1. Development of a Geomorphic Model to Predict Erosion of Pre-Dam Colorado River Terraces Containing Archaeological Resources Kate Thompson and Andre Potochnik Principal Investigators SWCA, Inc. Environmental Consultants and Gary O’Brien, Ron Ryel, and Lynn Neal

2. Problem Apparent rapid gully erosion of pre- dam terraces during the past two decades has caused loss of numerous cultural sites in Grand Canyon river corridor

3. Objectives of Study Test hypotheses: –Has erosion increased in the post dam period? –If so, is erosion climate driven or dam-related? Develop a model that predicts relative vulnerability of sites to erosion

4. Gully Erosion

5. Arroyo Development

6. Processes Driving and Resisting Erosion Illustrated by Gary O’Brien

7. Process Restoring Erosion

8. Cut and Fill (Cataract Canyon)

9. Study Locations

10. Comparison of Furnace Flats to Cataract Canyon

11. Cross Section of Terraces in Cataract Canyon

12. Part 1 Test Hypotheses

13. Arroyo at Paria River

14. Arroyo Aggradation (Paria River)

15. Null Hypothesis: Degree of gully erosion has remained unchanged from the pre-dam to post-dam period Test 1 - Use air photos to determine the degree of channel lengthening since 1965 Test 2 - Compare amount of gully erosion in Cataract Canyon (control section) to Furnace Flats section in Grand Canyon

16. Channel Lengthening Over Time n = 23

17. Gully Density/Depth of 3 Paired Areas

18. Climatic Variation Hypothesis: High precipitation anomalies in the post-dam period increases severity of gully erosion Test 1 - Evaluate previous research on variation of 20th century precipitation Test 2 - Investigate variation in monsoon season rainfall at equivalent time periods before and after closure of the dam

19. Decadal Variation in 20th Century Precipitation Webb et al. in prep.

20. Monsoon Precipitation for 13 Weather Stations, Colorado River Corridor (> 50 mm / month) Line of equal events

21. Monsoon Precipitation for 13 Weather Stations, Colorado River Corridor (>25 mm/day) Line of equal events

22. Base-Level Hypothesis: Reduction of sand supply and large floods in the post-dam period increases degree of gully erosion Test 1 - Report on rebuilding of high-elevation sand bars in both Grand Canyon and Cataract Canyon Test 2 - Assess catchment and river processes at each study site in Grand Canyon

23. 222 mile - 1923 (E.C. LaRue photo)

24. Granite Park 1963 1000 cfs (Belnap collection) Granite Park 1996 8000 cfs (Lisa Leap photo)

25. Old Unkar Camp 1963 (Belnap collections) Old Unkar Camp 1998

26. Cross Canyon March 1999 Cross Canyon August 1999

27. Rapid 12 March 1999 Rapid 12 August 1999

28. Percent of sites containing 1983 and pda deposits

29. Sites Supporting Base-Level Hypothesis n = 119 < 3 not supported = 3 weakly supported = 4 supported = 5 strongly supported

30. Part 2 Geomorphic Model for the Small-Catchment System

31. Steps to Building Geomorphic Model Classify catchments by geomorphic setting Construct process-based conceptual model Construct predictive mathematical model Use model to predict vulnerability of individual sites

32. Geomorphic Settings Illustrated by Gary O’Brien

33. Mathematical Model - Step 1 Quantify driving and resisting parameters Q = C*I*A (Am. Soc. Civil Engineers) Total runoff (m 3 ) upper catchment Axt = Wt * Dt Cross-sectional area of terrace segment

34. Add Geomorphic Factors to Model

35. Mathematical Model - Step 2 Vr = ln(Q)/ln[Axt * (1+TF)] Vr is raw vulnerability FVC i = (Vr * FVC i-1 )/100 FVC i is cumulative vulnerability thus: vulnerability rating of archaeological terrace = Vr of highest terrace and: vulnerability rating/catchment = mean FVC i

36. Vulnerability Plot 0 0.2 0.4 0.6 0.8 1 1.2 020406080100 Vulnerability of top terrace Gully depth ratio Grand Canyon Cataract Canyon n = 128 Threshold line

37. Mean Vulnerability by Geomorphic Setting

38. Conclusions (hypotheses testing) Gully erosion in terraces is more severe from 1978-1999 than 1942-1977 Gully erosion is more extensive in Grand Canyon today than in Cataract Canyon control site. Gully erosion is increased due to both a high precipitation anomaly and a decrease in sediment renewal. 78% of channels draining archaeological sites show most elements of restorative base-level process Eolian redistribution of fresh flood sand is on- going at about 50% of catchments

39. Conclusions (the predictive model) Process-based model works best for this small- catchment geomorphic system –it simplifies enormous variety and complexity of small catchments Statistically based model does not work well –poor correlation of gully depth/width to most measured parameters Highest vulnerabilities are function of large catchment area and narrow terrace width

40. Recommendations Site mitigation achieved by slowing erosion: – decrease stream power – increase terrace diffusivity Data recovery suggested at sites where: –outliers occur on plot –gully-depth ratios are close to 1.0 –there are few base-level controls Use vulnerability plot to: – identify high risk sites –use as a base to track how points shift in future

41. Future Work Integrate mathematical model results with mainstem studies (Wiele, 2000). Refine model through application and observation. Quantify drainage density of uppermost terrace: could be more important than gully depth and width. Eolian studies: quantify redistribution of sand. Repeat historic photography.