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Gully erosion of archaeological sites in Grand Canyon National Park: The question of causality

Gully erosion of archaeological sites in Grand Canyon National Park: The question of causality. Paul A. Petersen. Background Increased gully erosion of arch sites on Holocene terraces in Grand Canyon. (Thompson and Potochnik, 2000). Baselevel Hypothesis (Hereford et al, 1993).

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Gully erosion of archaeological sites in Grand Canyon National Park: The question of causality

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

  2. 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. DEM dataset Outside photogramcoverage 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

  8. 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%) Results Indian Canyon y = 0.0165x-0.4655

  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 37.4230 37.2017 0.2410 Min 0.0000 0.0000 -2.2826 Max 19302.5723 19162.3359 140.2363

  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

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