

reflect the likely erosional trajectory of biocrusted landscapes and 

archeological sites once the crust is locally disturbed and gullying 

has been initiated.

While investigating the effectiveness of aeolian sand in 

gully annealing, we have not addressed other factors related to 

changes in local base level that may also influence gully development (Hereford and others, 1993). The potential for gully erosion 

likely depends not only on whether aeolian sand is available to 

infill the gullies, but also on the base level to which gully incision 

is graded—that of terrace surfaces higher than the mainstem 

river channel, tributary-channel elevations, or the mainstem river. 

We have focused on the role of aeolian sand in gully prevalence 

because of its relevance for the present management objectives of 

the GCDAMP—increasing the aeolian supply through sandbar 

growth from controlled floods is a realistic management option, 

whereas modifying the base level of the mainstem river through 

major flood flows is not currently under consideration. However, 

the influence of local base level on gully erosion in the Colorado 

River corridor warrants further investigation in subsequent studies.

Section III - Landscape Change at 

Archeological Sites Receiving Sand 

Supply After Controlled Floods, Grand 

Canyon National Park

Background

As we have shown in the preceding sections, river-corridor 

archeological sites in Grand Canyon National Park vary in their 

potential to receive aeolian sand supply from fluvial deposits 

under the modern, dam-controlled flow regime. Sites also vary in 

their inferred resilience to gully erosion, as a function of aeolian 

sand activity on the surrounding landscape (Section II). Type 1 

sites (Section I) have the greatest connectivity between fluvial 

sand sources and upland sand-dominated landscapes, the greatest 

potential for modern aeolian sand supply, and thus the highest 

potential for natural mitigation of gully erosion. However, having 

high potential to receive modern aeolian sand supply does not 

necessarily mean that sites will be unaffected by gully erosion 

or other types of landscape degradation. Rain, wind, creep, and 

occasional rockfalls and debris flows continually reshape the 

river-corridor landscape, often altering the condition or integrity 

of cultural sites. Thus, even type 1 sites cannot be assumed to be 

stable and safe from erosive damage. To assess landscape change 

and stability at type 1 sites, we investigated the interplay of 

annealing and erosive forces on sites expected to receive additional 

aeolian sand supply owing to controlled floods.

We anticipated that type 1 sites, which receive aeolian sand 

from controlled-flood sandbars, would erode less rapidly than 

would other sites that lack windblown sand supply from modern 

(controlled-flood) sandbars. Thus, we quantified topographic 

change over several years at four type 1 sites in Grand Canyon 

National Park (Marble–Grand Canyon) that have substantial 

connectivity between fluvial sandbars and inland aeolian deposits, 

and compared these measurements to topographic change 

measured over a year at four sites in the Glen Canyon reach that 

do not receive modern sand supply (Section IV). We selected two 

groups of sites that represent end-member cases of modern sand 

supply. The four in Marble–Grand Canyon are within aeolian 

dune fields on top of debris fans that have sand-trapping eddies 
on their downstream sides. Sand from those eddy sandbars blows 

downwind directly into the dune fields containing the type 1 sites, 

with no substantial vegetation or topographic barriers to limit 

sand movement. In contrast, the Glen Canyon sites in Section IV 

are in and on Holocene terraces with essentially no modern sand 

supply, and very little aeolian sand activity on the terrace surfaces. 

The differences in topographic evolution of these two groups thus 

likely represent end members of site evolution; landscape change 

at sites with intermediate modern sand-supply conditions will be 

evaluated during a subsequent study.

The four type 1 sites in Marble–Grand Canyon are 

located approximately 50 to 100 river miles (80 to 160 km) 

from one another (fig. 29), thereby capturing a broad spectrum 

of the regional conditions in the canyons. Herein, we refer 

to each of the sites by their archeological-site identification 

number (AZ:C:05:0031, AZ:C:13:0321, AZ:B:10:0225, and 

AZ:G:03:0072US – the suffix “US” indicates that we monitored 

only the upstream half of the large archeological site). We used 

high-resolution landscape-change detection and high-resolution 

records of weather events to assess the cumulative impacts of 

weather-related changes and the effects of controlled floods on 

archeological-site change over a four-year monitoring period. 

The monitoring period brackets two controlled floods, one each 

in November 2012 and November 2013 (table 1). Our work 

complements previous studies at three of the same sites (Collins 

and others, 2009, 2012), thereby extending our understanding 

of the detailed, short-term geomorphic response of these sites to 

varying weather and river flow conditions.

Lake Powell

Lake Mead

Colorado River

Havasu Creek

Diamond

Cr

Tapeats

Cr

Shinumo

Cr

Bright Angel Cr

Little Colorado River

Paria R

Kanab Creek

GLCA

EmLCR

FF

UGG

SCA

GP

UTAH

NEVADA

ARIZONA

GRAND CANYONPARASHANT NM GRAND

CANYON

NP

ESCALANTE NM

KAIBAB

HAVASUPAI

NAVAJO 

HOPI HOPI

WUPATKI

NM

GLEN

CANYON

NRA

HUALAPAI 

Glen Canyon Dam

Lees Ferry

LAKE MEAD

NRA

Page

AZ:G:03:0072

AZ:B:10:0225

AZ:C:05:0031

AZ:C:02:0035

AZ:C:02:0032

AZ:C:02:0075

AZ:C:02:0077

AZ:C:02:0071

AZ:C:02:0070

AZ:C:13:0321

Weather

stations

Phantom

Ranch

EXPLANATION

National Park land

Other NPS land

Tribal lands

River mile

Archeological site

Study reach and 

identifier

Note:

NP=National Park

NM=National Monument

NRA=National Recreational Area

RM

150

GP

AZ:G:03:0072

RM

50

RM

100

RM

0

RM

150

RM

200

RM

250

114°30' 114° 113°30' 113° 112°30' 112° 111°30' 111°

37°

36°

30'

36°

35°

30'

Base map modified from National Atlas, National Hydrographic Dataset 

and other digital data, various scales. Coordinate reference system: 

NAD 1983 NSRS2007 StatePlane Arizona Central FIPS 0202. Horizontal 

datum is North American Datum of 1983. 

0 10 20 30 40 MILES

0 20 40 60 80 KILOMETERS

Figure 29. Location map (same area as in fig. 1B), showing 

approximate locations of archeological sites and weather 

stations discussed in Sections III and IV. The sites shown 

in Glen Canyon are near enough to each other not to be 

differentiable at this scale. Glen Canyon includes the canyon 

region from Lake Powell downstream to Lees Ferry (RM 0). 

Marble Canyon extends from RM 0–61 and Grand Canyon 

from RM 61–277.

Section III-Landscape Change at Archeological Sites Receiving Sand Supply After Controlled Floods 41
42 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

Research Questions

We evaluated the four, abovementioned type 1 archeological 

sites by posing two questions:

• Does aeolian sand supply from controlled-flood sandbars 

to type 1 archeological sites cause enough deposition 

to offset erosion, and thereby protect the archeological 

resources?

• In areas with modern aeolian sand supply, and with land 

surfaces undergoing both gully erosion and active aeolian 

sand transport, is there net sediment loss and topographic 

lowering such that cultural resources are affected?

We investigated both questions quantitatively by measuring the 

sediment volume gain or loss at the selected archeological sites, 

and linking these findings to landscape-altering wind and rainfall 

processes.

Methods

We used a combination of site-specific topographic, 

meteorological, and time-series photographic monitoring to 

document changes at the four selected type 1 archeological sites 

in Marble–Grand Canyon over a 2- to 4-year interval (2010–13; 

exact dates of data collection varied among the sites). The 

topographic data revealed the nature and location of landscape 

change occurring at each site, whereas the meteorological data 

identified the magnitude and timing of processes that potentially 

caused landscape change; the photographic data constrained the 

timing of landscape change. Although we collected only two 

new topographic data sets as part of this work (in May 2013 and 

May 2014), we compared these topographic surfaces to those 

collected at the same four sites in September 2010 (Collins and 

others, 2012). Partial meteorological and photographic datasets 

are available from 2010 to 2012; we used these to try to constrain 

the cause and timing of detected changes. During intervals without 

meteorological or photographic data, we estimated the probable 

causes of change based on field observations made during our site 

visits (for example, active dunes indicating aeolian transport).

Topographic Change Detection

Our methods for identifying site-specific landscape change 

at archeological sites followed protocols previously developed for 

monitoring in Grand Canyon National Park (Collins and others, 

2008, 2009, 2012). We used repeat, high-resolution, terrestrial 

(ground-based) lidar laser scanning to collect topographic data 

from each of the four archeological sites (fig. 30). The topographic 

data consist of millions of surveyed points (a point cloud) that 

capture the position and shape of any object within view of the 

laser instrument. In this study we used a Riegl Z420i scanner, in 

conjunction with a mounted and calibrated Nikon D300 digital 

camera, and a survey prism, to measure the x, y, z positions of 

each survey point, a red-green-blue-intensity determination of 

the pixel in which each point resides, and the x, y, z position of 

the laser instrument, respectively. By moving the laser scanner 

Figure 30. Terrestrial lidar 

data-collection setup in Grand 

Canyon National Park; laser 

scanner is at right. Optimal 

surveying locations must be 

identified to capture the area 

of interest—here, the gully 

partly filled with aeolian sand. 

A total station and survey prism 

were used to georeference 

the instrument location, along 

with temporary reflector 

control points (one visible in 

the background, mounted on a 

tripod).
around each site and collecting data from several vantage points, 

we quantified the three-dimensional shape of the ground surface. 

The final result was a colored point cloud in which objects (gullies, 

cliffs, vegetation, etc.) are easily identified and distinguished from 

one another.

Uncertainties in the topographic data arise from three 

sources: the laser, the registration process of collating data from 

multiple vantage points, and the georeferencing procedure that 

assigns real-world coordinates to each survey point. We adopted 

previously calculated empirical error determinations for the 

datasets based on established error-analysis methods developed 

in prior topographic surveys at some of the same sites (Collins 

and others, 2012). Average vertical errors in each final point 

cloud collected in September 2010, May 2013, and May 2014 

were determined to be 1.5 cm. Thus, we set a conservative 

change-detection threshold of 3 cm for each site, representing 

the limit of topographic change that can reasonably be detected 

and positively identified.

To calculate topographic differences from scans of the 

same site at different times, the point-cloud data were modeled 

by developing triangulated irregular network (TIN) surfaces 

and DEM grid surfaces. TINs typically represent terrain in 

better resolution, because the point data are used directly to 

form the corners of the triangular facets that define the surface. 

DEMs rely on extrapolation of the surface to the center of 

a grid cell, thereby introducing potential inaccuracies of the 

surface. However, DEMs also allow more robust changedetection algorithms, by computing the elevation difference 

between topographic surfaces resolved at different times. 

We used a minimum point density of 96 points/m2

 and a grid 

spacing of 5 cm to minimize potential errors in our surface 

modeling algorithms. For additional details of the data 

processing, see Collins and others (2009, 2012).

Meteorological Monitoring

To identify the role of weather, and the specific processes 

causing landscape change at the archeological sites, we installed 

remote weather stations at each site (fig. 29). The stations 

consisted of VaisalaTM WXT510/520 transmitters measuring wind 

direction, wind speed, maximum gust speed, air temperature, 

relative humidity, barometric pressure, and rainfall. Data loggers 

at each station recorded the parameters at 4-minute intervals, 

and were downloaded during field visits every few months. Full 

details of the weather stations and resulting data were provided 

by Dealy and others (2014) and Caster and others (2014); we 

refer here to general patterns and specific weather events recorded 

at these stations as necessary to interpret the landscape-change 

measurements discussed below.

Time-Series Photography

We collected high-resolution digital photographs of 

each archeological site and surrounding landscape to provide 

a means to verify and constrain the timing of the changes 

detected by the topographic monitoring. We used Cannon 

Rebel DSLR series T3/T4 cameras regulated with a Campbell 

Scientific CR200X data logger to take photographs every 2 

hours during daylight. Where we were able to locate a camera 

in close proximity to a site, we were able to identify subtle 

landscape changes, such as the formation of incipient gullies. 

However, in cases where there was no suitable location for a 

camera deployment near the site, and cameras could only be 

located across the river from a site of interest, the photographs 

were useful only for identifying large-scale changes such as 

sandbar formation. We refer to these photographic records 

as necessary to document the timing of landscape changes 

discussed below; more detail about these records is available 

in Caster and Sankey (2016).

Results

AZ:C:05:0031

Site Description

Site AZ:C:05:0031 (figs. 31, 32) is an ancestral Puebloan 

camp site dating to the 11th or early 12th century. The site 

contains evidence of roasting features, pottery sherds, stone 

tool-making debris, and low stacked-stone walls (Fairley 

and others, 1994), and is located immediately below a steep 

outcrop of limestone. Aeolian dune sand forms a ramp from 

the Colorado River up to the base of the outcrop, and the 

site itself is primarily located in and on this dune sand. A 

prominent gully (G1) bisects the dune field and bounds the 

archeological site to the south. The site is covered mostly by 

sand, but large (1- to 2-m diameter) boulders and isolated 

vegetation cover parts of the site as well. Vegetation includes 

prickly-pear cactus (Opuntia sp., including beavertail cactus, 

Opuntia basilaris), Mormon tea (Ephedra sp.), dropseed 

(Sporobolus sp.), Indian ricegrass (Achnatherum hymenoides), 

asters (Asteraceae family), dicoria (Dicoria canescens), and 

nonnative Russian thistle (tumbleweed; Salsola sp.). O’Brien 

and Pederson (2009a) identified gullying, aeolian transport, 

piping, and creep affecting this site, and noted that whereas 

revegetation efforts and aeolian inflation have stabilized some 

parts of the site, the archeological resources continue to be 

affected by creep. This site and its surrounding dune field are 

directly downwind of a fluvial sandbar that enlarged during 

high flows in the 1980s and also during each of the subsequent 

controlled floods.

We analyzed landscape change in the area of the 

archeological site below the limestone outcrop and in three 

areas of dune sand located between the outcrop and the 

Colorado River (figs. 31, 32). Focusing on this part of the 

site allowed us to detect any potential aeolian aggradation in 

response to sand supply from the controlled-flood sandbar 

located immediately upwind of the dunes.

Section III-Landscape Change at Archeological Sites Receiving Sand Supply After Controlled Floods 43
44 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

men16-3129_fig 31

Figure 32

G1

Colorado River

Wind

EXPLANATION

Survey area boundary

Archeological site boundary

Laser scan location

Shoreline at flow of 1,270 cubic

meters per second

Slope direction

0 50 100 FEET

0 20 40 METERS

Gully flow path and number

G1

AZ:C:05:0031

Figure 31. Site AZ:C:05:0031 survey-area map.
men16-3129_fig 32

G1

EXPLANATION

Gully flow path and number

G1

AZ:C:05:0031

Figure 32. Site AZ:C:05:0031 survey-area photograph, showing flow path of gully G1 (blue). View is toward the southwest.

Topographic Change (2010–13)

Topographic changes at site AZ:C:05:0031 between 2010 

and 2013 included both erosion and deposition, mainly related to 

aeolian sand transport (fig. 33A; table 2). The majority of change 

appears to have occurred from aeolian sand transport within the 

active inland dune field (as opposed to sand moving into the dune 

field from the fluvial sandbar). We infer sand transport from the 

southwest toward the northeast, coincident with the prevailing 

wind direction measured at this site (Draut and others, 2010). 

Erosion within the entire monitored area (that is, within the 

focused change-detection areas indicated by the green boundaries 

in fig. 33) totaled 27.72 m3

 and occurred over an area of 362.1 m2; 

deposition totaled 12.54 m3 and occurred over an area of 193.4 m2, 

yielding net erosion of 15.18 m3. The focused change-detection 

boundaries show where change is most likely to have occurred 

within areas of active aeolian sand deposits. The dominant 

erosional signal has a spatial pattern implying aeolian deflation, 

and indicates that sand is being transported by wind either into 

adjacent gullies (where it may partly offset overland-flow erosion) 

or out of the survey area entirely, where it has an unknown fate 

and unknown consequences for archeological-site stability. 

However, the deposition (aeolian inflation) that we detected 

within the archeological-site boundary indicates that active aeolian 

sand has likely increased site stability. In a previous monitoring 

campaign in 2010 (Collins and others, 2012), only erosion had 

been recorded at this site. Sand transport from the river margin 

toward the archeological site (toward the northeast) indicates that 

sandbar sediment, including some from controlled floods in 2008 

and 2012, may be increasing site stability (that is, burying part of 

the archeological site in sand; fig. 33). One area of erosion (ER4) 

and two areas of deposition (DEP4, DEP5) were detected nearest 

to the Colorado River, at the westernmost side of the surveyed 

area (fig. 33A), and may be linked to direct fluvial sediment 

mobilization and redeposition during controlled floods.

Topographic Change (2013–14)

Topographic changes at site AZ:C:05:0031 between 2013 

and 2014 showed similar patterns to those that occurred between 

2010 and 2013, and included both erosion and deposition related 

to aeolian sand transport (fig. 33B; table 2) and also fluvial erosion 

and deposition in the westernmost part of the surveyed area, 

by the river (regions ER4, DEP4, and DEP5). Erosion within 

the surveyed area totaled 8.57 m3

 and occurred over an area of 

135.7 m2; deposition totaled 9.04 m3 and occurred over an area 

Section III-Landscape Change at Archeological Sites Receiving Sand Supply After Controlled Floods 45
46 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

men16-3129_fig 33a

ER4

DEP5

DEP4

ER2

DEP2

ER3

DEP3

ER1

DEP1

EXPLANATION

Survey area boundary

Archeological site boundary

0 75 FEET

0 30 METERS

Focused change-detection areas

Deposition area and number

DEP1

Erosion area and number

ER1

30 to 50

10 to 30

Vertical distance between

surfaces, in centimeters

50 to 90

3 to 10

-3 to 3

-3 to -10

-10 to -30

-30 to -50

-50 to -70 Gully flow path and number

G1

A

Figure 33. Landscape change at site AZ:C:05:0031. A 5-cm gridded topographic change output shows erosion (warm colors, 

negative) and deposition (cool colors, positive) from (A) September 2010 to April 2013, and (B) April 2013 to May 2014. Topographic 

change was calculated only in focused areas noted. Identified change is grouped by area labels (ER, erosion; DEP, deposition) that 

are cross-referenced with table 2. “IG#” denotes incipient gullies detected from landscape-change analyses.
men16-3129_fig 33b

ER4

DEP5

DEP4

ER2

DEP2

ER3

DEP3

ER1

DEP1

IG1

IG2

EXPLANATION

Survey area boundary

Archeological site boundary

Vertical distance between

surfaces, in centimeters

0 75 FEET

0 30 METERS

Focused change-detection areas

Gully flow path and number

G1

Erosion area and number

Deposition area and number

ER1

DEP1

10 to 30

3 to 10

-3 to 3

-3 to -10

-10 to -30

-30 to -50

IG1 Incipient gullies

B

Figure 33.—Continued

Section III-Landscape Change at Archeological Sites Receiving Sand Supply After Controlled Floods 47
48 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

Table 2. Summary of topographic change at site AZ:C:05:0031.

