1. Gully erosion of archaeological sites in Grand Canyon National Park:
The question of causality
Paul A. Petersen

2. Increased gully erosion of arch sites on Holocene terraces
Background
Increased gully erosion of arch sites on Holocene terraces
in Grand Canyon
(Thompson and Potochnik, 2000)

3. Baselevel Hypothesis (Hereford et al, 1993)
Heavily criticized, neither verified nor falsified

4. 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

5. Area-Slope dataset
Measured slopes and contributing drainage areas at 30 gully
heads from 7 different sites in Grand Canyon National Park

6. Terrain Data Set High resolution photogrammetry (2 cm pixels) +
High resolution ground survey

7. D slope and area grids (Tarboton, 1997)
DEM dataset
Outside photogram coverage
Gully
Sandy alluvium
Coarse River
Deposits
Talus slope
10 cm DEMs, 4 sites
Spline tension
D slope and area grids (Tarboton, 1997)
Colorado
River
Indian Canyon

8. Results
Indian Canyon
y = x
1) Indian Canyon: 10 gullies observed, all predicted (100%)
2) Arroyo Grande: 19 gullies observed, 18 predicted (95%)
3) Granite Park: 19 gullies observed, 18 predicted (95%)
4) Gorilla Camp: 20 gullies observed, all predicted (100%)
Total of 4 sites: 68 gullies observed, 66 predicted (97%)

9. 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

10. Erosion index (d x s)
Shrub:Grass = high
Shrub:Grass = low
Difference map
Average
1.4844
1.4750
0.0095
Stdev
0.2410
Min
0.0000
Max

11. Discussion Thresholds change through space and time
(Montgomery and Dietrick, 1994)
Vegetation affects erosion threshold; grass to shrub = < Ic; > τ
(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)!

12. 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

13. 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