Effects of dam operations 

and vegetation management 

on the preservation and 

geomorphic condition of 

archaeological sites 

• Glen Canyon Dam Adaptive Management

Work Group Meeting 

• February 28-29, 2024, Phoenix, AZ 

• Helen Fairley1

, Joel B. Sankey1, Joshua 

Caster1, Lauren Tango1, Lonnie Pilkington2

,

Jennifer Dierker2, Ellen Brennan2, Amy E. 

East3 

• 1U.S. Geological Survey, Southwest Biological 

Science Center, Grand Canyon Monitoring and 

Research Center 

• 2U.S. National Park Service, Grand Canyon

National Park 

• 3U.S. Geological Survey, Pacific Coastal and

Marine Science Center 
 

 

Program Goals: Archaeological and Cultural Resources 

GCDAMP Regulatory Goals: 

• GCPA goal: Operate Glen Canyon Dam so as to protect, mitigate adverse impacts 

to, and improve ... natural and cultural resources... 

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

• LTEMP Goal 1: Maintain the integrity of potentially affected NRHP-eligible or listed 

historic properties in place, where possible, with preservation methods employed 

on a site-specific basis. 

Photo credit: J Caster (USGS) 
 

 

River management and 

archaeological site preservation 

• Native peoples occupied Grand 

Canyon for at least 9000 years, 

resulting in 100s of archaeological sites 

that are a tangible record of human 

history in this landscape 

• Burial of archaeological sites by riversourced aeolian sand provides a 

protective cover and resilient surface, 

reducing erosion potential 

• Important for site preservation 

• Long term reduction in sediment 

supply and increase in riparian 

vegetation since closure of Glen 

Canyon Dam has increased 

archaeological site erosion and 

decreased preservation potential 

Sankey et al., 2023, Journal of 

Environmental Management 

A. Prehistoric hearth B. Prehistoric bowl C. Slab structure D. Fire-altered rock 

in alluvial cutbank buried in sand eroding in gully exposed by deflation 
We use two 

different but 

complementary 

methods to 

monitor dam 

effects at 

archaeological 

sites 

Method 1: 

For entire population of sites within the Area of 

Potential Effect (n=362), we monitor changes in 

two classifications-- drainage evolution and fluvial 

sand connectivity -- at ~5-10-year intervals 

Method 2: 

For a sample of sites, we monitor change in 

topography (sediment deposition and erosion) 

using repeat lidar surveys, once every ~3 years 

 
Results: Fluvial Sediment Connectivity Classification 

From Sankey et al., 2023, Journal of Environmental Management 
 

 

 

Metric 1.3: Changes in aeolian and drainage classifications reflect 

changes in site stability and condition linked to dam operations 

• 89 sites (24%) changed 

to a “less connected” 

aeolian class 

• 20 sites (6%) changed to 

a more degraded 

drainage classification 

• 7 (2%) sites changed in 

both respects to a more 

degraded condition 

• Majority of sites (n=246, 

68%) did not change 

• No sites showed 

“improvement” in terms 

of their classifications 

Sankey et al., 2023, Journal of Environmental Management 
 

 

 

 

 

Metric 1.2: Lidar Topographic 

Change Detection 

• Currently, sample includes 27 sites with 

multiple repeat surveys that document 

net change in erosion or deposition 

• Net deposition documented at 33% of 

these sites (n=9); most are Aeolian 

Class 1 or 2 

• Approximately 55% of monitored sites 

have lost surface sediment (eroded) 

• 11% have neither aggraded nor eroded 

• Long-term effects of April 2023 HFE are 

not reflected in these results 

Preliminary results, please don’t cite 
 

 

 

 

 

 

Metric 1.1: Change in Integrity 

• Metric 1.1 is a recently added metric

that is specifically focused on Integrity 

• Integrity has a specific meaning in the

historic preservation field and the

National Historic Preservation Act: “the 

ability of a historic property to convey

its significance” 

• Integrity is not measurable. It is a 

professional judgment and is either

present or absent (i.e., there are no

“degrees” of integrity) 

• Metric 1.1 documents number of sites 

that have lost integrity during LTEMP 

• Currently all sites in the APE continue

to retain integrity, despite erosion

continuing to affect many sites 

Eroding archaeological sites in Grand Canyon, 2017 (photos by J. Sankey) 
 

 

 

 

Experimental management to increase wind transport of river-sourced sand 

Increase aeolian sediment supply: 

1. HFEs to rebuild river sandbars (Sankey et al., 2018, Aeolian Research) 

2. Lowering dam releases to expose sand that is normally underwater in the river channel & eddies (Sankey et al., 2022, JGR) 

Minimize vegetation blocking aeolian sand transport: 

3. Vegetation-management by NPS to remove riparian vegetation on sandbars (Pilkington et al., 2022, Park Science) 

2. Channel and eddy sand 

1. Sandbar sand 
Research question 

• Does removal of riparian vegetation barriers located 

between river sandbars and archaeological sites increase the 

resupply of aeolian sediment to sites? 

Wind 
 

 

 

 

 

 

 

 

Vegetation management for 

dunefield archaeological site 

restoration 

• Implemented in coastal 

dunefields around the world 

• Improve cultural, ecological, 

recreational resources 

• Protect infrastructure 

• Not common for aeolian 

dunefield and archaeological 

sites in river environments 

Photos taken before (top) and after (bottom) removal of 

invasive vegetation to restore a coastal dunefield at 

Doughboy Bay, Stewart Island, New Zealand (Konlechner et 

al., 2014, Dynamic Environments) 

An intact oven (top) in a 

coastal dunefield 

occupied by Maori 

ancestors, Mason Bay, 

New Zealand. The oven 

was exposed (top) and 

eroded (bottom) due to 

reduction in the supply of 

windblown coastal sand 

owing to encroachment of 

invasive vegetation (e.g., 

panel a). (Hilton and 

Konlechner, 2014, 

University of Otago) 
 

 

 

 

 

 

 

Lidar measured topographic 

change at archaeological sites 

downwind of sandbar vegetation 

management areas 

• Experimental removal of vegetation 

on river sandbars by NPS began in 

2019 

• Has contributed to some, but not all, 

of the associated downwind 

archaeological sites being buried by 

windblown river sand over time 

• Cumulative topographic changes 

likely dependent on frequency and 

timing of HFEs and vegetation 

management, as well as downwind 

distance to archaeological site, and 

other factors 

Preliminary results, please don’t cite 
Incipient aeolian dune on 

sandbar (wind blown river 

sand) that grew following 

the 2023 Spring HFE and 

migrated inland towards 

vegetation management 

area, downwind dunefield 

and archaeological sites. 

September 2023 Preliminary results, 

please don’t cite 
Monitoring for Potential Threats to Grand Canyon Rock Art 

In 2023 LTEMP Cultural PA 

requested USGS-GCMRC 

monitor C:06:0005 “Supai 

Man” rock writing site for 

degradation using LiDAR and 

photogrammetry 

September 1990 

Feature Map 

April 2023 

5 mm Lidar Relief Model 

1 m