[m/yyyy, month/year; m2, square meters; cm, centimeters; m3, cubic meters]

Site number and region Time period

(m/yyyy)

Area

(m2

)

Average depth 

(cm)

Volume

(m3)

AZ:C:05:0031 – ER1 9/2010–4/2013 169.2 -7 -11.72

AZ:C:05:0031 – ER2 9/2010–4/2013 50.6 -6 -2.85

AZ:C:05:0031 – ER3 9/2010–4/2013 85.4 -6 -4.78

AZ:C:05:0031 – ER4 9/2010–4/2013 56.9 -15 -8.37

AZ:C:05:0031 – DEP1 9/2010–4/2013 107.4 6 6.45

AZ:C:05:0031 – DEP2 9/2010–4/2013 25.8 6 1.51

AZ:C:05:0031 – DEP3 9/2010–4/2013 44.9 6 2.57

AZ:C:05:0031 – DEP4 9/2010–4/2013 12.7 5 0.62

AZ:C:05:0031 – DEP5 9/2010–4/2013 2.6 53 1.39

AZ:C:05:0031 – ER1 4/2013–5/2014 79.1 -6 -4.72

AZ:C:05:0031 – ER2 4/2013–5/2014 14.0 -5 -0.66

AZ:C:05:0031 – ER3 4/2013–5/2014 13.0 -6 -0.78

AZ:C:05:0031 – ER4 4/2013–5/2014 29.6 -8 -2.41

AZ:C:05:0031 – DEP1 4/2013–5/2014 43.6 5 2.19

AZ:C:05:0031 – DEP2 4/2013–5/2014 5.8 5 0.28

AZ:C:05:0031 – DEP3 4/2013–5/2014 16.0 5 0.84

AZ:C:05:0031 – DEP4 4/2013–5/2014 12.6 5 0.69

AZ:C:05:0031 – DEP5 4/2013–5/2014 43.6 12 5.04

of 121.6 m2. Whereas the volume of erosion was substantially 

less (69 percent) than that recorded over the longer time interval 

between 2010 and 2013, and the net volume change was positive 

between 2013 and 2014 (by 0.47 m3), the volume of deposition 

was similar (only 28 percent less over 2013–14), which may 

indicate positive short-term effects of the controlled floods in 2012 

and 2013. Although most of the new deposition was in the form 

of a fluvial sandbar (region DEP5), aeolian transport of this sand 

inland toward the archeological site is likely, given the prevailing 

northeastward wind at this site.

Despite aeolian processes having dominated the site 

response, precipitation-induced runoff also played an important 

role within the archeological-site boundary. At site AZ:C:05:0031, 

two incipient gullies (IG1 and IG2, fig. 33B) formed as a result of 

storms in August and September 2013. These summer monsoonal 

storms had maximum 10-minute rainfall intensities of 75 and 

61 mm/hr, respectively, both of which exceed the threshold for 

precipitation-induced runoff erosion identified to be important in 

this river-corridor environment (Collins and others, 2016). Lidar 

measurements in April 2014 indicated overall mean erosional 

depths of between 6 and 11 cm, with 0.57 m3 of total erosion. 

Analysis of time-lapse photographs taken by the stationary camera 

at this site indicated that the magnitude of gullying was even 

greater initially—comparison of photographs from September 

2013 with those from May 2014 showed that the incipient 

gullies partly annealed between September 2013 and May 2014, 

presumably by filling with aeolian sand (Caster and others, 2014).

AZ:C:13:0321

Site Description

Site AZ:C:13:0321 (figs. 34, 35) encompasses a large active 

dune field composed of source-bordering dunes directly inland 

from a Colorado River sandbar; the archeological-site boundary 

begins 30 m inland from the river. Because the dune field receives 

sand from an adjacent river sandbar enlarged by controlled floods, 

this site displays strong type 1 connectivity between fluvial and 

aeolian sand resources, burying and contributing to preservation of 

the archeological-site.

The site consists of a multi-component habitation area dating 

to the time period when ancestral Puebloan farmers occupied 

eastern Grand Canyon, approximately 700–1200 C.E. (Fairley and 

others, 1994; Fairley, 2003). Among the archeological remains are 

a rubble pile that appears to be a collapsed prehistoric masonry 

structure, at least four roasting features, several ceramic sherds, 

and fragments of ground stone. One of the roasting features 

(consisting of a circular mound of fire-cracked rocks, ash, and 

wood charcoal, encircled by upright stone slabs) is within the 

lowest part of the sand dunes adjacent to a channel-shaped swale 

formed by the dunes, and is intermittently buried by aeolian sand. 

Therefore, its protection and preservation are tied directly to the 

sandbar immediately upwind. The interdune swale slopes towards 

the river, but no indications of severe overland flow (that is, 

incised gullies) are readily evident (O’Brien and Pederson, 2009a) 
men16-3129_fig 34

Figure 35

Colorado River

Wind

EXPLANATION

Survey area boundary

Archeological site boundary

Laser scan location

Shoreline at flow of 1,270 cubic

meters per second

Slope direction

0 40 80 FEET

0 10 20 30 METERS

AZ:C:13:0321

Section III-Landscape Change at Archeological Sites Receiving Sand Supply After Controlled Floods 49

Figure 34. Site AZ:C:13:0321 survey-area map.
50 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

men16-3129_fig 35

AZ:C:05:0031

Figure 35. Site AZ:C:13:0321 survey area photograph. View is towards the north.

other than several areas of gravel-pebble slopewash located at the 

downslope end that we infer may be a lag deposit resulting from 

some past overland-flow episode. Honey mesquite trees (Prosopis 

glandulosa, some dead or dying, others healthy) surround the site. 

Within the site, a wide variety of isolated vegetation clusters are 

established within the dune sands and gravel substrate, including 

globemallow (Sphaeralcea sp.), arrowweed (Pluchea sericea), 

saltbush (Atriplex sp.), sand verbena (Abronia elliptica), and 

nonnative Russian thistle (Salsola sp.). O’Brien and Pederson 

(2009a) identified aeolian transport as the primary geomorphic 

process affecting this site. They noted that aeolian deflation can 

expose archeological artifacts there, making them vulnerable to 

anthropogenic impacts such as trampling.

We analyzed landscape change throughout the entire 

archeological site and adjacent sandbar to investigate potential 

linkages between site condition and the direction and quantity 

of windblown sand that the site receives. Dense vegetation at 

the top of the dunes prohibited detecting change in some areas; 

however, based on field observations over several years, we 

judged that it is unlikely that change greater than the uncertainty 

threshold (3 cm of vertical change) occurred beneath the 

vegetation in these areas.

Topographic Change (2010–13)

Topographic changes at site AZ:C:13:0321 between 2010 

and 2013 included erosion and deposition related mainly to aeolian 

sand transport (fig. 36A; table 3). Erosion within the surveyed 

area (which included approximately half of the archeologicalsite area; fig. 36) totaled 27.15 m3

 and occurred over an area of 

262.3 m2; deposition totaled 25.32 m3 and occurred over an area 

of 311.3 m2. These values yielded net erosion of 1.83 m3. The 

majority of change evidently occurred from aeolian transport of 

sand from an active fluvial sandbar (ER3) and associated inland 

dune deposits. Sand apparently was transported from the southsoutheast toward the north-northwest, apparently, judging from 

the region of erosion (ER3) at the riverward, upwind side of the 

survey area and depositional areas north, or downwind, of the 

eroded region (fig. 36A). This is broadly consistent with the northto northeastward prevailing wind direction in this area, measured 

at a weather station 700 m away (Draut and others, 2010; Caster 

and others, 2014) and inferred from dune slipface orientations 

immediately adjacent to the site. The volume of sand deposition 

within the archeological-site boundary (16.18 m3

 in region DEP1) 

was more than double that which exited (6.90 m3 in region ER1), 

and the roasting feature in the lowest part of the dunes was buried 

increasingly by sand between 2010 and 2013 as a result of this 

new deposition. However, parts of the interdune swale eroded, 

indicating that the processes affecting the site (aeolian inflation 

and deflation) are not spatially uniform. Regardless, the overall 

depositional signal within the archeological site between 2010 

and 2013 indicated that fluvial-sourced sand, potentially from 

controlled floods in 2008 and 2012 (floods that enlarged the 

sandbar upwind of this archeological site), may be reaching the 

archeological site downwind and thus achieving one objective of 

the controlled-flood management actions.
Topographic Change (2013–14)

Topographic changes at site AZ:C:13:0321 between 2013 

and 2014 followed similar patterns to those detected between 2010 

and 2013, and included both erosion and deposition related to 

aeolian sand-transport processes (fig. 36B; table 3). Erosion within 

the surveyed area (which included the entire archeological site) 

totaled 35.67 m3 and occurred over an area of 503.8 m2; deposition 

totaled 25.02 m3 and occurred over an area of 360.2 m2. These 

values yield net erosion of 10.65 m3 within the entire survey area. 

The survey area for 2013–14 was approximately twice as large 

as that surveyed for 2010–13; thus, only area-normalized change

can be compared between the datasets for each time interval.

The extended areal coverage in 2013–14 indicated that aeolian 

sand transport was widespread through the archeological site and 

neighboring areas. The overall pattern of sand transport remained 

unchanged from previous monitoring periods (see Collins and 

others, 2012, and results for 2010–13 reported herein), with broad 

patches of aeolian erosion and deposition (deflation and inflation) 

30 to 50

10 to 30

Vertical distance between

surfaces, in centimeters

3 to 10

-3 to 3

-3 to -10

-10 to -30

-30 to -50

Erosion area and number

Deposition area and number

ER1

DEP1

ER3

ER2

DEP2

ER1

DEP1

EXPLANATION

Survey area boundary

Archeological site boundary

0 25 FEET

0 10 METERS

A

Figure 36. Landscape change at site AZ:C:13:0321. A 5-cm gridded topographic change output shows erosion (warm colors, negative) and 

deposition (cool colors, positive) from (A) September 2010 to May 2013, and (B) May 2013 to May 2014. Note difference in scales and area 

surveyed. Identified change is grouped by area labels (ER, erosion; DEP, deposition) that are cross-referenced with table 3.

Section III-Landscape Change at Archeological Sites Receiving Sand Supply After Controlled Floods 51
52 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

men16-3129_fig 36b

ER1

DEP1

ER2

DEP2

2014 Survey area boundary

2013 Survey area boundary

0 50 FEET

0 20 METERS

30 to 50

10 to 30

Vertical distance between

surfaces, in centimeters

3 to 10

-3 to 3

-3 to -10

-10 to -30

-30 to -50

Erosion area and number

Deposition area and number

ER1

DEP1

EXPLANATION

Archeological site boundary

B

Figure 36.—Continued
Table 3. Summary of topographic change at site AZ:C:13:0321.

[m/yyyy, month/year; m2, square meters; cm, centimeters; m3, cubic meters]

Site number and region Time period

(m/yyyy)

Area

(m2

)

Average depth 

(cm)

Volume

(m3)

AZ:C:13:0321 – ER1 9/2010–5/2013 85.2 -8 -6.90

AZ:C:13:0321 – ER2 9/2010–5/2013 84.1 -11 -8.87

AZ:C:13:0321 – ER3 9/2010–5/2013 93.0 -12 -11.38

AZ:C:13:0321 – DEP1 9/2010–5/2013 176.2 9 16.18

AZ:C:13:0321 – DEP2 9/2010–5/2013 135.1 7 9.14

AZ:C:13:0321 – ER1 5/2013–5/2014 193.9 -6 -12.11

AZ:C:13:0321 – ER2 5/2013–5/2014 309.9 -8 -23.56

AZ:C:13:0321 – DEP1 5/2013–5/2014 123.4 7 8.89

AZ:C:13:0321 – DEP2 5/2013–5/2014 236.8 7 16.13

adjacent to one another—indications of a tightly coupled fluvial–

aeolian transport system. Notably, during the 2013–14 monitoring 

period, more sediment was eroded (12.11 m3) than was transported 

into (8.89 m3) the archeological site boundary. This pattern was 

opposite to that which occurred during the previous, but longer, 

monitoring period (2010–13; table 3). Whereas erosion rates were 

similar during these intervals (-0.69 cm/yr for 2010–13, compared 

to -0.72 cm/yr for 2013–2014), depositional rates varied, with the 

2010–13 rate (0.64 cm/yr) being approximately 28 percent greater 

than that in 2013–14 (0.50 cm/yr). Considering the net volumetric 

effects normalized for variably sized survey areas and time scales, 

0.05 cm/yr of sediment was eroded from the entire surveyed area 

between 2010 and 2013, and 0.21 cm/yr was eroded between 

2013 and 2014. Owing to variability in weather conditions and the 

timing of controlled floods that enlarge the sandbar upwind of the 

site, such fluctuations are expected. However, the most important 

conclusion from this site is that significant changes can be related 

directly to aeolian sand supply from the adjacent fluvial sandbar.

AZ:B:10:0225

Site Description

Site AZ:B:10:0225 is located within a partly vegetated 

aeolian sand ramp that is backed by steep and sometimes 

overhanging sandstone ledges (figs. 37, 38). The ledges 

formed shelters for the prehistoric people who formerly 

inhabited the area, and the slope below these shelters is littered 

with pottery sherds, lithic debitage, fire-cracked rocks, and 

other artifacts (Fairley and others, 1994). Two large gullies 

(G1 and G2) traverse the edges of the site and fall steeply over 

sandstone ledges, terminating near the edge of the river in a 

deposit of large cobbles and boulders (20 cm to 1 m diameter). 

The gullies are filled with sand, presumably transported from 

active dunes immediately upwind. The 74,000-m2

 aeolian dune 

field surrounding site AZ:B:10:0225 is one of the most active 

in Grand Canyon, with as much as 70 percent active aeolian 

sand area; this site is within the Stevens-Conquistador Aisles 

(SCA) reach, where wind directions transverse to the main 

canyon orientation result in substantial sand transport inland 

from the river margins (Section II). Sand sources feeding this 

particular dune field include a fluvial sandbar upwind of the 

archeological site that has been enlarged by recent controlled 

floods. The site may also receive some windblown sand from 

patchy fluvial sand deposits covering a cobble bar across the 

river from the site. Vegetation in the area surrounding the 

archeological site includes catclaw acacia (Senegalia greggii) 

bushes, prickly pear (Opuntia sp.), alkali goldenbush (Isocoma 

acradenia), Mormon tea (Ephedra sp.), brittlebush (Encelia

sp.), and perennial bunchgrasses such as Indian ricegrass 

(Achnatherum hymenoides). Tamarisk (Tamarix ramosissima) 

and arrowweed (Pluchea sericea) border the site near the 

edge of the river and lower dune areas. O’Brien and Pederson 

(2009a) identified gullying, overland flow, aeolian transport, 

and creep affecting this site, and noted that gullying and creep 

were incrementally eroding artifacts.

Section III-Landscape Change at Archeological Sites Receiving Sand Supply After Controlled Floods 53
54 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

men16-3129_fig 37

Figure 38

Colorado River

Wind

EXPLANATION

Gully flow path and number

Survey area boundary

Archeological site boundary

Laser scan location

Shoreline at flow of 1,270 cubic

meters per second

Slope direction

0 25 50 FEET

0 10 20 METERS

G1

G1

G2

l

AZ:B:10:0225

Figure 37. Site AZ:B:10:0225 survey-area map.
men16-3129_fig 38

EXPLANATION

Gully flow path and number

G1

G1

G2

AZ:B:10:0225

Figure 38. Site AZ:B:10:0225 survey-area photograph showing gully locations (G1 and G2). View is toward the southwest.

Topographic Change (2010–13)

Topographic changes at site AZ:B:10:0225 between 2010 

and 2013 included massive gully-related erosion, and both erosion 

and deposition from aeolian sand-transport processes (fig. 39; 

table 4). Erosion within the surveyed area (which included nearly 

the entire archeological site) totaled 93.71 m3 and occurred over 

an area of 275.5 m2; deposition totaled 5.21 m3 and occurred over 

an area of 72.5 m2, yielding net erosion of 88.50 m3. The vast 

majority of change occurred from overland-flow-induced erosion 

of gully G2. There, more than 90 percent of the volumetric erosion 

was associated with gully widening and deepening, including 

as much as 1.3 meters of new incision in region ER2. This area 

also had experienced extensive gullying previously, detected in 

surveys prior to 2010 (Collins and others, 2012), and the most 

recent episode of erosion continued to widen the gully to the 

east. Some effects of gullying during this time period affected the 

archeological site, to the west of the gully (figs. 37, 39A), but for 

the most part, we detected little change within the main part of 

the archeological site. Some minor aeolian and fluvial reworking 

of sediments also occurred in isolated patches within gully G2 

(region DEP2), but the overall signal in and around that gully was 

erosional. We also identified apparent aeolian reworking of the 

sand surface in the vicinity of gully G1 (regions ER1, DEP1), but 

found little change to the archeological site itself.

Topographic Change (2013–14)

Topographic changes at site AZ:B:10:0225 between 2013 

and 2014 provided evidence for continued widespread changes 

to the prominent gully (G2) located on the eastern edge of 

the site (figs. 37, 39B). Overall erosion within the surveyed 

area totaled 50.52 m3 and occurred over an area of 313.7 m2; 

deposition totaled 5.19 m3 and occurred over an area of 61.1 m2, 

yielding net erosion of 45.33 m3. Aeolian reworking of sand 

near gully G1 (regions ER1 and DEP1; fig. 39B) occurred with 

a slightly greater erosional signal (in terms of volume change 

per unit time) compared to that measured between 2010 and 

2013 (fig. 39A). We also detected some erosion within the 

archeological site boundary, inferred to have resulted from 

both gully-wall slope adjustment and aeolian transport. The 

area in and around gully G2, notable owing to the amount 

of erosional change documented in past monitoring periods 

(Collins and others, 2012, as well as documented herein for 

2010–13) continued to experience widespread runoff-induced 

erosion. Gullying widened and deepened this area, removing 

nearly 43 m3 of previously deposited sand that had likely 

infilled the gully through a combination of aeolian transport 

and backwasting of the unconsolidated sand walls of the gully. 

