Archaeological Sites are Eroding in 

Grand Canyon Owing to Six Decades 

of Glen Canyon Dam Operations: 

Floods, Low Flows and 

Vegetation Management Can Help

Glen Canyon Dam Adaptive Management Program 

Annual Reporting Meeting 2023

January 24-25, 2023, Phoenix, AZ

Joel B. Sankey1, H. Fairley1, J. Caster1, A. East2,

J. Dierker3, E. Brennan3, L. Pilkington, N. Bransky1, A. 

Kasprak1,4

1US Geological Survey, Southwest Biological Science 

Center, Grand Canyon Monitoring and Research 

Center, Flagstaff, AZ

2US Geological Survey, Pacific Coastal and Marine 

Science Center, Santa Cruz, CA

3National Park Service, Grand Canyon National Park, 

Flagstaff, AZ

4Fort Lewis College, Durango, CO

*This information is preliminary and is subject to revision. It is being provided to meet 

the need for timely best science. The information is provided on the condition that 

neither the U.S. Geological Survey nor the U.S. Government shall be held liable for 

any damages resulting from the authorized or unauthorized use of the information. Photo credit: J. Caster (USGS)
Program Goals: Archaeological and Cultural Resources 

GCDAMP Regulatory Goals:

• GCPA goal: protect, mitigate adverse impacts to, and improve ... natural and 

cultural resources...

• LTEMP and NHPA goal: “Preservation in place.”

• Maintain the integrity of potentially affected NRHP-eligible or listed historic 

properties in place, where possible, with preservation methods employed on a sitespecific basis. 

Photo credit: J Caster (USGS)
TWP Projects: Archaeological and Cultural Resources

Triennial Workplan (TWP) projects collectively examine whether and how dam operations 

and experiments like HFEs, vegetation management, and site management by NPS help 

to achieve the LTEMP goal of preservation in place:

• GCMRC TWP

• Project Element D.1. Dam Operations, Vegetation Management, 

Archaeological Sites (USGS, Sankey, Fairley)

• FY2021/2022/2023 ($258k/250k/266k)

• Project Element O.3. Aeolian Response to a Spring Pulse Flow (USGS, Sankey, 

Fairley)

• FY2021 ($10k)

• Reclamation TWP

• Project Element D.3. Cultural Resources Monitoring – Grand Canyon (NPS, 

Brennan, Dierker)

• Project Element C.7. GRCA Experimental Vegetation Treatment (NPS, Pilkington) 

Photo credit: J. Caster (USGS)
Human Activity and Archaeology Along the 

Colorado River in Grand Canyon

• People have used resources on the Colorado River in Grand Canyon for at least 9,000 years

• Indigenous peoples have inhabited region periodically since time immemorial

• European explorers first visited the canyon 480 years ago 

• Today, evidence of those ancestral peoples and recent historic activities is displayed in hundreds of 

archaeological sites along the river in Grand Canyon National Park 

Multi-room structures Large donut-shaped roasting 

feature

Historic boat Images pecked into rock

Photo credits: J. Dierker and other National Park Service staff

https://www.nps.gov/grca/learn/historyculture/arch.htm
Geomorphic Setting of River Corridor

Archaeological Sites in Grand Canyon National Park

• Colorado River-sourced sediment 

deposits contain much of the river 

corridor archaeological record in 

Grand Canyon

Photo credits: J. Sankey (USGS)
NPS and USGS Monitor and Research Effects of Humans

and Natural Processes on Archaeological Site Condition

Photo credits: J. Dierker and other National Park Service staff

Examples of erosion documented by NPS monitoring site visits

A) Cutbank erosion in alluvial terrace exposes prehistoric hearth, B) Rainfallrunoff erosion and lack of burial by sand expose prehistoric bowl, C) Gully 

eroding base coarse of structure, D) Wind erosion and lack of burial by 

sand result in loss of matrix surrounding fire-altered rock
NPS and USGS Monitor and Research Effects of Humans and Natural 

Processes on Archaeological Site Condition

Photo credit: J. Caster (USGS)
Lack of floods & decrease in windblown (aeolian) river sand supply have

resulted in expanded gullying at archaeological sites from pre- to post-dam time

River Mile -0.5

La Rue, 1923

River Mile -0.5

Fairley & Fairley, 2021
Gullying at archaeological sites: some examples

Photo credits: 

J. Caster and J. 

Sankey (USGS)
Long-term increases in riparian vegetation on sandbars have decreased

windblown (aeolian) sand supply from sandbars to archaeological sites

River Mile 194, River right

Borden-Weeden, 1973

River Mile 194, River right

Fairley-Fairley, 2021

River Mile 219, River right

Borden-Weeden, 1973

River Mile 219, River right

Fairley-Fairley, 2021

Photo Sources: 1973 (Weeden), 2019 (Fairley and Fairley), 

1984 and 2021 (GCMRC Aerial Overflight Image Archives) 
Thus, archaeological sites are especially vulnerable to erosion from gullying

processes, but burial by windblown river sand is the ongoing contemporary

mechanism that can offset that gully erosion and potentially preserve sites in-situ

Gully

Cultural Feature

Cultural Feature

Cook et al., 2019; East et al., 2016
Assess changes to the geomorphic condition of Colorado River archaeological sites 

along the entire river corridor throughout Grand Canyon National Park.

