Gully erosion of archaeological sites in Grand Canyon

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Gully erosion of archaeological sites in Grand
Canyon National Park The question of causality
Paul A. Petersen
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Background Increased gully erosion of arch sites
on Holocene terraces in Grand Canyon
(Thompson and Potochnik, 2000)
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Baselevel Hypothesis (Hereford et al, 1993)
Heavily criticized, neither verified nor falsified
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Our Approach

• Test for upcatchment control
• Common area-slope threshold?
• Link between vegetation and infiltration?
• 3) How do all these factors relate?

field data and simple models
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Area-Slope dataset

• Measured slopes and contributing drainage areas
  at 30 gully
• heads from 7 different sites in Grand Canyon
  National Park

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Terrain Data Set
High resolution photogrammetry (2 cm
pixels) High resolution ground survey
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DEM dataset
Outside photogram coverage
Gully
Coarse River Deposits
Sandy alluvium
Talus slope

• 10 cm DEMs, 4 sites
• Spline tension
• D? slope and area grids (Tarboton, 1997)

Colorado River
Indian Canyon
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Results
Indian Canyon
y 0.0165x-0.4655
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Shifting gears Modeling erosion response to
vegetation change

• Quantify ground cover (1 site)
• 2) Separate veg zones and clip grids
• 3) Create ground cover grids, merge
• 4) Create K grid based on ground cover
• 5) Calculate infiltration excess from storm
• 6) Accumulate infiltration excess (depth)
• 7) Multiply accumulation grid and slope grid
• 8) Repeat with different ground cover input

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Discussion
Thresholds change through space and time
(Montgomery and Dietrick, 1994) Vegetation
affects erosion threshold grass to shrub lt Ic
gt t (Abrahams et al., 1995) Only small veg
change needed to affect erosion (Rogers and
Schumm, 1991) Climate change during late 1970s
in Grand Canyon (Hereford and Webb, 1992)!
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Conclusions Baselevel and erosion potential
Gullies in Grand Canyon defined by area-slope
parameters Baselevel change not necessary to
account for head position and extent Climate
and vegetation change feedbacks can change
threshold, rejuvenate system
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Questions? Comments?
Acknowledgements
Funding Grand Canyon Monitoring and Research
Center Geological Society of America Colorado
Scientific Society Field, lab, and thought
assistance Joel Pederson, David Chandler, Wally
McFarlane, Jen Dierker, Jay Norton, Stacy
Petersen, Sammie McFarlane, Isaac Larsen, Jesse
Allen, Lynn, Thomas, Scott Cragun, Jack Schmidt,
Tom Monaco Moral support and encouragement
Stacy Petersen