Weather-station data and time-lapse photographs indicated that 

storms in July and August 2013, with associated maximum 

Section III-Landscape Change at Archeological Sites Receiving Sand Supply After Controlled Floods 55
56 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

men16-3129_fig 39a

X X'

ER2

DEP1

ER1

DEP2

G1

G2

Gully flow path and number

G1

X X' Trace of section

Survey area boundary

Deposition area and number

DEP1

Erosion area and number

ER1

EXPLANATION

Archeological site boundary

30 to 50

10 to 30

Vertical distance between

surfaces, in centimeters

3 to 10

-3 to 3

-3 to -10

-10 to -30

-30 to -50

-50 to -130

0 25 FEET

0 10 METERS

A

Figure 39. Landscape change at site AZ:B:10:0225. A 5-cm gridded topographic change output shows erosion (warm colors, negative) 

and deposition (cool colors, positive) from (A) September 2010 to May 2013, and (B) May 2013 to May 2014. Identified change is grouped 

by area labels (ER, erosion; DEP, deposition) that are cross-referenced with table 4. Line X–X ́ shows location of cross sections in (C). (C) 

Cross-sections across gully G2, showing sequential topographic change between September 2007 and May 2014. Data for September 

2007 and September 2010 sections are from Collins and others (2012).
men16-3129_fig 39b

X X'

ER2

DEP1

ER1

DEP2

G1

G2

ER3

DEP3

Gully flow path and number G1

X X' Trace of section

Deposition area and number

DEP1

Erosion area and number

ER1

EXPLANATION

Archeological site boundary

0 25 FEET

0 10 METERS

30 to 50

10 to 30

Vertical distance between

surfaces, in centimeters

3 to 10

-3 to 3

-3 to -10

-10 to -30

-30 to -50

-50 to -100

2014 Survey area boundary

2013 Survey area boundary

50 to 60

B

Figure 39.—Continued

Section III-Landscape Change at Archeological Sites Receiving Sand Supply After Controlled Floods 57
58 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

men16-3129_fig 39c

2007

2010

2013

2014

EXPLANATION

0

6

8

10

2

4

12

Distance along cross-section, in meters

0 4 8 16 20 24 28

Vertical distance, in meters

X X'

C

Figure 39.—Continued

Table 4. Summary of topographic change at site AZ:B:10:0225.

[m/yyyy, month/year; m2, square meters; cm, centimeters; m3, cubic meters]

Site number and region Time period

(m/yyyy)

Area

(m2

)

Average depth 

(cm)

Volume

(m3)

AZ:B:10:0225 – ER1 9/2010–5/2013 10.6 -9 -0.90

AZ:B:10:0225 – ER2 9/2010–5/2013 264.9 -35 -92.81

AZ:B:10:0225 – DEP1 9/2010–5/2013 36.3 8 2.70

AZ:B:10:0225 – DEP2 9/2010–5/2013 36.2 7 2.51

AZ:B:10:0225 – ER1 5/2013–5/2014 35.3 -8 -2.73

AZ:B:10:0225 – ER2 5/2013–5/2014 264.3 -18 -46.52

AZ:B:10:0225 – ER3 5/2013–5/2014 14.1 -9 -1.27

AZ:B:10:0225 – DEP1 5/2013–5/2014 16.9 7 1.20

AZ:B:10:0225 – DEP2 5/2013–5/2014 40.8 9 3.71

AZ:B:10:0225 – DEP3 5/2013–5/2014 3.4 8 0.28
10-minute rainfall intensities of 35–42 mm/hr, likely generated 

overland flows that enhanced the gullying, and that strong (>2 

m/s) winds in September and October 2013, and also during 

spring 2014, led to aeolian smoothing and reworking of the 

ground surface elsewhere at the site (Caster and Sankey, 2016).

Since the first lidar measurements at this site in September 

2007 (Collins and others, 2012), more than 260 m3 of sand have 

been lost from the G2 gully system. Whereas episodic stormdriven overland flow events have removed more than 1 m of 

sediment (vertically) in distinct locations throughout the gully, the 

overall result has been to shift the gully thalweg toward the east, 

away from the archeological-site boundary (fig. 39). The slope 

bordering the archeological site has remained at approximately 

32° inclination (near the angle of repose for dune sand; West, 

1995) as the gully has widened eastward. It is unclear whether this 

trend will continue, or whether the thalweg will avulse westward, 

in which case it could undermine the existing slope that forms 

the archeological-site boundary. However, it is clear that the area 

surrounding this site is undergoing rapid change and that aeolian 

infilling of gully G2 has not kept pace with runoff-induced erosion. 

New aeolian sand inputs from fluvial sandbars have not been 

detected definitively, and we infer that any new deposition has 

been a result of aeolian and runoff-generated reworking of existing 

sand at and near the site. Thus, although this type 1 site apparently 

has connectivity between an upwind fluvial sandbar and an active 

dune field downwind (containing the archeological site), and thus 

has the potential to receive aeolian sand supply that could increase 

the sand volume at the site itself, we have not resolved such a 

result at the site. The land surface within the archeological-site 

boundary, which contains moderate vegetation cover and biologic 

soil crust (fig. 38), has been unusually stable during the monitoring 

interval given its location in an otherwise relatively active dune 

field, which elsewhere contains well defined dune crests and slip 

faces of active, uncrusted, and unvegetated aeolian sand. This 

may represent an example of the site-detection bias mentioned in 

Section II, in that the exposure of archeological artifacts at the land 

surface is probably greater, and thus sites are more likely to be 

documented, in relatively inactive sand landscapes.

AZ:G:03:0072

Site Description

Site AZ:G:03:0072 is located within Colorado River sand 

deposits, approximately 30 m inland from the river (figs. 40, 41). 

The site is a prehistoric and protohistoric habitation area that 

contains sparse surface artifacts and numerous large mounds of 

fire-cracked rocks representing the remains of agave-roasting pits 

(Fairley and others, 1994). The upstream part of the archeological 

site (the focus of our analysis, herein named AZ:G:03:0072 US) 

consists of a convex hillslope situated between an alluvial terrace 

and the Colorado River. The hillslope is further bounded on one 

side by a boulder-choked overland-flow channel, and on the other 

side by steep talus slopes and basalt cliffs. Three gullies traverse 

this part of the site; two (G1 and G2) are founded on cobbles and 

boulders, whereas the other (G3) cuts through an active aeolian 

dune field. The dune field extends from gully G2 to the west edge 

of the site near the boulder-choked channel. Brush check dams 

were installed in gully G2 in May 2006 in an attempt to slow 

erosion (O’Brien and Pederson, 2009b). Vegetation in and around 

the site consists of native bunchgrasses, shrubs, and prickly pear 

cactus (Opuntia basilaris), along with taller groups of ocotillo 

(Fouquieria splendens), buckhorn cholla cactus (Cylindropuntia 

acanthocarpa), and creosote bush (Larrea tridentata) and catclaw 

acacia trees (Acacia greggii) several meters high. O’Brien and 

Pederson (2009a,b) identified gullying, creep, aeolian transport, 

and overland flow as the geomorphic processes affecting this site, 

and they noted that archeological features were actively being 

eroded by these processes.

Although the position of site AZ:G:03:0072 relative to an 

adjacent, upwind fluvial sandbar indicates type 1 classification 

presently, the riparian zone upwind of this site has experienced 

rapid vegetation growth over recent years (fig. 13). If the vegetation 

expansion continues, the site soon will need to be reclassified as 

type 2a, in which vegetation barriers are assumed to reduce the 

connectivity between fluvial and aeolian sedimentary systems, or 

type 3 if no open sand exists riverward of the vegetation.

Section III-Landscape Change at Archeological Sites Receiving Sand Supply After Controlled Floods 59
60 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

men16-3129_fig 40

Figure 41

Colorado River

G1

G2

G3

Wind

EXPLANATION

Survey area boundary

Archeological site boundary

Laser scan location

Shoreline at flow of 1,270 cubic

meters per second

Gully flow path and number

G1

0 40 80 FEET

0 10 20 30 METERS

Slope direction

AZ:G:03:0072

Figure 40. Site AZ:G:03:0072 survey-area map. 
men16-3129_fig 41

G1 G2

G3

EXPLANATION

Gully flow path and number

G1

Figure 41. Site AZ:G:03:0072 US survey-area photograph, showing gully locations (G1–G3). View is toward the west.

Topographic Change (2010–13)

Topographic changes at site AZ:G:03:0072US between 

2010 and 2013 included erosion and deposition related to 

both aeolian and overland flow processes (fig. 42A; table 5). 

Erosion within the entire surveyed area (which includes only 

the upstream part of the archeological site; figs. 40, 42) totaled 

6.27 m3 and occurred over an area of 78.2 m2. Deposition 

within this 2010–13 survey area totaled 5.86 m3 and occurred 

over an area of 99.6 m2, yielding net erosion of 0.41 m3. 

Widespread gully erosion had been documented previously at 

this site (Collins and others, 2009, 2012), and the volume loss 

measured over the 2010–13 interval was consistent with those 

previous observations. However, only one gully (G3, regions 

ER2 and DEP2) changed between 2010 and 2013, suggesting 

that storms during this time interval did not cause widespread 

runoff. Instead, inspection of topographic change along the 

gully walls revealed that topographic smoothing (backwasting) 

of the previously formed near-vertical sidewalls (see Collins 

and others, 2012) probably caused the wall erosion and 

deposition within the thalweg.

We detected widespread aeolian sand transport signals in 

two regions (ER1-DEP1 and ER3-DEP3), indicating deflation 

and inflation of the land surface. Comparison of the shapes, 

locations, and associated volumetric change between patches 

of erosion and deposition indicates that net sand transport 

was toward the northeast, coincident with the predominant 

wind direction measured here (Draut and others, 2010; 

Caster and others, 2014). Sand from region ER1 likely was 

deposited exclusively in region DEP1, and sand from region 

ER3 was transported to region DEP3. However, whereas the 

volumetric change in regions ER1 and DEP1 were nearly the 

same (within 10 percent; table 5), suggesting simple aeolian 

erosion from region ER1 and redeposition immediately 

downwind in region DEP1, the deposition in region DEP3 was 

approximately 25 percent greater than the erosion detected 

within the ER3 patches. This indicates that sand from outside 

the survey boundary (from a location southward, upwind and 

closer to the river, presumably the fluvial sandbar that receives 

new deposition during controlled floods) may have been 

introduced to the site area during this time interval.

Topographic Change (2013–14)

Topographic changes at site AZ:G:03:0072US between 

2013 and 2014 were similar to those detected during 2010–

2013, but with indications that precipitation-induced runoff 

was more active. Erosion within the entire surveyed area (a 

larger area than that surveyed in 2010) totaled 6.89 m3 and 

occurred over an area of 123.7 m2. Deposition within the 

2013–14 survey area totaled 5.11 m3 and occurred over an 

area of 102.0 m2, yielding net erosion of 1.78 m3. Erosion and 

deposition in all three gullies was detected and in gully G3, 

the volume of sediment eroded (from region ER4) was nearly 

three times as great as the volume deposited (in region DEP4). 

In gullies G1 and G2, small areas of deposition may indicate 

sediment storage above check dams that were installed in the 

early 2000s to slow archeological-site erosion (Pederson and 

others, 2006), or from natural landscape features providing 

temporary storage behind boulders or vegetation. We inferred 

aeolian reworking in the mid-site area (regions ER3 and 

DEP4) and south of gully G1 (regions ER5 and DEP5). Only 

in the far southern, upwind part of the site (regions ER6 and 

DEP6) were the volumes of mobilized sediment dissimilar, 

with three times as much erosion as deposition (table 5). 

Overall, the results suggest that little new sand came into 

the site (for example, from the upwind fluvial sandbar), 

despite the wind conditions having been favorable for aeolian 

transport from river level toward the archeological site (Caster 

and others, 2014).

Section III-Landscape Change at Archeological Sites Receiving Sand Supply After Controlled Floods 61
62 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

men16-3129_fig 42a

G1

G2

G3

DEP1

ER1

ER2

DEP2

DEP3

ER3

EXPLANATION

0 25 FEET

0 10 METERS

Gully flow path and number

G1

Survey area boundary

Archeological site boundary

Deposition area and number

DEP1

Erosion area and number

ER1

Vertical distance between

surfaces, in centimeters

10 to 30

3 to 10

-3 to 3

-3 to -10

-10 to -30

-30 to -50

A

Figure 42. Landscape change at site AZ:G:03:0072US. A 5-cm gridded topographic change output shows erosion (warm colors, negative) 

and deposition (cool colors, positive) from (A) October 2010 to May 2013, and (B) May 2013 to May 2014. Note difference in scales and area 

surveyed. Identified change is grouped by area labels (ER, erosion; DEP, deposition) that are cross-referenced with table 5.
men16-3129_fig 42b

G1

G2

ER2

DEP2

ER1

DEP1

DEP3

ER3

DEP4

ER4

ER5

DEP5

ER6

DEP6

G3

EXPLANATION

Gully flow path and number

G1

2014 Survey area boundary

2013 Survey area boundary

Archeological site boundary

Vertical distance between

surfaces, in centimeters

10 to 30

3 to 10

-3 to 3

-3 to -10

-10 to -30

Deposition area and number

DEP1

Erosion area and number

ER1

0 50 FEET

0 20 METERS

B

Figure 42.—Continued

Section III-Landscape Change at Archeological Sites Receiving Sand Supply After Controlled Floods 63
64 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

Table 5. Summary of topographic change at site AZ:G:03:0072US.

[m/yyyy, month/year; m2, square meters; cm, centimeters; m3, cubic meters]

Site number and region Time period

(m/yyyy)

Area

(m2

)

Average depth 

(cm)

Volume

(m3)

AZ:G:03:0072US – ER1 10/2010–5/2013 52.6 -8 -4.15

AZ:G:03:0072US – ER2 10/2010–5/2013 10.3 -12 -1.25

AZ:G:03:0072US – ER3 10/2010–5/2013 15.3 -6 -0.87

AZ:G:03:0072US – DEP1 10/2010–5/2013 58.9 6 3.78

AZ:G:03:0072US – DEP2 10/2010–5/2013 16.2 6 0.99

AZ:G:03:0072US – DEP3 10/2010–5/2013 24.5 4 1.09

AZ:G:03:0072US – ER1 5/2013–5/2014 0.6 -8 -0.05

AZ:G:03:0072US – ER2 5/2013–5/2014 3.6 -6 -0.23

AZ:G:03:0072US – ER3 5/2013–5/2014 27.3 -6 -1.57

AZ:G:03:0072US – ER4 5/2013–5/2014 15.7 -7 -1.13

AZ:G:03:0072US – ER5 5/2013–5/2014 47.2 -5 -2.49

AZ:G:03:0072US – ER6 5/2013–5/2014 29.3 -5 -1.42

AZ:G:03:0072US – DEP1 5/2013–5/2014 0.5 6 0.03

AZ:G:03:0072US – DEP2 5/2013–5/2014 2.0 6 0.12

AZ:G:03:0072US – DEP3 5/2013–5/2014 40.9 5 1.96

AZ:G:03:0072US – DEP4 5/2013–5/2014 7.3 6 0.41

AZ:G:03:0072US – DEP5 5/2013–5/2014 40.9 5 2.10

AZ:G:03:0072US – DEP6 5/2013–5/2014 10.4 5 0.49

Discussion: Landscape Change and Response to 

Controlled Floods

The dominant landscape-change signal in the four type 1 

sites that we analyzed in Marble–Grand Canyon has been that of 

erosion (fig. 43). Since monitoring began at these sites between 

2006 and 2010 (depending on the particular site), 74 percent of 

the detected change (as measured by the percent surface area per 

year) has been from erosion. Even at these type 1 sites, which 

represent ‘best-case scenarios’ for maintaining sediment cover 

through potential aeolian sand supply from fluvial sources, overall, 

more site area underwent erosion than deposition over any given 

monitoring interval (fig. 43). Over nearly a decade of landscapechange detection, our research group has documented only two 

instances when sand deposition slightly outpaced erosion (by area) 

in any site-monitoring interval (at site AZ:C:13:0321 between 

2010 and 2013, and at site AZ:G:03:0072US between 2010 and 

2013; fig. 43). During the interval reported herein (2010–14), the 

volume of upland landscape erosion (245.72 m3

, which excludes 

probable fluvial erosion that occurred within the AZ:C:05:0031 

survey area but not within or near the archeological site boundary; 

areas ER4, DEP4, and DEP5 in fig. 33), was nearly three times 

the volume of deposition (85.55 m3). The finding that volumetric 

change from aeolian inflation and deflation (193.07 m3) was 40 

percent greater than that from overland flow (138.19 m3) indicates 

the importance of aeolian transport in shaping landscapes in which 

archeological sites have high connectivity to fluvial sand sources. 

This conclusion is supported by the areal landscape-change data 

(the percent of the total changed area per year, again not including 

apparently fluvial changes at AZ:C:05:0031). These data indicate 

that the proportion of area influenced by aeolian processes since 

monitoring began is more than double that influenced by runoff 

processes (70 percent compared to 30 percent; fig. 43). This 

difference reflects the fact that changes caused by runoff generally 

are focused on isolated, narrow gully channels, whereas aeolian 

inflation and deflation act across wider regions and greater area 

overall.

We can use the detailed topographic changes detected at 

these four type 1 archeological sites in Marble–Grand Canyon 

to address the science questions posed at the beginning of this 

section. Regarding the potential for the aeolian sand supply from 

controlled-flood sandbars to reach type 1 sites, we found conclusive 

evidence at one site (AZ:C:13:0321), and potential linkages at the 

three others (AZ:C:05:0031, AZ:B:10:0225, and AZ:G:03:0072), 

that direct aeolian transport pathways between fluvial sandbars 

and downwind dune fields can provide archeological sites with 
men16-3129_fig 43

EXPLANATION

Runoff Aeolian

Runoff Aeolian

Erosion

Induced by:

Deposition

10 to 50 50 to 100 100 to 200 200 to 400 400 to 1,000

Rate of change,in square meters per year (m2/y)

AZ:C:05:0031

AZ:C:13:0321

AZ:B:10:0225

AZ:G:03:0072US

(2,500 m2)

(3,200 m2)

(1,700 m2)

(2,600 m2)

Mar.JuneSept.Mar.JuneSept.Mar.JuneSept.Mar.JuneSept.Mar.JuneSept.