Compile the results of two classification metrics applied to the population of 362 

archaeological sites within the area of potential effect of operations of Glen Canyon 

Dam over multiple decades, along with repeat lidar survey topographic change 

detection metrics from a sample of these sites.

Objective and Methods

Photo credit: J Caster (USGS)
Methods: Site type classification system for extent 

of gullying at archaeological sites

• The drainage classification 

system assesses the 

maximum local maturity of 

gully networks. 

• River-based (D4) and sidecanyon-based (D3) drainages 

are graded to the lowest 

possible local base level in 

this system and represent the 

evolutionary endpoint of 

drainage development. 

• Terrace-based (D2) drainages 

represent an intermediary 

stage of development and 

may, in the future, become 

river-based or side-canyon 

based drainages.

(adapted from East et al., 2016, 2017)
Results: Drainage Classification 

(Proposed LTEMP Performance Metric) 

Preliminary results, subject to change, please don’t cite
Methods: Site type classification system for windblown 

(aeolian) sand supply from sandbars to archaeological sites

Fluvial Sediment 

Connectivity (FSC) 

(a.k.a. “Aeolian 

Classification”) is a 

ranked classification of 

the relative potential for 

archaeological sites to 

receive windblown sand 

from upwind river 

sandbar deposits which 

might keep sites buried 

with a protective cover of 

sand that can potentially 

offset erosion that 

otherwise occurs. 

(adapted from East et al., 2016, 2017)
Results: Fluvial Sediment Connectivity Classification 

(Proposed LTEMP Performance Metric) 

Preliminary results, subject to change, please don’t cite
Methods: Lidar Topographic Change Detection

Repeat lidar surveys are 

used to accurately measure 

topographic changes 

associated with erosion and 

deposition of sediment over 

time within archaeological 

sites and surrounding 

landscape

(Caster et al., 2022)
Results: Lidar Topographic Change Detection 

(Proposed LTEMP Performance Metric) 

Caster et al., 2022
Summary

• Condition of 362 Colorado River archaeological sites

assessed

• River-sourced aeolian sand decreased since 1973,

making most sites more erosion-prone

• Proportion of sites eroding by gully processes has

increased since 2000

• Erosion is counter to management goal to maintain

or improve site integrity in situ (preservation in place)

• Environmental management opportunities: floods, 

low flows, riparian plant removal

Photo credit: J Caster (USGS)

Preliminary results, subject to change, please don’t cite
Management opportunities

Three ways to increase wind transport of river-sourced sand to muchhigher-elevation locations of archeological sites to help decrease or 

reverse erosion and increase potential for preservation in place

Increase supply of upwind bare sand by:

1. HFE high-elevation sandbar deposition

2. lowering dam releases to expose sand that is normally underwater

Minimize vegetation on sandbar blocking wind transport of sand: 

3. ongoing NPS/GCMRC LTEMP vegetation-management experiments

1. HFE sand

2. Low sand

Photo credit: J. Sankey (USGS)
Management opportunity: Floods

HFE sand

1. Increase supply of upwind bare sand by HFE high-elevation sandbar 

deposition

• Windblown river sand deposited in dunefield archaeological sites 

increases cumulatively when upwind river sandbars are 

resupplied by consecutive annual HFEs (Sankey et al., 2018)

Photo credit: J. Sankey (USGS)
Management opportunity: Low Steady Flows

Low sand

2. Increase supply of upwind bare sand by lowering dam releases to expose sand that 

is normally underwater

• Low steady flows and wind offer alternative to flooding for maintaining sandy 

landscapes in Grand Canyon (Sankey et al., 2022)

• Lowering river discharge to ~ 4,000 CFS during the 2021 Spring 

Disturbance flow exposed ~26,000 m2 of sand per kilometer of river; 

>100% increase in source area for windblown sand supply (Kasprak, 

2021)

• 2-3 days of low steady flow duration sufficient for river sand to dry and 

be transported by wind (Sankey et al., 2022)

Photo credit: J. Sankey (USGS)
Management opportunity: Riparian vegetation removal

Preliminary results, subject to change, please don’t cite

3. Ongoing NPS/GCMRC LTEMP vegetation management experiments 

remove the barrier of invasive riparian vegetation on sandbars that blocks 

wind transport to archaeological sites. 

• Initially Implemented 2019, repeated 2020/2021/2022...

• Project report on findings in this year (FY23 Workplan Deliverable)

• Preliminary results indicate more windblown sand deposition on some 

sandbar campsites and downwind archeological sites than before 

vegetation removal (Pilkington, 2022; Caster ARM 2023 Poster)

• However, implemented based on assumption we’d have HFEs!

Photo credit: J. Sankey (USGS)
Conclusions

• Majority of Colorado River archaeological sites are eroding

and are more erosion prone owing to six decades of Glen

Canyon Dam Operations

• Erosion is counter to management goal to maintain or

improve site integrity in situ (preservation in place)

• Environmental management opportunities to increase in-situ

preservation potential by slowing down or reversing site

erosion are floods, low flows, and riparian plant removal

• When in-situ preservation fails, site excavations become the

management option

Photo credit: J. Caster (USGS)
Key References and 2022 Deliverables

Photo credit: J Caster (USGS)