Spatial and Temporal Patterns of Erosion in Western China and Tibet Amanda C. Henck A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Washington 2010 Program Authorized to Offer Degree: Geological Sciences In presenting this dissertation in partial fulfillment of the requirements for the doctoral degree at the University of Washington, I agree that the Library shall make its copies freely available for inspection. I further agree that extensive copying of this dissertation is allowable only for scholarly purposes, consistent with “fair use” as prescribed in the U. S. Copyright Law. Requests for copying or reproduction of this dissertation may be referred to ProQuest Information and Learning, 300 North Zeeb Road, Ann Arbor, MI 48106- 1346, 1-800-521-0600, to whom the author has granted “the right to reproduce and sell (a) copies of the manuscript in microform and/or (b) printed copies of the manuscript made from microform.” Signature__________________________ Date______________________________ University of Washington Abstract Spatial and Temporal Patterns of Erosion in Western China and Tibet Amanda C. Henck Chairs of the Supervisory Committee: David R. Montgomery, Professor Department of Earth and Space Sciences Bernard Hallet, Professor Department of Earth and Space Sciences Two parallel debates are ongoing about how tectonics and climate interact to set erosion rates and how humans alter these erosion rates; these issues form the basis for my dissertation. Using the eastern margin of the Tibetan Plateau as a natural laboratory to inform these two debates, I conduct detailed studies of millennial and decadal erosion rates in southwest China and southeast Tibet, focusing specifically on the Three Rivers Region (TRR) as well as other rivers in Tibet and Yunnan. The TRR is where the Salween, Mekong, and Yangtze Rivers flow parallel to one another incised in 3000-m deep gorges. To further understand controls on annual sediment yield, I calculate effective discharges for the same region. I find two patterns in the millennial erosion rates. First is an along river trend of increasing erosion rates as the rivers leave the dry, flat Tibetan Plateau and enter the wet, steep gorges of the TRR. In the Salween and Mekong Rivers, erosion rates are well correlated with relief and stream power, while in the Yangtze River they are correlated with rainfall. Second is a pattern across the rivers. In the lower basins of the TRR the erosion rates in the Salween are 200 times higher than the erosion rates in the Yangtze River across a region with little variation in stream power and no variation in relief or rainfall, suggesting a strong tectonic control on erosion rates. Using daily sediment concentration and discharge records, I calculate mean annual sediment yield for 44 stations in Tibet and Yunnan. I find that sediment yield is weakly correlated with mean annual rainfall and fraction of land under cultivation and that there are no systematic trends in the sediment yield data over time. To further investigate the controls on these sediment yields, I calculate effective discharge for all of these stations. The effective discharge calculations shed light on a long-standing debate about whether small and frequent or large and infrequent events transport more sediment. In this monsoon region, I find that effective discharge recurs throughout the monsoon and is controlled by monsoon base flow rather than individual storm events. TABLE OF CONTENTS LIST OF FIGURES ................................................................................................. iii LIST OF TABLES ................................................................................................... iv Chapter 1: Introduction .............................................................................................. 1 Tectonic and geomorphic context .......................................................................... 2 Chapters 2-4 ........................................................................................................... 4 Conclusions ............................................................................................................ 6 Notes for chapter 1 ................................................................................................. 7 Chapter 2: Spatial controls on erosion in the Three Rivers Region ......................... 10 Introduction .......................................................................................................... 10 Samples and methods ........................................................................................... 15 Results .................................................................................................................. 22 Discussion ............................................................................................................ 28 Conclusion ........................................................................................................... 37 Acknowledgements .............................................................................................. 39 Figures for chapter 2 ............................................................................................ 40 Tables for chapter 2 .............................................................................................. 47 Notes for chapter 2 ............................................................................................... 65 Chapter 3: The myth of Communist land degradation: new evidence from local erosion and basin-wide sediment yield in SW China and SE Tibet ......................... 76 Introduction .......................................................................................................... 76 Methods ................................................................................................................ 82 Results .................................................................................................................. 85 Discussion ............................................................................................................ 89 Conclusions .......................................................................................................... 94 Acknowledgements .............................................................................................. 96 i Figures for chapter 3 ............................................................................................ 97 Tables for chapter 3 ............................................................................................ 106 Notes for chapter 3 ............................................................................................. 114 Chapter 4: Monsoon control of effective discharge, Yunnan and Tibet ................ 121 Introduction ........................................................................................................ 121 Methods .............................................................................................................. 124 Results and discussion ....................................................................................... 126 Conclusions ........................................................................................................ 127 Acknowledgments .............................................................................................. 129 Figures for chapter 4 .......................................................................................... 130 Tables for chapter 4 ............................................................................................ 132 Notes for chapter 4 ............................................................................................. 134 Bibliography ........................................................................................................... 138 ii LIST OF FIGURES Figure 2.1: Context map of the study region ........................................................... 40 Figure 2.2: Location of samples and topography for the TRR. ............................... 41 Figure 2.3: Erosion rates measured for the TRR ..................................................... 42 Figure 2.4: Geomorphic parameters for the region. ................................................. 43 Figure 2.5: Relationship between erosion and geomorphic parameters .................. 44 Figure 2.6: Basin area vs. cumulative sediment yield for each river. ...................... 45 Figure 2.7: Longitudinal profiles for main stem of each river. ................................ 46 Figure 3.1: Location of the study region .................................................................. 97 Figure 3.2: Details on location and quality of sediment yield data. ......................... 98 Figure 3.3: Geomorphic and anthropogenic data about the region. ......................... 99 Figure 3.4: Scatter plots showing the relation between various geomorphic and anthropogenic metrics and sediment yield. ...................................................... 100 Figure 3.5: Sediment yield and sediment rating curve parameters as a function of time ................................................................................................................... 102 Figure 3.6: Erosion rate calculated from sediment yield as a function of erosion rate measured over millennial timescales from 10Be data. ............................................ 104 Figure 3.7: Cumulative sediment