2006 2007 2008 2009 2010

Mar.JuneSept.Mar.JuneSept.Mar.JuneSept.Mar.JuneSept.

2011 2012 2013 2014

Archeological 

site and area

Figure 43. Summary of landscape change at four type 1 archeological sites in the Colorado River corridor of Marble–Grand Canyon over 

various monitoring intervals (data for 2006–10 from Collins and others, 2009, 2012). Pie size indicates the total area of erosion and deposition, 

normalized by the length of time between measured changes. Hatching pattern indicates the primary geomorphic process inferred to have 

caused the change. Values in parentheses indicate average total area where change detection was investigated. Dashed lines are dates of data 

collection; pie locations indicate measured change between times represented by adjacent dashed lines.

new sand. At site AZ:C:13:0321, direct effects of the recent 

controlled floods in building a fluvial sandbar adjacent to the 

site, combined with a near-optimal prevailing wind direction, 

resulted in net sediment influx to the site between 2010 and 2013 

(fig. 36A, table 3). Although that site lost sand volume overall 

between 2013 and 2014, our results indicate that at least some sand 

continued to move into the site (fig. 36B). At the other three sites 

some aeolian deposition also occurred, but it was unclear whether, 

during these time intervals, deposition resulted from reworking of 

existing aeolian sand at the sites (which ultimately derives from 

fluvial deposits), or whether sand from newly formed sandbars 

was transported into the site area. Regardless, we identified 

transport evidence with wind directions indicating that sand 

could be transported to these sites from fluvial sandbars formed 

during controlled floods. Examples of this are most clear at sites 

AZ:C:05:0031 and AZ:G:03:0072 (figs. 33A,B, 42B).

However, our measurements indicate that the presence of 

a recent controlled-flood sandbar upwind of a type 1 site does 

not necessarily lead immediately to net depositional conditions 

at the downwind archeological site (our first science question of 

Section III), although we have shown that situation to be possible 

at some sites over some time intervals (fig. 43). Only one site 

(AZ:C:13:0321) shows clear evidence that new sand coming 

from outside the site, sourced from a controlled-flood sandbar, 

has deposited in such a way that it covered previously exposed 

archeological features over our measurement time scales. At the 

other sites, cultural features may have been protected by new 

aeolian sand deposition, but there is insufficient data to determine 

whether this is from aeolian reworking of existing sand sourced at 

or near the site (which ultimately originated from fluvial sandbars, 

but took longer than our annual measurement interval to travel 

from the sandbar to the downwind site) or a direct result of recent 

controlled-flood sandbar growth. Answering this part of the 

question may require either longer-term monitoring, or additional 

controlled floods at different flow conditions and seasons to 

maximize sandbar building and subsequent aeolian sand transport.

Our second science question of Section III, whether sites 

with active aeolian deposition and substantial gully incision 

undergo net sediment loss and topographic lowering such 

that archeological features degrade, can be answered using 

the two sites that experienced extensive gullying over the 

monitoring period—AZ:B:10:0225 and AZ:G:03:0072. At 

site AZ:B:10:0225, repeated overland flow events in gully G2 

removed more than 260 m3

 of sediment over approximately 

7 years, lowering the surface elevation immediately east of 

the archeological site by as much as 2.2 m in some places 

Section III-Landscape Change at Archeological Sites Receiving Sand Supply After Controlled Floods 65
66 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

(fig.39C). That gully drains a steep bedrock watershed, and 

pours over sandstone ledges before forming a gully through 

active dune sand, thereby representing a landscape likely to 

undergo repeated overland-flow incision. Despite the large 

magnitude of erosion in the gully itself, we detected relatively 

little change within the boundaries of the adjacent archeological 

site, which underwent only minor erosion along its west 

boundary related to gully bank slumping (fig. 39A) and minor 

aeolian reworking of the surface (fig. 39B). Instead, most 

erosion focused on the side of the gully located away from the 

archeological-site boundary (fig. 39C), which could be related 

either to the presence of weaker, more erosive substrate that 

underwent backwasting or to slight realignment of the incoming 

source of overland flow. Depending on variations in these 

processes, topographic lowering eventually could affect the 

archeological site (for example, if the gully flowpath changes 

orientation again). At site AZ:G:03:0072, repeated erosion and 

incision have occurred in gully G3 over 8 years (this study and 

Collins and others, 2009, 2012). Recent monitoring (fig. 42) 

indicates that incision has lowered the thalweg elevation by 

30 cm locally and consequent bank slumping has widened the 

active gully channel by several meters since the initial survey in 

2006. Thus, site AZ:G:03:0072 provides an example of repeated 

gully incision causing overall topographic lowering of parts of 

the archeological site.

At both sites AZ:B:10:0225 and AZ:G:03:0072, our 

observations imply that without enhanced aeolian sand input to 

the gully areas, overland flow erosion and gully backwasting will 

continue to cause net sediment loss, likely with eventual negative 

effects for the integrity of the archeological sites. Thus, to answer 

our second question—site erosion in and adjacent to gullies 

is tied to a complex set of factors including substrate erosive 

susceptibility and the tendency of gullies to avulse or meander 

away from a static channel alignment in one direction or another, 

processes with some degree of stochasticity that makes detailed 

site-scale assessments or predictions difficult.

Section IV - Landscape Change at 

Archeological Sites in a SedimentStarved Reach: Glen Canyon 

Background

In Section III, we showed that in some cases the erosion of 

river-corridor archeological sites in sand-dominated landscapes 

can be counteracted partly by deposition of aeolian sand supplied 

from fluvial sandbars. The sites where we measured landscape 

change in Grand Canyon National Park were identified as bestcase scenarios for site preservation—sites that maintain high 

connectivity with active fluvial sand sources (type 1 sites in our 

classification system; Section I) and with active aeolian sand 

movement (Section II). However, we also showed that landscape 

change at type 1 sites can include erosion by overland flow and 

gullying, aeolian deflation, or both, and that erosion and sediment 

loss exceeded deposition and sediment gain at most sites over 

most intervals. The interplay between erosion and annealing 

processes is paramount to identifying why, where, and how rapidly 

archeological sites erode.

In this section, we investigated topographic change to 

infer landscape processes at archeological sites in a sedimentstarved section of the Colorado River, where aeolian inflation 

is expected to be minimal or absent—in Glen Canyon, the 

Colorado River reach immediately downstream from Glen 

Canyon Dam. We studied four sites in Holocene terrace deposits 

of Glen Canyon, in which mainstem fluvial sediment interbeds 

with slopewash material (for example, Pederson and others, 

2011). The geomorphic setting of the Glen Canyon sites forms 

an end-member case to contrast with the type-1 sites in Marble–

Grand Canyon, in terms of the likelihood of gully annealing by 

aeolian sand activity. We contrasted sites in these two endmember settings to evaluate whether the type 1 sites would 

erode less rapidly than would the Glen Canyon sites, which are 

essentially disconnected from sediment supply from modern 

sandbars—sites exhibiting the best- and worst-case scenarios 

for modern preservation potential, respectively.

The Glen Canyon reach appears particularly prone to 

widespread gully erosion, owing to a combination of inherent 

geomorphic conditions and pronounced sediment starvation 

in postdam time (Section II). The geomorphic setting of Glen 

Canyon (including a lack of large debris fans, and thus no 

strong segmenting of hydraulic control) resulted in deposition of 

extensive Holocene fluvial terraces. The terraces, which are close 

to the river and contain a high concentration of archeological sites, 

are vulnerable to gully erosion and today have extremely low 

potential for either aeolian inflation of the terrace surfaces or gully 

annealing by aeolian sand transport, because this reach has almost 

no modern aeolian sand supply (with very few modern fluvial 

sandbars) and a low proportion of active aeolian sand area (Section 

II; figs. 23, 24). The inactive sand within the Glen Canyon reach 

contains more gully area per unit sand area than do any of the 

reaches we studied in Marble–Grand Canyon (fig. 24). Gullies in 

Glen Canyon have grown very large locally, and in some places 

have incised entirely through the sedimentary terraces to bedrock, 

degrading and removing archeological material. This is generally 

consistent with the findings of Pederson and O’Brien (2014) in 

Marble–Grand Canyon that wide Holocene terrace deposits tend 

to host cultural sites with acute gully erosion.

In this section we report our findings from high-resolution 

landscape-change detection at four archeological sites in Glen 

Canyon, and concurrent measurements of weather events to 

which landscape changes can be linked. The four sites, numbered 

AZ:C:02:0032, AZ:C:02:0035, AZ:C:02:0075, and AZ:C:02:0077 

(fig. 29), all exhibit representative landscape processes occurring 

commonly in the Glen Canyon reach. The sites were monitored 

twice with terrestrial lidar, once in mid-September 2012 and again 

in early November 2013, thereby bracketing the November 2012 

controlled flood (table 1). Our methods were similar to those 

in Section III for landscape-change detection and examination 
Section IV - Landscape Change at Archeological Sites in a Sediment-Starved Reach: Glen Canyon 67

of possible controlled-flood effects. We analyzed geomorphic 

factors that might influence comparison of site responses between 

Glen Canyon and Marble–Grand Canyon. We also analyzed 

and compared historical rainfall records from Glen Canyon and 

Marble–Grand Canyon, to investigate whether the intensive 

gullying in Glen Canyon may relate to regional precipitation 

trends. Understanding climatic and geomorphic process potential 

more fully will allow for future development of site-monitoring 

and mitigation plans that best address the unique geomorphic 

setting and likely trajectory for river-corridor cultural sites in Glen 

Canyon National Recreation Area.

Research Questions

• How do rates and processes of landscape change at Glen 

Canyon archeological sites compare to those measured in 

Marble–Grand Canyon, where the modern sand supply is 

greater?

• Are weather conditions in Glen Canyon more conducive to 

causing gully erosion than in Marble–Grand Canyon?

Methods

Similar to the methods outlined in Section III, we used a 

combination of site-specific topographic, meteorological, and 

time-series photographic monitoring to document changes at the 

four selected archeological sites in Glen Canyon. In contrast to the 

multi-year time interval analyzed in Marble–Grand Canyon, our 

Glen Canyon data-collection interval spanned just longer than one 

year (September 2012 to November 2013).

Topographic Change Detection

Our methods for identifying site-specific landscape change 

at Glen Canyon used identical terrestrial-lidar data collection, 

processing, and analysis protocols as described in Section III. 

We also collected airborne lidar data of the Glen Canyon sites in 

July 2013 as part of an effort to analyze the geomorphology of 

a longer section of Glen Canyon. Those results are presented by 

Collins and others (2014) and are not included here. However, 

we rely on the geomorphologic interpretations in that report to 

infer likely causes of change that we detected during the present 

study.

Geomorphologic Analysis

We conducted comparative qualitative and quantitative 

geomorphic assessments at the four type-1 archeological sites 

studied in Marble–Grand Canyon (Section III) and the four 

Glen Canyon sites investigated herein. We compared site 

substrates, gully geometry, site topography, and the watershed 

area draining into each of these sites. Using these parameters, 

we assessed whether site geomorphic setting, taken as a 

whole, could explain any differences in net topographic 

change between our best-case (type 1) and worst-case (Glen 

Canyon terrace) study areas.

Lacking consistent infiltration measurements, we assessed 

substrate differences qualitatively through observations of 

generalized surface sediment. We compared gully geometry 

quantitatively by using gully width and depth measurements from 

our terrestrial-lidar data. Similarly, we compared topographic 

slope and drainage area quantitatively among the sites by 

analyzing available lidar data, including 1-m digital elevation 

models (DEMs) of the Colorado River corridor (for example, 

Kaplinski and others, 2014) and 10-m DEMs for watershed 

areas outside the river corridor (http://nationalmap.gov/elevation.

html). After calculating flow direction and accumulation using 

the 10-m-resolution data, we defined pour points at each of the 

survey-area boundaries and calculated watershed area upslope 

from those points. Using the 1-m DEM data, we then calculated 

flow direction and accumulation within the river-corridor area of 

each site. We integrated the watershed polygons based on these 

accumulations with those previously obtained for the 10-m data 

to calculate an up-watershed area for each site boundary. We 

finalized these watershed areas by using Google EarthTM imagery 

and high-resolution orthoimagery (http://earthexplorer.usgs.gov/) 

to define apparent drainage pathways above the river corridor. 

Where apparent drainage paths differed in distance or direction 

from the 10-m-DEM estimates, the watershed was modified to 

match the most probable contributing area; this final step allowed 

inclusion of rock fractures that were visible in aerial imagery 

but not adequately represented in the 10-m DEM. This analysis 

yielded conservative estimates of contributing watershed area, 

while representing the clearest flow paths from upper watersheds 

to the site boundaries.

Meteorological Monitoring

We deployed one weather station in Glen Canyon and 

one near Lees Ferry in 2013 with identical characteristics to 

those described earlier (Section III; Caster and others, 2014). 

Although incomplete records exist from these stations for the 

time bracketing the landscape-change data collection, we used 

the available weather data to identify wind and rain events that 

might have caused landscape change.

In addition to relating weather events to specific topographic changes, we analyzed spatial variability in weather 

by using data from the stations deployed in Glen and Marble–

Grand Canyons (hereafter referred to as USGS inner-canyon 

stations; Draut and others, 2009a,b, 2010; Caster and others, 

2014; Dealy and others, 2014), and three National Weather 

Service Cooperative Observer Program (COOP) stations 

located in and near the canyon (fig. 1; National Oceanic and 

Atmospheric Administration, 2014). We analyzed rainfall 

data for three time intervals: 2013–2014, to overlap with the 

landscape-change analyses of this study; October 2007 to 

October 2010, the interval from which USGS inner-canyon 

records are most extensive; and 1952–2012, the most complete 

COOP regional climate record.
68 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

This analysis focused specifically on wind speed and 

rainfall. We analyzed rainfall amount and intensity, factors that 

contribute to overland-flow erosion (Collins and others, 2016). 

Although daily rain accumulation (depth) can be calculated 

directly from USGS inner-canyon stations and COOP stations, 

only the USGS inner-canyon data has sufficient resolution 

to permit calculation of subdaily rainfall intensity. To 

estimate rainfall intensity at COOP stations for the purpose 

of understanding long-term rainfall variability, we developed 

regression relations between daily rain depth and maximum 

daily 10-minute intensity, for each rain season recorded at 

the USGS inner-canyon stations (Caster and Sankey, 2016). 

Rain seasons follow the definitions of Hereford and others 

(2014) for Grand Canyon, with the so-called cool season from 

October 8 to March 31, dry season from April 1 to June 31, 

and warm (monsoon) season from July 1 to October 7. The 

seasonal depth-intensity regression equations were applied 

to long-term National Weather Service COOP daily rainfall 

records to estimate maximum daily 10-minute intensities from 

1952 to 2012. Additional details of methods that we used to 

estimate the relationships at the USGS inner-canyon stations 

and apply them to the COOP records are presented by Caster 

and Sankey (2016).

Maximum wind-gust speed was compared for 

92,355 4-minute intervals concurrently recorded during 

approximately one year (February 13, 2014 to February 

5, 2015), at six USGS inner-canyon stations within Glen 

and Marble–Grand Canyon. In addition to comparing these 

parameters, we assessed aeolian sand-transport potential by 

calculating the amount of time that maximum gust speed 

exceeded three possible sand-transport thresholds: 2 m/s 

(Draut and Rubin, 2005, 2008), 5 m/s (Stout, 2004; Liu 

and others, 2015), and 8 m/s (Baudat and Breed, 1999), 

with the lowest examined wind speed (2 m/s) representing 

the typical threshold for dry sand transport within Grand 

Canyon (assuming a grain size of fine sand and Bagnold-type 

entrainment; Bagnold, 1941; Draut and Rubin, 2005, 2008).

Time-Series Photography

Although we did not initiate time-series photographic 

monitoring in Glen Canyon, we did utilize a pre-existing 

dataset of the AZ:C:02:0032 archeological-site area dating 

back to 1992, provided by the Aquatics Program and Cultural 

Resources Program staff at Glen Canyon National Recreation 

Area. The dataset includes images (at varying temporal 

intervals, often daily) showing most of the archeological site 

and the steep riverbank that forms one of the boundaries of 

the site. We used these photographs to examine the effects 

of fluvial inundation on the toe of the river cutbank during 

previous controlled floods.

Topographic Change Detection

AZ:C:02:0077

Site Description

Site AZ:C:02:0077 consists of a sparse scatter of flaked 

stone distributed on the surfaces of two adjoining fluvial 

terraces, with an erosional cutbank (terrace scarp) separating 

the two terrace surfaces (figs. 44, 45). The site also contains 

buried archeological features, particularly in the upper 

terrace, and may contain some of the oldest cultural remains 

in Glen Canyon (Anderson, 2006; Spurr and Collette, 2007; 

Collins and others, 2014). The terraces are covered mainly 

with inactive, biocrusted river-derived sand and some 

gravel, with active aeolian sand patches to the north of the 

site. Previous archeological investigations mentioned active 

aeolian processes at the site, as well as rodent burrowing and 

social trailing from a nearby campground (Neff and Wilson, 

2002). Aeolian dune forms are not well developed on the 

terrace surfaces, however. Owing to the upwind presence of 

a Colorado River shoreline that has received controlled-flood 

sand deposition in the past, one of the few such places in Glen 

Canyon, as well as a dense band of vegetation between the 

shoreline and the site, we considered this to be a type 2a site 

(upwind fluvial sand source but with vegetation barrier; fig. 9). 

The upper terrace edge is approximately 100 m landward from 

the edge of the river, and vegetation covers the riverward 75 

percent of the lower terrace. The vegetation is differentiated 

qualitatively by the two terrace surfaces, with Tamarisk trees 

(Tamarix ramosissima), Drummond’s goldenbush (Isocoma 

drummondii), and fourwing saltbush (Atriplex canescens) 

found predominantly on the lower terrace. Fourwing saltbush 

(Atriplex canescens) is also found on the upper terrace, along 

with grasses, Mormon tea (Ephedra sp.), and snakeweed 

(Gutierrezia sp.) shrubs, puncture vine (Tribulus terrestris), 

and Engelmann’s hedgehog cacti (Echinocereus engelmannii).

Several gullies are incised into the upper (presumably 

older) terrace scarp, and sediment from these gullies forms 

small alluvial fans on the lower, younger terrace surface. 

Collins and others (2014) found recent gully incision to be 

minor over the majority of the terrace with the exception of 

the deeper gully (G3) located to the south of the site, which 

drains a larger watershed emanating from two deep clefts in 

the adjacent bedrock cliffs. The smaller gullies, G1 and G2, 

originate in terrace sediment, not in bedrock catchments. 

The lack of severe gully incision at this site, especially given 

the position of G3 at the outlet of bedrock catchments, was 

attributed to high soil infiltration capacity in the lower terrace 

(Collins and others, 2014).
Section IV - Landscape Change at Archeological Sites in a Sediment-Starved Reach: Glen Canyon 69

men16-3129_fig 44

G1

G2

G3

Wind

Figure 45

EXPLANATION

Survey area boundary

Archeological site boundary

Laser scan location

Upper terrace boundary

Slope direction

Gully flow path and number G1

0 50 100 150 FEET

0 25 50 METERS

AZ:C:02:0077

Figure 44. Site AZ:C:02:0077 survey-area map.
70 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

men16-3129_fig 45

G2 G1

G3

Colorado River

EXPLANATION

Gully flow path and number

G1

Figure 45. Site AZ:C:02:0077 survey-area photograph, showing gully locations (G1–G3). View is toward the west.

Topographic Change (2012–13)

We analyzed landscape change at site AZ:C:02:0077, 

spanning the entire archeological-site boundary and adjacent 

terrace areas (fig. 46) including an apparently active aeolian 

sand area northwest of the site, to assess the potential for 

aeolian sand transport to the site (the dominant wind direction 

being generally toward the south; fig. 44). Overall, less than 

0.1 percent of the archeological-site area underwent change 

that exceeded our detection threshold (3 cm). Topographic 

changes observed between 2012 and 2013 consisted of small 

(<1 m2) erosional areas within the archeological-site boundary 

(regions ER3, ER4, ER5) and a larger area of aeolian erosion 

and deposition at the north boundary of the site (regions ER1, 

DEP1; table 6).

The somewhat large magnitude of erosional downcutting 

(9 to 19 cm) compared to the small gully area over which we 

detected change (limited to a few square meters in unnamed 

gullies; fig. 46) suggests that downcutting by overland flow was 

limited to surficial erosion of the steeper areas of the terrace edge, 

at least over this measurement interval. Moreover, neither the ER3 

nor the ER4 erosion area connects to a larger upstream gully, and 

neither the G1 nor the G2 gully is incised; these gullies are subtle, 

shallow, ephemeral features (Collins and others, 2014). These 

observations, together with a lack of substantial gullying below 

the terrace edge separating the lower and upper terraces, indicate 

that precipitation-induced gullying is not a prevalent geomorphic 

process at this site, with the exception of flow paths directly fed by 

bedrock-sourced overland flow (gully G3, which did not change 

during this monitoring interval).

The spatial pattern of the larger areas of change identified 

in the northwestern part of the survey area (ER1 and DEP1; 

fig. 46) suggests aeolian processes, with change having occurred 

over 422.8 m2

. Areas of change consist of neighboring patches 

of erosion and deposition (deflation and inflation), and the 

volume of aeolian erosion (in region ER1) and deposition (in 

region DEP1) are nearly identical (approximately 17 m3 each; 

table 6). The spatial distribution of deposition in an arcuate zone 

south of the main area of erosion suggests a generally north-tosouth wind direction, consistent with that measured by a nearby 

meteorological station during fall and winter 2013 (Caster and 

others, 2014), which indicates a wind vector-sum direction 

from due north. This area is part of a broad, active aeolian sand 

region that extends approximately 250 m north of the site. 

However, this area of inferred aeolian sand activity is entirely 

above the inundation stage of controlled floods (approximately 

1,200 m3/s), and there are no geomorphic indications that 

controlled-flood sand has been transported through the 

vegetation into the area that neighbors the archeological site. 

Thus, we infer no connectivity between modern fluvial sand 

deposits and the downwind aeolian sand landscape in the 

vicinity of the archeological site. Instead, the aeolian sand 

activity likely reflects reworking of a predam flood deposit, 

and indeed wind-scour features that we observed on the terrace 

surface around region ER1 imply that the wind is removing 

predam flood sediment there and redepositing it downwind.

Table 6. Summary of topographic change at site AZ:C:02:0077.

[m/yyyy, month/year; m2, square meters; cm, centimeters; m3, cubic meters]

Site number and region Time period (m/yyyy) Area (m2) Average depth (cm) Volume (m3)

AZ:C:02:0077 – ER1 9/2012–11/2013 168.0 -10 -16.97

AZ:C:02:0077 – ER2 9/2012–11/2013 0.6 -19 -0.11

AZ:C:02:0077 – ER3 9/2012–11/2013 0.6 -15 -0.09

AZ:C:02:0077 – ER4 9/2012–11/2013 2.7 -9 -0.25

AZ:C:02:0077 – DEP1 9/2012–11/2013 254.8 7 17.33
Section IV - Landscape Change at Archeological Sites in a Sediment-Starved Reach: Glen Canyon 71

men16-3129_fig 46

G1

ER1 ER2

ER3

ER4

DEP1

G2

G3

Gully flow path and number

G1

EXPLANATION

Survey area boundary

Archeological site boundary

Vertical distance between

surfaces, in centimeters

30 to 50

10 to 30

3 to 10

-3 to 3

-3 to -10

-10 to -30

-30 to -50

Deposition area and number

DEP1

Erosion area and number

ER1

0 75 FEET

0 30 METERS

0 15 FEET

0 5 METERS

Figure 46. Landscape change at site AZ:C:02:0077. A 5-cm gridded topographic change output showing erosion (warm colors, 

negative) and deposition (cool colors, positive) between September 2012 and November 2013. Identified change is grouped by area 

labels (ER, erosion; DEP, deposition) and cross-referenced with table 6.
72 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

AZ:C:02:0075

Site Description

Site AZ:C:02:0075 is just downstream from site 

AZ:C:02:0077 in an area where the lower and upper terraces 

narrow to form a 20-m-wide zone between the Colorado River 

and adjacent bedrock cliffs (figs. 47, 48). Two areas of artifacts 

and buried cultural features are located within the terrace deposits, 

separated by a deeply incised gully (G3), and erosion probably 

has removed a substantial amount of archeological material from 

the site. The terrace surface is similar in composition to that at 

site AZ:C:02:0077, and near the river margin the terrace surface 

includes inactive, as well as possibly active, aeolian sand deposits. 

Terrace vegetation is similar to that at site AZ:C:02:0077, and 

includes taller (1 to 2 m) Tamarisk trees (Tamarix ramosissima) 

near the river on the remnants of the lower terrace and irregularly 

spaced grasses, Mormon tea (Ephedra sp.) shrubs, and 

Engelmann’s hedgehog cacti (Echinocereus engelmannii) on the 

upper terrace.

Wind

Figure 48

G1

G2

Colorado River

EXPLANATION

Survey area boundary

Archeological site boundary

Laser scan location

Shoreline at flow of 1,270 cubic

meters per second

Slope direction

Gully flow path and number

G1

Bedrock gully

0 50 100 FEET

0 20 40 METERS

AZ:C:02:0075

Figure 47. Site AZ:C:02:0075 survey-area map.
Section IV - Landscape Change at Archeological Sites in a Sediment-Starved Reach: Glen Canyon 73

men16-3129_fig 48

G1

G2

EXPLANATION

Gully flow path and number

G1

AZ:C:02:0075

Figure 48. Site AZ:C:02:0075 survey-area photograph showing gully locations (G1, G2). View is toward the west.

A deep (approximately 3 m) gully traverses much of the 

site, and is incised entirely through the terrace sediment to 

bedrock in the lower parts of gullies G1 and G2; the upper 

parts of these gullies traverse the terrace surfaces but do not 

incise all the way to bedrock. Previous investigations identified 

widespread gullying as the predominant geomorphic process 

active at the site, but also mentioned aeolian processes, bedrocksourced overland flow, rodent burrowing, and visitor impacts 

by way of social trails from a nearby campground (Neff and 

Wilson, 2002; Collins and others, 2014). Although a narrow 

sandbar is sometimes subaerially exposed upwind of the site, 

we have not observed modern controlled-flood sand deposition 

there. Thus, we classify this site as type 3 (having an upwind 

shoreline but no recently deposited fluvial sand; fig. 9). Fluvial 

erosion by the Colorado River is also possible here at high 

flows; water would have pooled in the bedrock-floored part 

of the gullies during the high flows of 1983, based on our 

observations at this site during the 2012 controlled flood.

Topographic Change (2012–13)

We analyzed topographic change over the entire 

archeological site and adjacent terrace area (fig. 49), focusing 

especially on the steep sidewalls of the gullies that traverse 

the site. Over this monitoring interval, nearly 10 percent of the 

archeological-site area underwent change greater than 3 cm. 

Measured topographic changes between 2012 and 2013 were 

all erosional and occurred over 211.5 m2 of surface area.

The majority of erosion (in region ER3) occurred as a 

result of channelized gullying in gully G2, where 25.74 m3 of 

the 2-m-tall vertical gully cut bank fell away. We measured no 

deposition at this site, indicating that gully flow transported all 

of the newly eroded material directly into the Colorado River. 

Although rainfall data are incomplete at the station closest to 

the site for the September 2012 to November 2013 timeframe 

in which this gullying occurred, the partial record (Caster and 

others, 2014) and unpublished data from local rain gages (D.J. 

Topping, USGS Grand Canyon Monitoring and Research 

Center, written commun., 2015) indicate that several large, 

high-intensity storms occurred in summer 2013. The 10-minute 

maximum rainfall intensities exceeded 30 mm/hr on at least 4 

days during this time and reached 72 mm/hr during one event 

in late August (Caster and others, 2014). This intensity exceeds 

the threshold (64 mm/hr) determined by Collins and others 

(2016) to cause gullying at other archeological sites in Grand 

Canyon that have river-derived sediment as the substrate. In 

addition, a storm on August 2, 2013, caused the largest flooding 

documented in 14 years (2001–14) in Water Holes Canyon, 

a tributary that joins the Colorado River approximately 5 km 

from site AZ:C:02:0075 (D.J. Topping, written commun., 2015), 

suggesting that flow depths may have been substantial in the 

bedrock-incised gullies during that August 2013 event.

The two other areas of erosion (regions ER1 and ER2) 

were immediately adjacent to the gullies, but on the terrace 

surface rather than in the gully bottom. The spatial pattern 

of erosion indicated that it occurred by aeolian reworking of 

on-site sediment. Erosion removed 13.52 m3

 of sediment, with 

mean erosion depths of 7 and 11 cm for regions ER1 and ER2, 

respectively (table 7). Given the prevailing wind direction 

from the north (Caster and others, 2014), we might have 

expected deposition south of the site, but detected no new 

deposition anywhere within the surveyed area. Therefore, we 

assume that aeolian deflation exported sand from the survey 

area, possibly with some blown into the gullies and then 

removed by overland flow into the Colorado River.
74 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

men16-3129_fig 49

G2

G1

ER2

ER1

EXPLANATION

Gully flow path and number

G1

Survey area boundary

Archeological site boundary

Vertical distance between

surfaces, in centimeters

0 to -3

-3 to -10

-10 to -30

-10 to -30

-30 to -50

Erosion area and number

ER1

Bedrock gully

0 75 FEET

0 25 METERS

ER3

Figure 49. Landscape change at site AZ:C:02:0075. A 5-cm gridded topographic change output showing erosion (warm colors, 

negative) from September 2012 to November 2013. No deposition was detected. Identified change is grouped by area labels (ER, 

erosion) and cross-referenced with table 7.

Table 7. Summary of topographic change at site AZ:C:02:0075. 

[m/yyyy, month/year; m2, square meters; cm, centimeters; m3, cubic meters]

Site number and region Time period (m/yyyy) Area (m2) Average depth (cm) Volume (m3)

AZ:C:02:0075 – ER1 9/2012–11/2013 56.5 -7 -3.79

AZ:C:02:0075 – ER2 9/2012–11/2013 85.4 -11 -9.73

AZ:C:02:0075 – ER3 9/2012–11/2013 69.6 -37 -25.74
Section IV - Landscape Change at Archeological Sites in a Sediment-Starved Reach: Glen Canyon 75

AZ:C:02:0032

Site Description

Site AZ:C:02:0032 is located along and at the top of a terrace 

adjacent to an approximately 11-m-tall cutbank (height measured 

while river flow was 226 m3/s) bordering the Colorado River (figs. 

50, 51). Archeological material includes several lenses of charcoalstain deposits at the site, along with grinding slabs, ceramics, and 

a probable hearth (Anderson, 2006; Spurr and Collette, 2007; 

Pederson and others, 2011). These cultural features, together 

with geoarcheological analyses (Pederson and others, 2011), 

indicate that the site has undergone at least three different cutand-fill episodes, with each set of fill preserving different periods 

of human occupation—the oldest dates to approximately 500 

B.C.E. and possibly as old as around 1500 B.C.E. (Pederson and 

others, 2011). The terrace is composed of interbedded slopewash 

and mainstem Colorado River fluvial deposits, with the top 30 

cm indicating aeolian reworking (Pederson and others, 2011). 

However, our mapping showed little evidence of modern aeolian 

Figure 51

G1

G2

G3 G4

G5

G6

Colorado River

EXPLANATION

Survey area boundary

Archeological site boundary

Shoreline at flow of 1,270 cubic

meters per second

Slope direction

Gully flow path and number

G1

Laser scan location (three additional 

laser scan locations positioned 

to the south)

0 50 100 150 FEET

0 25 50 METERS

AZ:C:02:0032

Figure 50. Site AZ:C:02:0032 survey-area map. Prevailing wind direction is not shown on this site map because this region is in a transition

zone between dominantly upstream and dominantly downstream winds (inferrered from weather stations in fig. 29); we observed no 

geomorphic features that indicated a clear prevailing wind direction at this site. 
76 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

men16-3129_fig 51

Colorado River

EXPLANATION

Gully flow path and number

G1

AZ:C:02:0032

G1

G2

G3

G4

G6 G5

Figure 51. Site AZ:C:02:0032 survey-area photograph, showing gully locations (G1–G6). View is toward the south.

sand activity in this area, and dune forms are not present on the 

terrace surface. The major erosional processes affecting the site 

are overland-flow gully erosion across the terrace surface, and 

terrace bank collapse associated with high flows of the Colorado 

River. Terrace bank erosion at this site has been severe in the 

past (Burchett, 1996; Grams and others, 2007) and continues to 

threaten archeological-site stability (Pederson and others, 2011). 

We categorized the site as type 4, having no upwind shoreline 

corresponding to a recent controlled flood (fig. 9).

Six gullies traverse this terrace, incised to varying 

degrees (fig. 51; Collins and others, 2014). Four are shallow 

swales (<40 cm deep) as they cross the terrace; the other 

two have incised 40–60 cm. This geomorphology is similar 

to what was mapped in Marble–Grand Canyon by Hereford 

and others (1993), with some gullies being terrace-based 

(that is, thalwegs that do not reach the Colorado River) and 

others river-based, with a clear drainage path to the river. The 

two river-based gullies at site AZ:C:02:0032 have incised 

notches in the terrace bank edge, with one incising through 

nearly half of the terrace thickness (the gully incises through 

5.2 m of the 11-m-thick terrace). Site vegetation consists of 

grasses, fourwing saltbush (Atriplex canescens), puncture 

vine (Tribulus terrestris), Mormon tea (Ephedra sp.) shrubs, 

and prickly pear cacti (Opuntia polyacantha var. erinacea); 

vegetation appears to offer only minor erosional resistance to 

the overland flow generated on the neighboring talus slope. 

The severity of gully erosion is related to episodic runoff from 

the talus slope, but fluvial bank erosion also might determine 

the incision rate near the terrace edge (Collins and others, 

2014). Additional site details, including a more in-depth 

examination of gullying processes at this site, are included in 

Collins and others (2014).

Topographic Change (2012–13)

We focused our change-detection analyses on both the 

terrace surface and terrace cutbank (fig. 52), analyzing the 

terrace cutbank in three-dimensional space (as opposed to 

plan view). The results for this area are shown in a rotated 

view to assist with visualization, using a 45-degree oblique 

view (fig. 52). We identified 18 areas of erosion over the 

entire survey area (fig. 52, table 8), encompassing 115.6 m2, 

which account for 1.3 percent of the plan-view surface area 

within the archeological-site boundary. We found no areas 

where deposition was evident at site AZ:C:02:0032 over this 

monitoring interval.

Whereas overland-flow-induced erosion was active in the 

deepest, most incised gully crossing the terrace (G3; regions ER14 

through ER18), the total sediment volume eroded from the terrace 

surface (0.85 m3) was much less than the erosion that occurred 

along the terrace cutbank. There, along a 200-m length of the 

terrace edge, 13 generalized areas of erosion (regions ER1 through 

ER13) totaled 20.23 m3 of erosion. The average plan-view depth 

of erosion among all erosional areas was 15 cm; terrace erosion 

was confined to distinct parts of the terrace cutbank rather than 

whole-height collapses (fig. 52). Although the cause of the terraceedge erosion cannot be identified with certainty, we consider the 

November 2012 controlled flood the most likely agent because 

erosion was widespread along the cutbank and appeared not to be 

directly related to particular spillover points from the gullies on 

the terrace surface. Time-lapse photographic documentation by 

National Park Service (NPS) staff (T. Baker, written commun., 

2013) shows that during other controlled floods, the toe of 

the cutbank has been inundated by rising water levels. After 

saturation of the cutbank toe, and upon water levels subsequently 

dropping as controlled floods recede, soil piping likely occurs, 

wherein water seeps out of the cutbank and destabilizes its toe. 

This mechanism is akin to a rapid drawdown effect that must be 

designed for, or restricted by all means, in earthen-dam operations. 

Destabilization by seepage forces may undercut the cutbank and 

(or) alter the stress condition of overlying exposed areas of the 

cutbank, wherein discrete areas of the cutbank fall away. If erosion 

occurs during recession of a controlled flood, the river flow may 

export the eroded material. If erosion occurs at some time soon 

after, piles of debris (that is, deposition) may accumulate at the toe 

of the cutbank. Because vegetation obscures most of the cutbank 

toe at this site, our lidar measurements could not resolve whether 

such deposits remain there.
Section IV - Landscape Change at Archeological Sites in a Sediment-Starved Reach: Glen Canyon 77

men16-3129_fig 52

ER14

ER15

ER16

ER17

ER18

G1 G2 G3

G4

G5

G6

ER1 ER2 ER3

ER4 ER5

ER6

ER7

ER8

ER9

ER10 ER11

ER12 ER13

Gully flow path and number

G1

EXPLANATION

Survey area boundary

Archeological site boundary

Erosion area and number

ER1

Cut-bank boundary

Vertical distance between

surfaces, in centimeters

0 to -3

-3 to -10

-10 to -30

-30 to -50

-50 to -100

Obligue view: Rotated 45 degrees

from plan view

Obligue view: Rotated 45 degrees

from plan view

0 100 FEET

0 40 METERS

0 100 FEET

0 40 METERS

Figure 52. Landscape change at site AZ:C:02:0032. A 5-cm gridded topographic change output showing erosion (warm colors, negative) 

from September 2012 to November 2013. No deposition was detected. Identified change is grouped by area labels (ER, erosion) and 

cross-referenced with table 8.
78 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

Table 8. Summary of topographic change at site AZ:C:02:0032.

[m/yyyy, month/year; m2, square meters; cm, centimeters; m3, cubic meters]

Site number and region Time period (m/yyyy) Area (m2) Average depth (cm) Volume (m3)

AZ:C:02:0032 – ER1 9/2012–11/2013 3.6 -12 -3.13

AZ:C:02:0032 – ER2 9/2012–11/2013 1.0 -18 -0.56

AZ:C:02:0032 – ER3 9/2012–11/2013 0.7 -16 -0.45

AZ:C:02:0032 – ER4 9/2012–11/2013 0.6 -29 -0.20

AZ:C:02:0032 – ER5 9/2012–11/2013 1.9 -13 -0.30

AZ:C:02:0032 – ER6 9/2012–11/2013 3.1 -12 -0.50

AZ:C:02:0032 – ER7 9/2012–11/2013 2.9 -9 -0.31

AZ:C:02:0032 – ER8 9/2012–11/2013 16.6 -9 -2.00

AZ:C:02:0032 – ER9 9/2012–11/2013 1.2 -15 -0.18

AZ:C:02:0032 – ER10 9/2012–11/2013 7.5 -9 -1.09

AZ:C:02:0032 – ER11 9/2012–11/2013 16.2 -9 -3.02

AZ:C:02:0032 – ER12 9/2012–11/2013 3.1 -33 -1.10

AZ:C:02:0032 – ER13 9/2012–11/2013 47.5 -10 -7.39

AZ:C:02:0032 – ER14 9/2012–11/2013 3.2 -6 -0.21

AZ:C:02:0032 – ER15 9/2012–11/2013 2.9 -6 -0.18

AZ:C:02:0032 – ER16 9/2012–11/2013 1.4 -18 -0.24

AZ:C:02:0032 – ER17 9/2012–11/2013 1.9 -9 -0.18

AZ:C:02:0032 – ER18 9/2012–11/2013 0.3 -13 -0.04

By dividing the volumes of erosion (table 8) by the threedimensional surface area of the erosion regions at the site 

(approximately three times the plan view area reported in table 

8), we calculated the average erosion into the cutbank to be 

10 cm (ranging between 5 and 24 cm) at the areas undergoing 

change. This calculation is not an overall indication of average 

terrace retreat, which must be calculated using the entire areal 

extent of the cutbank. Using the surveyed cutbank height 

(7.6 m) and length (200 m), and over the 1.1-year time frame 

bracketing the repeat topographic surveys, this volume of 

erosion yields a short-term terrace-retreat rate of 0.01 m/yr. 

This rate is an order of magnitude less than that documented 

by NPS staff (0.11 m/yr, the mean of four surveyed control 

points with one showing no erosion and the others showing 

between 10 and 25 cm of erosion) at the site between 1991 

and 2003 (Wulf and Moss, 2004). However, our retreat 

rate is based on a shorter measurement interval (1.1 years). 

Compared to long-term retreat rates measured in other nearby 

areas of the same terrace, our calculated bank-retreat rates are 

low. Grams and others (2007) showed that the terrace in the 

vicinity of this same archeological site eroded approximately 

90 m between 1952 and 1984 (2.8 m/yr). At a location <1 km 

downriver, repeat measurements at surveyed cross-sections 

indicate that the terrace eroded 34 m horizontally between 

1959 and 1965 (5.8 m/yr) and that another 12 m eroded 

between 1990 and 2000 (1.3 m/yr; Grams and others, 2007). 

Although Grams and others (2007) noted that this area of 

Glen Canyon may have undergone anomalously high rates of 

erosion compared to other sections, the recent measurements 

of ongoing terrace erosion are a reminder that the terrace 

bank at this site is dynamic, and has not achieved a stable 

configuration with respect to ongoing dam operations.

AZ:C:02:0035

Site Description

Site AZ:C:02:0035 is located just downriver from site 

AZ:C:02:0032 on a continuation of the predam fluvial terrace 

that borders the river in this stretch of Glen Canyon. The 

terrace at site AZ:C:02:0035 narrows to the east (downstream), 

and large boulders from rockfalls are situated on and 

immediately adjacent to the terrace surface, which grades 

into a talus slope (figs. 53, 54). The area is archeologically 

important as a prehistoric ancestral Puebloan site with two 

concentrations of artifacts. Previous fluvial geomorphological 

research (R. Hereford, unpub. data; Anderson, 2006) indicates 

that the deposits were emplaced during and prior to the 

Puebloan period (700–1200 C.E.) and that higher, older 

terraces are preserved by the talus deposits that cover part 

of the site. The main terrace is similar to that forming site 

AZ:C:02:0032 and is composed of interbedded slopewash and 

mainstem Colorado River fluvial sediment, although at the toe 
Section IV - Landscape Change at Archeological Sites in a Sediment-Starved Reach: Glen Canyon 79

men16-3129_fig 53

Figure 54

G1

G2

G3

G4

Colorado River

EXPLANATION

Survey area boundary

Archeological site boundary

Shoreline at flow of 1,270 cubic

meters per second

Slope direction

Gully flow path and number

G1

Laser scan location (three additional 

laser scan locations positioned 

to the south)

0 50 100 FEET

0 20 40 METERS

AZ:C:02:0035

Figure 53. Site AZ:C:02:0035 survey-area map. Wind direction is not indicated because this region is in a transition zone between 

dominantly upstream and dominantly downstream winds (inferrered from weather stations in fig. 29); we observed no geomorphic 

features that indicated a clear prevailing wind direction at this site.
80 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

men16-3129_fig 54

Colorado River

G1

G2

G3

EXPLANATION

Gully flow path and number G1

AZ:C:02:0035

Figure 54. Site AZ:C:02:0035 survey-area photograph showing gully locations (G1–G3). View is toward the southwest.

of the cutbank that forms the river-side of the terrace, these 

deposits are underlain by a 3-m-tall bedrock outcrop that was 

likely exposed during previous high flows released from Glen 

Canyon Dam (Grams and others, 2007). No active aeolian 

sand presently exists at this site; similar to site AZ:C:02:0032, 

gullying erosion is the dominant erosional process. Unlike the 

situation near site AZ:C:02:0032, the terrace area riverward 

of site AZ:C:02:0035 has not undergone as extensive cutbank 

collapse during recent controlled floods, owing to the erosional 

protection offered by the bedrock outcrop at the terrace toe. 

However, the terrace at this site retreated approximately 100 m 

between 1952 and 1984 (Grams and others, 2007), and the 

cross-section-based rapid retreat rates mentioned previously 

(5.8 m/yr between 1959 and 1965, and 1.3 m/yr between 1990 

and 2000) were obtained even closer to this site than to site 

AZ:C:02:0032. We categorized site AZ:C:02:0035 as type 4 

(no upwind shoreline corresponds with a recent high flow); 

there is no active aeolian sand source upwind of this site.

Four gullies traverse the terrace, incised to varying degrees 

(figs. 53, 54; Collins and others, 2014). The two gullies with the 

deepest incision (G3 and G4) begin high on the talus slopes inland 

and upslope of the terrace, and have larger contributing drainage 

area than do the four less-incised gullies, potentially linked to 

overland flow inputs from the cliffs and cliff-top areas. Vegetation 

is similar to, but sparser than, site AZ:C:02:0032, with grasses, 

fourwing saltbush (Atriplex canescens), Mormon tea (Ephedra sp.) 

shrubs, and prickly pear cacti (Opuntia polyacantha var. erinacea) 

interspersed on the terrace surface.

Topographic Change (2012–13)

We analyzed topographic change over the entire 

archeological site and adjacent terrace area (fig. 55), including 

the cutbank that forms the south edge of the site. Within the 

archeological-site boundary we documented only 0.1 percent 

of surface area change, all caused by precipitation-induced 

erosion in gully G3 (erosion areas ER1 and ER2). Substantial 

gullying also occurred outside the archeological-site boundary, 

farther downstream along gully G3 and within the other 

deeply incised gully (G4). In these locations, between 21 and 

33 cm of vertical downcutting occurred (regions ER3, ER4, 

ER5; table 9). Over the entire survey area, the eroded volume 

totaled 1.53 m3

 and occurred over an area of 8.4 m2. We did 

not detect evidence for aeolian inflation or deflation of the land 

surface, nor for any gully filling by aeolian sand.

The most notable erosion occurred in gully G3, where 

we measured mean erosion depths of between 28 and 33 cm 

(regions ER3 and ER4; table 9) and from which more than 1 m3

in sediment volume was lost. Detailed analysis of these erosion 

areas revealed that the gully widened (as opposed to merely 

downcutting), with some parts of the gully sidewalls having been 

undercut by as much as 38 cm and forming overhanging sections 

that are likely to collapse. We detected no deposition in any part 

of the site, suggesting that all eroded sediment was exported 

downstream by the Colorado River. The same storms identified 

to have caused gullying at sites AZ:C:02:0075 and AZ:C:02:0032 

likely were responsible for the erosion here. Storms with high 

rainfall intensity (10-minute intensity > 60 mm/hr) likely exceeded 

the soil infiltration capacity threshold at this site (Collins and 

others, 2016). Because erosion occurred only in the largest, 

most deeply incised gullies (G3 and G4), the erosion observed 

at AZ:C:02:0035 could have been a result of the potentially 

larger catchment area of these gullies (with consequently greater 

overland flow), the steeper gradient (because these gullies are 

already deeply incised, and grade to river level; Collins and others, 

2014), or possibly the lower infiltration capacity of the gully 

bottoms. In all regards, it should come as no surprise that erosion 

occurred in locations where gullies had formed in the past.
Section IV - Landscape Change at Archeological Sites in a Sediment-Starved Reach: Glen Canyon 81

men16-3129_fig 55

G1

G2

G3

G4

ER1

ER4

ER3

ER2

ER5

Gully flow path and number

G1

EXPLANATION

Survey area boundary

Archeological site boundary

0 75 FEET

0 30 METERS

Vertical distance between

surfaces, in centimeters

0 to -3

-3 to -10

-10 to -30

-30 to -50

-50 to -100

Erosion area and number

ER1

Figure 55. Landscape change at site AZ:C:02:0035. A 5-cm gridded topographic change output showing erosion (warm colors, 

negative) from September 2012 to November 2013. No deposition was detected. Identified change is grouped by area labels (ER, 

erosion) and cross-referenced with table 9.

Table 9. Summary of topographic change at site AZ:C:02:0035.

[m/yyyy, month/year; m2, square meters; cm, centimeters; m3, cubic meters]

Site number and region Time period (m/yyyy) Area (m2) Average depth (cm) Volume (m3)

AZ:C:02:0035 – ER1 9/2012–11/2013 1.3 -5 -0.06

AZ:C:02:0035 – ER2 9/2012–11/2013 2.0 -4 -0.09

AZ:C:02:0035 – ER3 9/2012–11/2013 3.9 -28 -1.09

AZ:C:02:0035 – ER4 9/2012–11/2013 0.4 -33 -0.13

AZ:C:02:0035 – ER5 9/2012–11/2013 0.8 -21 -0.16
82 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

Landscape Change in Glen Canyon 

Topographic-change analyses indicate that erosion 

processes dominated strongly over depositional processes 

at our four study sites in Glen Canyon (fig. 56). At all four 

sites overland-flow erosion incised gullies, in some cases 

lowering the land surface by more than 2 m over 14 months. 

Topographic changes included gully-wall undercutting and 

widening, in addition to some gully downcutting—particularly 

at gradient changes and knickpoints at terrace edges. Direct 

effects of Colorado River flows caused terrace-bank erosion at 

one site (AZ:C:02:0032)—a process which has been ongoing 

over the past five decades since construction of Glen Canyon 

Dam and now appears to be slowing, but is still indicative of 

a system that has not reached a stable configuration. At only 

one site did we detect any deposition (AZ:C:02:0077; fig. 

46). Although the spatial pattern of that deposition suggested 

aeolian inflation (fig. 46), it could not be linked to any fluvial 

sand source formed under the current flow regime, and we 

inferred redistribution by wind of local, relict terrace sediment 

as the most likely process. Aeolian transport at the other sites 

was not entirely absent, but where present (AZ:C:02:0075; fig. 

49), resulted only in erosion and net sand loss.

The four Glen Canyon sites showed a greater 

dominance of erosional processes compared to depositional 

processes than did our four Marble–Grand Canyon study 

sites over the temporal intervals measured (compare 

fig. 43 with fig. 56). Whereas gross erosion rates (the area 

of change per year per site) were actually higher at the 

Marble–Grand Canyon sites (an average of 143 m2/yr/site 

during 2010–14) than at the Glen Canyon sites (117 m2/yr/

site during 2012–13), deposition offset some of that erosion 

in Marble–Grand Canyon such that net erosion rates were 

higher at the Glen Canyon sites (58 m2/yr/site) than at the 

Marble–Grand Canyon sites (51 m2/yr/site). Comparing 

volumetric change for eroding regions yields similar results. 

Whereas gross erosion volumes (the volume of change per 

year per site, using the same time periods as for the areabased calculations) were similar between the Marble–Grand 

Canyon sites (average of 17.9 m3

/yr/site) and Glen Canyon 

sites (18.3 m3/yr/site), net erosional volumes were again 

higher in Glen Canyon (14.3 m3/yr/site) than in Marble–

Grand Canyon (11.4 m3/yr/site). Moreover, whereas some 

aeolian deposition occurred at all four study sites in Marble–

Grand Canyon during both monitoring intervals (2010–13 

and 2013–14; fig. 43), aeolian deposition occurred at only 

one site in Glen Canyon (AZ:C:02:0077; fig 46), and that 

deposition was apparently caused by reworking of relict 

(predam) rather than newly supplied sediment.

This type of direct comparison between landscape evolution 

in Glen and Marble–Grand Canyons is undoubtedly limited 

because the processes responsible for changes are different, the 

measurements spanned different time intervals, and the total 

surveyed area varied among sites. In addition, geomorphic 

setting varied somewhat from site to site. The first three factors 

(landscape process, measurement timing, and surveyed area) 

arguably will have limited impact on inferred differences between 

the two groups of sites: the process types are similar, the temporal 

intervals were not vastly different—1.1 yr and 3.6 yr, respectively, 

for Glen Canyon and Marble–Grand Canyon—and although 

the total surveyed area is 83 percent greater in Glen Canyon, 

that is unlikely to affect interpretations substantially because 

the areas where we detected landscape change are smaller and 

spatially isolated compared to the total area surveyed. However, 

the geomorphic settings must be closely compared to determine 

whether the Glen Canyon sites may be more predisposed (by 

factors other than their lack of aeolian-sand activity) to erosion.

We identified similarities and differences in geomorphic 

parameters between the two groups of sites. Topographic slope 

at the Glen Canyon sites compares favorably to that at the 

Marble–Grand Canyon sites (average values of 15° and 13°, 

respectively; fig. 57A) and up-watershed substrates are generally 

composed of talus slopes with likely similar infiltration capacity. 

Watershed area, on the other hand, is roughly double at the 

sites in Glen Canyon compared to those in Marble–Grand 

Canyon (averages of approximately 29,000 m2

 and 14,400 m2, 

respectively; fig. 57B). Whereas this could lead to either more 

EXPLANATION

0 to 50

50 to 100

100 to 200

200 to 400

Rate of change,in square 

meters per year (m2/y) (14,900 m2)

(9,900 m2)

(21,200 m2)

(12,400 m2)

JuneSept.Mar.JuneSept.

2012 2013

Archeological 

site and area

AZ:C:02:0077

AZ:C:02:0075 

AZ:C:02:0032

AZ:C:02:0035

Runoff erosion

Runoff Aeolian

Erosion

Induced by:

Fluvial

Runoff Aeolian

Deposition

Fluvial

Figure 56. Summary of landscape change measured at four 

archeological sites in the Colorado River corridor through the Glen 

Canyon reach, by area, over the 2012–2013 monitoring interval. Pie 

size indicates the total area of erosion and deposition, normalized 

by the length of time between measured changes. Hatching 

pattern indicates the primary geomorphic process inferred to 

have caused the change. At site AZ:C:02:0032, fluvial-related 

change occurred in the form of terrace cutbank erosion likely 

related to toe inundation from a controlled flood in 2012. Values in 

parentheses indicate average total area where change detection 

was investigated. Dashed lines are dates of data collection; pie 

locations indicate measured change between times represented 

by adjacent dashed lines.
Section IV - Landscape Change at Archeological Sites in a Sediment-Starved Reach: Glen Canyon 83

men16-3129_fig 57

Glen Canyon Marble - Grand Canyon

Glen Canyon Marble - Grand Canyon

A

B

Archeological site

20

10

30

40

50

60

70

80

Drainage area, in thousands of square meters Slope, in degrees

AZ:C:

02:0077

AZ:C:

02:0075

AZ:C:

02:0032

AZ:C:

02:0035

AZ:C:

05:0031

AZ:C:

13:0321

AZ:B:

10:0225

AZ:G:

03:0072US

60

55

50

45

40

35

30

25

20

15

10

5

0

Figure 57. (A) Maximum and 

mean slope calculated within the 

survey area of each of the eight 

archaeological sites investigated 

in Sections III and IV. Error bars 

represent the standard deviation 

of the mean slope. (B) Contributing 

drainage area upslope of the survey 

boundary for each site. 

frequent or more deeply incised gullies through greater overland 

flow, a closer look at the data raises doubts that this is the only 

relevant factor. The sites with the largest and smallest overlandflow erosion volume per year—AZ:B:10:0225, in Grand 

Canyon, with 37.4 m3/yr, and AZ:C:02:0077, in Glen Canyon, 

with 0.4 m3/yr during the total monitoring period at each site—

have similar contributing watershed area—the area draining 

into those sites is within 3,000 m2

 (20 percent of the standard 

deviation between all watershed-area measurements) of each 

other. Thus, although two of the Glen Canyon sites have the 

largest watershed areas (fig. 57B), it is unclear to what extent 

this contributes to the observed differences in surface change 

between Glen and Marble–Grand Canyons.

Factors such as site substrate and gully-incision magnitude 

are not helpful in identifying parameters that might be 

responsible for the observed differences in erosion rates among 

the study sites. Aeolian sand is a more common substrate at 

the Marble–Grand Canyon sites than in the Holocene terraces 

of Glen Canyon, but this is expected given the design of the 

study (in Marble-Grand Canyon, we purposefully selected type 

1 sites, whereas in Glen Canyon, none of the sites has apparent 

connectivity to modern aeolian sand supply). Similarly, there 

are more (3.8 versus 2.0 gullies/site) and deeper (1.4 m versus 

0.9 m) gullies in the Glen Canyon sites compared to the MarbleGrand Canyon sites, but again, these differences existed before 

our study (and were factored into the study design), and it is 

unclear whether these pre-existing differences affected erosion 

rates measured during the study.

The contrast between landscape change measured at the 

Marble–Grand Canyon sites and that measured at the Glen 

Canyon sites illustrates the best- and worst-case scenarios for 

site-preservation trajectory in the Colorado River corridor, given 

the range in connectivity to modern sand supply and geomorphic 

context in this system. Sand deposition at the type 1 Marble–Grand 

Canyon archeological sites increases the potential for erosion to 

be counteracted either by aeolian inflation or gully annealing, 

as discussed in Section III. With little or no deposition at Glen 

Canyon sites, in contrast, there is less possibility for inflation and 

volume gain on terrace surfaces, or for gullies to anneal. Thus, 

even undergoing nearly the same event-scale or annual-scale 

erosion rates as in Marble–Grand Canyon, archeological sites in 

Glen Canyon are more likely to become more highly dissected and 

degraded. These process links are perhaps best exemplified by the 

changes measured at site AZ:C:02:0035 in Glen Canyon, where 

the most-incised gullies also showed the most erosion during the 

study period. No substantial net aeolian inflation or aeolian gully 

annealing is likely to occur in Glen Canyon owing to the lack of 

connectivity to modern fluvial sand sources; therefore, erosion is 

likely to continue there essentially unabated if present conditions 

and processes persist.
84 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

Climatologic Comparison of Glen and Marble–

Grand Canyons

We investigated whether the extensive gully erosion in 

Glen Canyon could be caused or exacerbated by a subregional 

climate that might include substantially more precipitation, 

and more intense storms capable of causing overland flow. 

Below, we address the second science question of Section IV 

with regard to whether weather patterns could cause substantially different landscape-change trajectories in Glen Canyon 

than in Marble–Grand Canyon.

Glen Canyon received less annual rainfall than did most 

of Marble–Grand Canyon over the 3-year interval when the 

greatest number of USGS inner-canyon weather stations 

operated concurrently and from which we also have National 

Weather Service COOP-station records (October 2007 to 

October 2010; fig. 58). Long-term, 60-year mean annual 

rainfall records from COOP stations at Phantom Ranch (Grand 

Canyon), Lees Ferry (boundary of Marble and Glen Canyons), 

and Page (on the southeast rim, above Glen Canyon) similarly 

indicate that the Glen Canyon region receives less annual 

rainfall, on average, compared to farther downriver in Grand 

Canyon (fig. 59A). During the drought year of 2014, however, 

below-average annual rainfall was observed at all locations, 

and rainfall in Glen Canyon and at Lees Ferry was slightly 

greater than at the Phantom Ranch COOP station (fig. 59B), 

a situation that appears atypical in the context of longer-term 

records (Hereford and others, 2014).

Decadal-scale records (1958–2012) show that rainfall 

intensity and the frequency of days with high-intensity rainfall 

in Glen Canyon appear to be typically less, and certainly not 

greater, than in Grand Canyon (fig. 60). When compared to the 

Phantom Ranch COOP station (at RM 87, Grand Canyon), the 

estimated maximum daily 10-minute rainfall intensity values 

at Lees Ferry and Page generally have been lower—less than 

85 mm/hr at Lees Ferry and Page (although one event in 1964 

exceeded 90 mm/hr at Page), compared to more than 116 mm/

hr at Phantom Ranch (fig. 60). Moreover, approximately half 

as many moderate to strong storms (defined as individual days 

having 10-minute maximum rainfall intensity >32 mm/hr; 

Collins and others, 2016) occurred at Lees Ferry (35) and Page 

(32) compared to Phantom Ranch (65) during 1958–2012. 

Long-term records from three stations surely do not represent conditions over the entire, vast canyon area, especially 

given that a 60-year record exists from only one point in 

Grand Canyon (Phantom Ranch), and that spatial variations 

in rainfall amount and intensity often are substantial in this 

river corridor (for example, Draut and Rubin, 2008; Caster and 

others, 2014). However, the available long-term data imply 

that storms capable of gully erosion have been less frequent in 

Glen Canyon since 1958.

Wind speeds within Glen Canyon did not differ 

substantially from those recorded in Marble–Grand Canyon 

between February 2014 and February 2015 (the interval from 

which data are available for all USGS inner-canyon stations). 

Thus, the wind data we analyzed did not explain differences in 

the prevalence of aeolian geomorphic processes between the 

two study areas—rarity of active aeolian sand in Glen Canyon 

(Section II), and less aeolian inflation or deflation measured 

at the four sites there compared to those in Marble–Grand 

Three-year mean annual

Three-year mean warm-season

Three-year mean cool-season

EXPLANATION

COOP Page (on

canyon rim)

COOP Lees Ferry Paria River

COOP Phantom Ranch

Little Colorado

River

Glen

Canyon

Grand

Canyon

Marble

Canyon

50

0

100

150

200

250

300

350

400

0 50 100 150 200

River mile

Rainfall, in millimeters

Figure 58. Spatial distribution of annual and seasonal rainfall within the Colorado River corridor and an adjacent rim location. Mean values 

were calculated from data collected between October 8, 2007, and October 7, 2010, the period with greatest number of concurrently operating 

stations, at 8 U.S. Geological Survey inner-canyon stations and 3 National Weather Service Cooperative Observer Program (COOP) stations 

(Phantom Ranch, Grand Canyon; Lees Ferry, at the boundary of Glen and Marble Canyons; and Page, on the southeast rim above Glen Canyon). 

Major tributary confluences are shown for reference. Error bars represent standard error of the 3-year mean.
Section IV - Landscape Change at Archeological Sites in a Sediment-Starved Reach: Glen Canyon 85

Paria River

Little Colorado

River

Paria River

Little Colorado

River

Glen

Canyon

Grand

Canyon

Marble

Canyon

Glen

Canyon

Grand

Canyon

Marble

Canyon

COOP Lees Ferry COOP Lees Ferry

EXPLANATION

EXPLANATION

Mean annual

Mean warm-season

Mean cool-season

2014 annual

2014 warm-season

2014 cool-season

60-year annual mean

A

B

50

0

100

150

200

250

300

350

400

50

0

100

150

200

250

300

350

400

Rainfall, in millimeters Rainfall, in millimeters

COOP Page (on canyon rim)

COOP Phantom Ranch

GCMRC AZ:C:02:0071GCMRC AZ:C:02:0070

0 50 100

0 50 100

Figure 59. Temporal summaries of mean seasonal and annual rainfall measured at National Weather Service Cooperative Observer Program 

(COOP) stations Phantom Ranch (Grand Canyon), Lees Ferry (boundary of Glen and Marble Canyons), and Page (on the southeast rim, above 

Glen Canyon) by distance downstream from Glen Canyon Dam. (A) 60-year (1952–2012) annual and seasonal mean rainfall. Major tributary 

confluences are shown for reference. Error bars represent standard error of the 60-year mean. (B) Seasonal and annual rainfall totals from 

October 8, 2013, through October 7, 2014, for U.S. Geological Survey inner-canyon weather stations AZ:C:02:0070 and AZ:C:02:0071 (locations 

on fig. 29; Caster and others, 2014) and COOP stations at Phantom Ranch and Page. The COOP station at Lees Ferry was removed in 2012. The 

60-year annual means are provided for reference.
86 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

1950 1960 1970 1980 1990 2000 2010

20

40

60

80

100

120

0

0

Ten-minute intensity, in millimeters per hour

20

0

40

60

80

100

120

Ten-minute intensity, in millimeters per hour

20

40

60

80

100

120 Ten-minute intensity, in millimeters per hour

Year

A. Page (rim)

B. Lees Ferry

C. Phantom Ranch

Figure 60. Estimated maximum daily 10-minute rainfall intensities (in millimeters per 

hour) measured by National Weather Service Cooperative Observer Program (COOP) 

weather stations at (A) Page (on the southeast rim above Glen Canyon), (B) Lees Ferry 

(boundary of Glen and Marble Canyons, RM 0) and (C) Phantom Ranch (within Grand 

Canyon). Daily maximum intensity estimates were based on seasonal equations (Caster 

and Sankey, 2016).
Section IV - Landscape Change at Archeological Sites in a Sediment-Starved Reach: Glen Canyon 87

Canyon (Sections III, IV). In this discussion we focus on 

4-minute maximum gust speeds rather than on 4-minuteaveraged wind speeds because the former provide more 

information about aeolian sand-transport potential (4-minute 

average speed being low or near zero much of the time). The 

maximum 4-minute gust speed recorded in Marble–Grand 

Canyon was 16.2 m/s, at site AZ:B:10:0225, slightly lower 

than the maximum gust speed of 17.1 m/s recorded in Glen 

Canyon (fig. 61A). Mean and median 4-minute-maximum 

gust speeds in Glen Canyon were similar to those in Marble–

Grand Canyon (1.9–2.1 m/s and 1.6–1.8 m/s, respectively; 

fig. 61A). The proportion of time when maximum 4-minute 

gusts exceeded the assumed sand-transport threshold of 2 m/s 

was also similar, being only 5 percent greater, on average, at 

the Marble–Grand Canyon stations compared to those at Glen 

Canyon or Lees Ferry (fig. 61B; station AZ:C:05:0031, in 

Marble Canyon, recorded substantially lower wind speeds than 

did any other station). The longest continuous measurement 

of maximum gusts >2 m/s was recorded in Grand Canyon (at 

site AZ:C:13:0345; Caster and others, 2014) and exceeded 

one full day (386 consecutive 4-minute records). The longest 

interval with wind gusts >2 m/s at all other stations (including 

Glen Canyon) ranged from 170 to 200 consecutive records. 

Wind gusts >5 m/s occurred 6.5 percent of the time in Marble–

Grand Canyon and 5.4 percent of the time in Glen Canyon, 

on average (fig. 61B). Finally, wind gusts >8 m/s were 

uncommon, occurring less than 1 percent of the time at all 

stations (fig. 61B).

Based on the available data, we infer that Glen Canyon 

and Marble–Grand Canyon have similar wind conditions. 

Wind and aeolian sand transport are, however, highly variable 

and dynamic processes, such that detecting and quantifying 

spatial and temporal patterns is notoriously difficult, even with 

much greater measurement resolution than was available in 

this study (Bauer and others, 1996). Long-term wind records 

from within Glen Canyon do not exist, so it is not possible 

to assess whether the comparison of wind conditions in these 

regions over 2014–15 is representative of other years.

Thus, our climatologic analysis did not indicate that 

rainfall or wind conditions differ sufficiently to produce 

4

0

0

8

12

16

20

Wind gust speed, in meters per second

0.1

0.2

0.3

0.4

0.5

AZ:C:

02:0071

AZ:C:

02:0070

AZ:C:

05:0031

AZ:C:

13:0345

AZ:B:

10:0225

AZ:G:

03:0072

Weather station

Proportion of time above threshold

Greater than 2 meters per second

Greater than 8 meters per second

Greater than 5 meters per second

4-minute maximum gust

EXPLANATION

Maximum

Median

Mean

EXPLANATION

A

B

Figure 61. Summary statistics for 

wind-speed measurements at U.S. 

Geological Survey inner-canyon 

weather stations. (A) Maximum, 

mean, and median values, calculated 

from 92,355 concurrent 4-minute 

maximum-gust-speed records 

collected between February 13, 2014, 

and February 5, 2015. (B) Proportion 

of time that maximum gust speed was 

above the 2, 5, and 8 m/s thresholds 

for sand transport. See text for 

explanation of threshold choice.
88 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

consistently more erosive conditions in Glen Canyon 

compared to Marble–Grand Canyon. The available records 

indicate that conditions are typically drier, with fewer highintensity storms, in Glen Canyon than in Marble–Grand 

Canyon. Wind conditions within Glen Canyon, including 

the frequency and duration of wind speeds with potential 

to transport sand, were not notably different from those 

in Marble–Grand Canyon during the assessment period. 

Therefore, we see no indication that climatic differences 

would have caused the greater prevalence of gully erosion and 

lack of aeolian sand activity in Glen Canyon. Rather, we infer 

that these differences in landscape processes and topographic 

evolution result from inherent geomorphic characteristics (the 

large terraces in Glen Canyon, and lack of eddy sand deposits) 

combined with especially pronounced effects of sediment 

starvation there in the postdam era.

Section V - Synthesis and Conclusions

Landscape evolution in the Colorado River corridor 

demonstrates not only the importance of connectivity among 

fluvial, aeolian, and hillslope processes, but also the relative 

roles of common geomorphic processes and extreme events in 

shaping landscapes—a balance that scientists have long sought to 

understand in various geomorphic contexts. Sedimentary terraces 

of the Colorado River corridor formed largely from episodic flood 

deposition over thousands of years, and were modified by wind 

and hillslope runoff (overland flow) between large, landscaperesetting floods. Fluvial, aeolian, and hillslope processes all 

occur with a range of magnitudes and frequencies; of those three, 

fluvial processes have been altered the most by dam operations. 

In the postdam river, hillslope runoff and wind action occur with 

presumably natural or near-natural magnitude and frequency, such 

that gullying and aeolian sand transport alter the landscape on 

seasonal to annual time scales. Major wind events move sand (in 

places with active, mobile sand surfaces) on seasonal or shorter 

time scales, and rainfall runoff erodes gullies commonly every 

2–3 years at most sites. Thus, hillslope and aeolian processes 

modify the landscape much as they did in predam time, albeit 

with reduced sediment supply to the landscapes they affect. In 

contrast, the magnitude and frequency of river flows have changed 

profoundly as a function of dam operations. The loss of decadal- to 

centennial-scale sediment-rich floods, which provided a largescale mechanism to anneal gullies and resupply upland dunes, 

together with the loss of low flows that exposed much fluvial sand 

area to wind, and substantial riparian-vegetation encroachment, 

have left aeolian and hillslope processes as disproportionately 

important factors modifying river-corridor landscapes and 

archeological sites.

Modern controlled floods (flows of around 1,200 m3

/s, lasting 

several days), which are relatively small and brief compared to 

even the annual predam spring flood, and much smaller than 

decadal-scale predam floods, cannot be expected to restore natural 

geomorphic evolution of river-corridor sedimentary deposits. 

Nevertheless, because controlled floods do form or enlarge some 

sandbars, controlled flooding does provide some modern aeolian 

sand supply to some upland areas. In our evaluation of how this 

modern sand supply affects archeological resources, we have 

shown that only a small minority of river-corridor archeological 

sites are ideally situated to receive aeolian sand supply from 

sandbars of recent controlled floods. Whereas three-fourths of 

the 358 river-corridor archeological sites we examined include 

Colorado River-derived sand as an integral component of their 

geomorphic context, only 32 sites—9 percent of the total number, 

or just under 12 percent of the sand-dependent sites—appear 

to have a high degree of connectivity between modern fluvial 

sandbars and a downwind aeolian sand landscape (type 1 sites 

in our classification system). In total, we identified 232 sanddependent sites that we consider to be potentially influenced by 

dam operations, because they are downwind from river shorelines 

reached by flows of 1,270-m3

/s (site types 1–3). A site need not 

necessarily be type 1 to receive some aeolian sand supply from 

controlled-flood sandbars; for example, such connectivity has 

been documented previously for a type 2b site, where aeolian sand 

crossed a topographic barrier and deposited at an archeological site.

The number and proportion of type 1 archeological sites, 

which have the greatest potential to receive windblown sand 

from modern fluvial sand sources, decreased over each time 

step we evaluated (1973, 1984–85, 1996, and 2012–14), and 

now constitute one-third the number evident in 1973. Therefore, 

controlled floods have had a limited, and decreasing, influence 

on aeolian sand supply to archeological sites in the river corridor. 

We infer that the decrease in type 1 sites resulted from (1) a lack 

of sediment-rich flows large enough to both deposit sandbars at 

elevations above the controlled-flood stage and to remove riparian 

vegetation, (2) vegetation growth having covered formerly open 

sand sources and impeded sand entrainment and transport, and (3) 

fluvial erosion of some formerly open (as well as some vegetated) 

sand deposits.

Aeolian sand activity in the Colorado River corridor varies 

substantially as a function of reach morphology and dominant 

wind direction relative to the river-corridor orientation, factors 

that control accommodation space for river-derived sand and 

the modern aeolian sand supply to source-bordering dunes in 

the absence of large floods. The active aeolian sand ratio in the 

Upper Granite Gorge and Stevens-Conquistador Aisle reaches 

is 2–3 times greater than that in the Eminence–Little Colorado 

River and Furnace Flats reaches, and nearly an order of magnitude 

greater than in the Glen Canyon and Granite Park reaches. In 

places where the wind blows at an angle orthogonal to the river, 

source-bordering dunes and sand ramps generally have more 

active aeolian sand than in other reaches where the wind direction 

is nearly parallel to the river. These attributes, together with a 

negative correlation between aeolian sand activity and gully 

occurrence, imply substantial variation in net long-term gullyerosion risk for sediment deposits and associated archeological 

sites in different regions of the river corridor.

Gully erosion is less severe in active aeolian sand 

landscapes than in those that are inactive with respect to aeolian 

transport; gullies terminate more commonly in active sand. 

We infer that these characteristics largely result from aeolian 
References Cited 89

sand transport being an effective gully-annealing mechanism, 

and active aeolian sand being a poor retainer of gully-channel 

morphology. Aeolian infilling occurs in conjunction with 

backwasting of noncohesive sand in gully walls, which provides 

an additional mechanism leading to the obliteration of gully 

morphology in active aeolian areas. Seasonal- to annualscale topographic-change detection, and limited examples of 

annealed gullies in the aerial photographic record, indicate that 

both gullying and infilling of gullies are not rare (though gully 

formation is much more common than annealing), but rather are 

common processes altering river-corridor landscapes.

The prevalence of inactive sand landscapes in Marble–Grand 

Canyons indicates that aeolian annealing is likely to occur over 

only spatially limited regions—currently 5–35 percent of the 

sand landscape area in Marble–Grand Canyon has active aeolian 

surfaces where gully annealing likely could occur (the proportion 

varies spatially, for reasons of geomorphology and wind direction). 

Thus, over most of the Colorado River corridor, including some of 

the archeologically richest regions, there is too little aeolian sand 

activity to anneal gullies effectively.

At archeological sites where landscape change has been 

measured at high resolution, sand loss by overland flow (gully 

erosion) and aeolian deflation generally exceeds deposition, such 

that erosional signals typically dominate landscape change over 

most monitoring intervals—even at type 1 sites with modern 

connectivity to new sand supply. The rate of landscape degradation 

in places having both active aeolian transport and gully erosion is 

a complex result of gully meandering, incision, and aeolian sand 

flux, processes with enough stochasticity, or randomness, to make 

site-specific predictions difficult.

The Glen Canyon reach of the river corridor appears 

especially vulnerable to gully erosion. Gully prevalence in the 

large terrace deposits of Glen Canyon is comparable to the 

most-intensively gullied parts of Grand Canyon (large, predam 

flood deposits of the Furnace Flats or Granite Park reaches). Glen 

Canyon also has proportionally less active aeolian sand area than 

in any of the five reaches we studied in Marble–Grand Canyon, 

indicating extremely low potential for aeolian sand to anneal 

gullies. Within the sample of archeological sites that we monitored 

in detail, and over the temporal change-detection intervals we 

used, erosional processes dominated over depositional processes 

to a greater degree at Glen Canyon sites than at Marble–Grand 

Canyon sites. A relative lack of depositional processes led to 

greater net erosion at the Glen Canyon sites compared to those in 

Marble–Grand Canyon. Having found no differences in weather 

patterns to suggest greater erosive forcing in Glen Canyon, we 

attribute the greater erosion observed there to a combination of (1) 

inherent geomorphic context, which includes extensive predam 

fluvial terraces, and precludes retention of eddy sandbars that 

could serve as aeolian sand sources; and (2) pronounced effects of 

postdam sediment-supply limitation (sand deficit and base-level 

lowering).

In this study, we sought to determine whether archeological 

sites in the Colorado River corridor through Glen, Marble, and 

Grand Canyons are eroding or changing faster or in a substantially 

different manner than they would if Glen Canyon Dam were 

operated differently than it has been. We conclude that most rivercorridor archeological sites are subject to increased risk of net 

gully erosion under present dam operations. For the more than 260 

archeological sites that have river-derived sand as an integral part 

of their geomorphic context, we infer elevated erosion risk owing 

to a combination of reduced sand supply (both fluvial and aeolian) 

through (1) the lower-than-natural flood magnitude, frequency, and 

sediment supply of the controlled-flooding protocol; (2) reduction 

of open, dry sand area available for wind redistribution under 

current normal (nonflood) dam operations, which do not include 

flows as low as natural seasonal low flows; and (3) impeded 

aeolian sand entrainment and transport owing to increased riparian 

vegetation growth in the absence of larger, more-frequent floods.

If dam operations were to increase the supply of sand 

available for windblown transport—for example, through larger 

floods, sediment augmentation, or increased fluvial sandbar 

exposure by low flows—and also decrease riparian vegetation, the 

prevalence of active aeolian sand landscapes likely would increase 

over time. We suggest that in such a situation, the prevalence 

of gully development through those landscapes and associated 

archeological sites would be less than will occur if management 

of flow, sediment, and riparian vegetation remains unchanged. 

Ultimately, the river-corridor landscape context of many cultural 

sites is altered fundamentally by the lack of large, sediment-rich 

floods (flows on the order of 5,000 m3

/s, with decadal-scale return 

intervals). Although structural considerations of Glen Canyon 

Dam preclude designing such large controlled floods, some 

combination of sediment-rich flows above 1,270 m3/s, seasonal 

flows below 226 m3/s, and riparian-vegetation removal likely 

would increase the preservation potential for sand-dependent 

archeological resources in the Colorado River corridor.

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Appendix
Appendix. Classification of 358 river-corridor archaeological sites in Grand Canyon National Park with respect to potential for aeolian 

sand supply. Sites are listed in order from upstream to downstream. Two sites (AZ:B:09:0314 and AZ:A:16:0159) were not classified for 

time intervals when recent high flows potentially could have inundated the site area, according to the modeled flood shorelines of 

Magirl and others (2008), but for which sand deposition could not be confirmed; those two sites are beneath bedrock overhangs and so 

are not visible in aerial imagery.

Site number 1973 1984–85 1996 2012–14

C:02:0098 3 1 3 3

C:02:0097 3 1 3 3

C:02:0094 3 1 3 3

C:02:0092 3 3 3 3

C:02:0096 3 1 1 3

C:02:0101 3 1 3 3

C:06:0003-1 2b 2b 2b 2b

C:06:0003-2 1 1 1 1

C:06:0002 5 5 5 5

C:06:0005 5 5 5 5

C:06:0008 5 5 5 5

C:06:0010 5 5 5 5

C:06:0004 5 5 5 5

C:05:0005 5 5 5 5

C:05:0031 1 1 1 1

C:05:0004 3 1 2b 2b

C:05:0033 5 5 5 5

C:05:0037 1 1 2a 2a

C:05:0009 5 5 5 5

C:05:0039 5 5 5 5

C:09:0065 5 5 5 5

C:09:0064 2b 2b 3 2b

C:09:0004 5 5 5 5

C:09:0005 5 5 5 5

C:09:0088 1 1 3 3

C:09:0034 2c 3 3 3

C:09:0083 2c 3 3 3

C:09:0032 5 5 5 5

C:09:0030 5 5 5 5

C:09:0030 2c 2c 2c 2c

C:09:0031 2c 2c 2c 2c

Unassigned 2a 2a 2a 2a

C:09:0068 2a 2a 2a 2a

C:09:0056 5 5 5 5

C:09:0058 5 5 5 5

C:09:0085 5 5 5 5

C:09:0050 4 4 4 4

C:09:0053 1 3 3 3

C:09:0033 4 4 4 4

C:09:0069 2a 3 3 3

C:09:0062 2b 3 3 3

C:09:0080 5 5 5 5

C:09:0186 5 5 5 5

C:09:0059 2a 2a 3 3

C:09:0061 2a 2a 3 3

C:09:0051 2a 2a 3 3

C:09:0082 2a 2a 3 3

C:09:0060 5 5 5 5

C:09:0184 5 5 5 5

C:09:0188 5 5 5 5

C:09:0189 5 5 5 5

C:09:0187 5 5 5 5

C:09:0001 5 5 5 5

C:09:0052 2a 2a 3 3

C:09:0071 2a 2a 2a 2a

C:09:0185 1 1 1 1

C:09:0072 2a 2a 2a 2a

C:09:0073 2a 2a 2a 2a

Appendix 99
100 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

Appendix.—Continued

Site number 1973 1984–85 1996 2012–14

C:09:0054 2c 2c 2c 2c

C:09:0070 5 5 5 5

C:09:0075 5 5 5 5

C:09:0074 5 5 5 5

C:09:0076 5 5 5 5

C:09:0067 5 5 5 5

C:09:0084 2a 2a 3 3

C:13:0368 2c 2c 2c 2c

C:13:0370 4 4 4 4

C:13:0372 4 4 4 4

C:13:0365 2b 2b 2b 2c

C:13:0329 1 1 3 3

C:13:0331 5 5 5 5

C:13:0006 2b 2b 2b 2b

C:13:0353 1 1 3 2c

C:13:0374 2c 3 3 3

C:13:0376 5 5 5 5

C:13:0380 5 5 5 5

C:13:0371 1 1 1 1

C:13:0375 5 5 5 5

C:13:0360 5 5 5 5

C:13:0003 1 1 2a 2a

C:13:0486 5 5 5 5

C:13:0033 5 5 5 5

C:13:0007 2c 2c 2a 2a

C:13:0384 2c 2c 2a 2a

C:13:0355 4 4 4 4

C:13:0098 4 4 4 4

C:13:0099 1 4 4 4

C:13:0100 1 4 4 4

C:13:0101 1 4 4 4

C:13:0272 1 4 4 4

C:13:0334 4 4 4 4

C:13:0336 1 4 4 4

C:13:0332 4 4 4 4

C:13:0337 4 4 4 4

C:13:0335 4 4 4 4

C:13:0373 4 4 4 4

C:13:0333 4 4 4 4

C:13:0274 5 5 5 5

C:13:0273 4 4 4 4

C:13:0339 4 4 4 4

C:13:0341 2b 2b 2c 2c

C:13:0323 1 1 2a 2a

C:13:0324 1 1 2a 2a

C:13:0340 2a 2a 2a 2a

C:13:0327 2a 2a 2a 2a

C:13:0008 2a 2a 2a 2a

C:13:0338 2a 2a 2a 2a

C:13:0009-1 2b 2b 2b 2b

C:13:0009-2 1 1 1 1

C:13:0321 1 1 1 1

C:13:0092 3 3 3 3

C:13:0325 4 4 4 4

C:13:0342 1 2a 3 3

C:13:0344 1 2a 3 3

C:13:0346 1 2a 3 3

C:13:0348 1 2a 3 3

C:13:0350 1 2a 3 3

C:13:0352 1 2a 3 3

C:13:0349 1 2a 3 3

C:13:0345 2a 2a 3 3
Appendix.—Continued

Site number 1973 1984–85 1996 2012–14

C:13:0351 2a 2a33

C:13:03434444

C:13:03471333

C:13:00691 2a 2a 2a

C:13:03225555

C:13:03641333

C:13:00101111

C:13:02911133

C:13:07765555

C:13:00015555

C:13:07874444

C:13:03774444

C:13:07864444

C:13:07883333

C:13:07903333

C:13:07791133

C:13:078011 2b 2b

C:13:037911 2b 2b

C:13:00701133

C:13:03851113

C:13:038611 2b 2c

C:13:0387111 2a

C:13:03545555

C:13:03591 2b3 2c

C:13:0381 2b 2b 2b 2b

C:13:03615

555

C:13:03891111

C:13:03901111

C:13:0391 2b 2b 2b 2b

C:13:03631333

C:13:03825555

C:13:03835555

C:13:0005 2b 2b33

C:13:0392 2b 2b33

C:13:03931133

B:16:0001 2a 2a 2a 2a

B:16:02575555

B:16:03655555

B:16:0364 2b 2b 2b 2c

B:16:02585555

B:16:02591133

B:16:01705555

B:16:0911 2b 2b 2b 2b

B:16:0261113 2a

B:16:00035555

B:15:00965555

B:15:01245555

B:15:0128 2b 2b 2b 2b

B:15:00975555

B:15:00011111

B:15:01393333

B:15:01324444

B:15:0133

5555

B:15:01345555

B:15:01255555

B:15:01185555

B:15:01195555

B:15:00735555

B:15:01235555

B:15:01435555

B:15:01261133

B:15:01381111

Appendix 101
102 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

Appendix.—Continued

Site number 1973 1984–85 1996 2012–14

B:15:0127 2b 2b 2b 2b

B:15:0135 2b 2b 2b 2c

B:14:0107 2b 1 2b 2c

B:14:0105 1 1 1 2a

B:14:0093 1 1 1 1

B:14:0094 1 1 1 1

B:14:0095 1 1 1 1

B:14:0108 1 1 1 1

B:10:0261 1 1 1 1

B:10:0263 1 1 1 1

B:10:0260 2b 2b 2b 2b

B:10:0253 4 4 4 4

B:10:0111 2b 2b 2b 2b

B:10:0224 1 1 1 1

B:10:0225 1 1 1 1

B:11:0280 2b 2b 2b 2b

B:11:0283 5 5 5 5

B:11:0273 5 5 5 5

B:11:0279 2b 2b 2b 2b

B:11:0284 5 5 5 5

B:11:0278 3 3 2b 2b

B:11:0282 1 1 1 1

B:11:0272 2b 2b 2c 2c

B:11:0359 5 5 5 5

B:11:0276 2b 2b 2c 2c

B:11:0277 1 1 2b 2b

B:11:0281 1 1 2b 2b

B:11:0275 2b 2b 2b 2b

B:11:0271 2b 2b 2b 2b

B:11:0002 5 5 5 5

B:10:0004 2b 2b 3 3

B:10:0252 2b 2b 2b 2b

B:10:0229 5 5 5 5

B:10:0231 5 5 5 5

B:10:0001 4 4 4 4

B:10:0250 5 5 5 5

B:10:0226 1 2a 2a 2a

B:10:0237 1 1 1 1

B:10:0238 1 1 1 1

B:10:0121 1 1 1 1

B:10:0249 3 3 3 3

B:10:0264 1 1 1 1

B:10:0227 5 5 5 5

B:10:0266 5 5 5 5

B:10:0228 5 5 5 5

B:10:0223 5 5 5 5

B:09:0316 1 1 1 1

B:09:0314 N/A 4 4 4

B:09:0317 2b 1 2b 2b

B:13:0001 2b 2b 2b 2b

B:13:0002 3 3 4 4

A:16:0160 2c 2a 2c 3

A:16:0168 5 5 5 5

A:16:0223 4 4 4 4

A:16:0149 2b 2b 3 3

A:16:0153 1 1 3 3

A:16:0150 3 2c 3 3

A:16:0162 2a 2a 2a 3

A:16:0154 1 2a 2a 2a

A:16:0151 2b 2b 3 2c

A:16:0163 2b 2c 2c 2c

A:16:0157 4 4 4 4
Appendix.—Continued

Site number 1973 1984–85 1996 2012–14

A:16:0180 2a 2a33

A:16:0173 2a 2a 2a 2a

A:16:01673 2c33

A:16:0158111 2a

A:16:01741113

A:16:016111 2a 2a

A:16:01481133

A:16:0164 2b 2b3 2c

A:16:0155 2a 2a33

A:16:00011313

A:16:0159 N/A N/A44

A:16:0003 2b 2b 2b 2b

A:16:01775555

A:16:0184 2a 2a 2a 2a

A:16:01654444

A:16:0004 2b 2b 2b 2b

A:16:0176 2a 2a 2a 2a

A:16:0169 2b 2b 2b 2b

A:16:0175 2a 2a 2c3

A:16:0185 2b 2b 2c3

A:16:0171 2b 2b33

A:15:0026 2c 2b33

A:15:0022 2a 2a33

A:15:00443 2a3 2a

A:15:0035 2a 2a 2a 2a

A:15:0031 2a 2a 2a 2a

A:15:002011 2b 2b

A:15:00195555

A:15:0023 5555

A:15:00183333

A:15:000311 2b 2b

A:15:0027 2a 2a33

A:15:00211 2c 2c3

A:15:0024 2c 2c 2c3

A:15:0028 2c 2c 2c3

A:15:0032 2a 2a 2a 2a

A:15:00255555

A:15:00345555

A:15:0048 2c 2c 2c 2c

A:15:0052 5555

A:15:00565555

A:15:003611 2a3

A:15:0040 2a 2a 2a3

A:15:0005 2b 2b 2b 2c

A:15:0029 2a 2a 2a3

A:15:00331113

A:15:0039 2c 2c 2c3

A:15:0037 2a 2a 2a 2a

A:15:0043 2a 2a 2a 2a

A:15:0001 2c 2c 2c 2c

A:15:0042 2a 2a 2a 2a

A:15:0038 2a 2a 2a3

A:15:0055

5555

A:15:00471111

A:15:0051 2c 2c 2c 2c

A:15:0004 2a 2a 2a 2a

G:03:005611 2a 2a

G:03:0038111 2a

G:03:00425555

G:03:00043333

G:03:00374444

G:03:00461 2a3 2a

Appendix 103
104 Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor

Appendix.—Continued

Site number 1973 1984–85 1996 2012–14

G:03:0060 2a 2a 2a 2a

G:03:0040 2a 2a 2a 3

G:03:0064 2a 2a 3 3

G:03:0041 2b 2b 3 3

G:03:0071 3 2a 3 3

G:03:0002 2b 2b 3 2c

G:03:0024 2b 2b 3 2c

G:03:0081 2b 2b 3 3

G:03:0025 2b 2b 3 2c

G:03:0028 2b 2b 3 2c

G:03:0026 2b 2b 3 2c

G:03:0003 1 1 3 2a

G:03:0043 1 1 3 3

G:03:0055 1 1 1 1

G:03:0059 1 1 1 1

G:03:0063 1 1 1 1

G:03:0034 1 1 2b 3

G:03:0076 1 1 1 3

G:03:0032 2b 2b 2b 2c

G:03:0044 1 1 1 2a

G:03:0020-1 2b 1 2b 2b

G:03:0020-2 2b 2b 2b 2b

G:03:0030 1 1 1 3

G:03:0045 3 3 3 3

G:03:0049 1 2a 2a 2a

G:03:0029 3 2b 3 2b

G:03:0036 3 2a 2a 2a

G:03:0053 3 3 3 3

G:03:0073 2b 2b 3 3

G:03:0052 2b 2b 3 3

G:03:0066 1 1 1 1

G:03:0077 5 5 5 5

G:03:0082 5 5 5 5

G:03:0057 5 5 5 5

G:03:0061 3 3 3 3

G:03:0065 5 5 5 5

G:03:0048 5 5 5 5

G:03:0006 1 1 1 2a

G:03:0067 1 2b 2b 2c

G:03:0083 5 5 5 5

G:03:0058 1 1 2a 3

G:03:0062 5 5 5 5

G:03:0080 2b 2b 2b 2b

G:03:0054 1 1 1 2c

G:03:0085 3 3 3 3

G:03:0023 2b 2b 2b 2b

G:03:0072 1 1 1 1

G:02:0102 2b 2b 2b 2b

G:02:0101 5 5 5 5

G:02:0106 3 3 3 3

G:02:0100 1 1 1 1

G:02:0105 5 5 5 5
Menlo Park Publishing Service Center

Manuscript approved for publication March 29, 2016

Edited by Katherine Jacques

Cartography and graphics by Jeanne DiLeo and Jacqueline Olson

Layout by Cory D. Hurd
East and others—Conditions and Processes Affecting Sand Resources at Archeological Sites in the Colorado River Corridor—Professional Paper 1825

ISSN 2330-7102 (online)

http://dx.doi.org/10.3133/pp1825