COMPARISON OF LANDSLIDES AND THEIR RELATED OUTBURST FLOOD DEPOSITS, OWYHEE RIVER, SOUTHEASTERN OREGON _________________________________________________ A Thesis Presented to The Graduate Faculty Central Washington University _________________________________________________ In Partial Fulfillment of Requirements for the Degree Master of Science Geology _________________________________________________ by Shannon Marian Othus May 2008 CENTRAL WASHINGTON UNIVERSITY Graduate Studies We hereby approve the thesis of Shannon Marian Othus Candidate for the degree of Master of Science APPROVED FOR THE GRADUATE FACULTY _________________ ________________________________________ Dr. Lisa L. Ely, Committee Chair _________________ ________________________________________ Dr. Beth Pratt-Sitaula _________________ ________________________________________ Dr. Karl Lillquist _________________ ________________________________________ Dr. Wendy Bohrson, Geology Department Chair _________________ ________________________________________ Dean of Graduate Studies ii ABSTRACT COMPARISON OF LANDSLIDES AND THEIR RELATED OUTBURST FLOOD DEPOSITS, OWYHEE RIVER, SOUTHEASTERN OREGON by Shannon Marian Othus May 2008 Numerous landslides have entered the Owyhee River canyon north of Rome, Oregon. As the river flows through different lithologic units, the style of mass wasting changes from large slump events and small rock falls to large earthflows. The change in mechanism of mass wasting from upstream to downstream seems to depend on several factors: (1) the ratio of the basalt cap to the exposed underlying sediments, (2) the composition of underlying sediments, (3) the canyon geometry, and (4) the extent and frequency of mass wasting. All three mechanisms of mass wasting have the ability to block the river channel and display associated flood deposits downstream. Only earthflows show aggradation deposits upstream, suggesting greater longevity than other types of mass wasting. Also, based on the frequency and size of related outburst flood deposits, smaller rock falls could possibly have the greatest cumulative effect on river channel evolution. iii ACKNOWLEDGMENTS Assistance was provided by many people and organizations during this project. Funding was generously provided by the National Science Foundation grant EAR- 0617234, the CWU Department of Graduate Studies, and the Geological Society of America. Special thanks go to my field assistants, Tabitha Trosper and Leslie Hickey, for endless help in the field carrying appliances and tools back and forth across the river and hiking up giant hills in 100°F heat. Thanks also go to P. Kyle House, Liz Safran, Jim E. O’Connor, Gordon Grant, and Rose Wallick for providing hours of fun in the field and meaningful and diverse interpretations of geomorphic deposits and landforms. Thanks also go to Megan Mills-Navoa and Scott Anderson for providing terrace profiles for the Artillery reach, singing in the field, and the butt-rock. Thank you to Cassandra Fenton for providing the cosmogenic dates presented in this paper. Thanks also go to my committee and other faculty and staff of the Department of Geological Sciences, especially Craig Scrivner for putting some much needed computer programs on my laptop. Immense gratitude goes to Vicki Potts for Friday afternoon conversation involving my many students in White Swan, for always being on top of my paperwork, and knowing all of the ins-and-outs related to the bureaucracy of CWU. I would also like to thank the WATERS program for increasing my standard of living exponentially. To my fellow students of 2008, thanks for the many venting sessions regarding the many frustrations as a result of TA-ing and pagination. And my final thanks go to my advisor, Dr. Lisa Ely, whose help in writing and supporting data is immeasurable. iv TABLE OF CONTENTS Chapter Page I INTRODUCTION............................................................................................. 1 Location…. ................................................................................................. 2 II BACKGROUND .............................................................................................. 7 Geologic Setting.......................................................................................... 7 Landslides and Landslide Dams in Fluvial Systems................................. 10 Effects of Landslide Dams on River Profiles ........................................... 12 Outburst Flooding..................................................................................... 14 III METHODS ..................................................................................................... 17 Geologic Mapping.................................................................................... 17 Characterizing Landslide Events and Composition of Geology Beneath Local Basalt Cap....................................................................................... 18 Relative and Absolute Dating ................................................................... 22 Flood Boulder Analysis ............................................................................ 26 Terrace Characteristics and Longitudinal Profiles.................................... 29 IV RESULTS AND INITIAL INTERPRETATIONS......................................... 33 Mapping .................................................................................................... 33 Unit Descriptions (Plate 1)........................................................................ 35 Unit Descriptions (Plate 2)........................................................................ 43 Characterizing Landslide Events and Composition of Geology Beneath Local Basalt Cap....................................................................................... 48 Absolute Dating........................................................................................ 53 Flood Boulder Analysis ............................................................................ 57 Terrace Composition and Longitudinal Profiles....................................... 63 V DISCUSSION................................................................................................. 69 Artillery Reach.......................................................................................... 69 Artillery Landslide.................................................................................... 76 Sedimentology of Tsv Sediments at Artillery Landslide.......................... 78 v TABLE OF CONTENTS (continued) Chapter Page Hell’s Gate Rock Fall................................................................................ 80 Artillery Reach Outburst Flood Deposits ................................................. 83 The Hole in the Ground ............................................................................ 86 Earthflow Dams ........................................................................................ 89 Sedimentology of East Spring Earthflow ................................................. 95 Aggradation Deposits and Flood Bars of the Hole in the Ground Reach......................................................................................................... 97 Landslide Comparison ............................................................................ 104 Dam Breach and Related Geomorphic Surfaces..................................... 107 Flood Boulder Analysis .......................................................................... 110 VI SUMMARY AND CONCLUSIONS ........................................................... 115 Future Work............................................................................................ 120 REFERENCES CITED................................................................................. 122 APPENDICES .............................................................................................. 126 Appendix A—Hand Sample Descriptions.............................................. 126 Appendix B—Thin Section Descriptions ............................................... 129 Appendix C—Terrace Long Profiles...................................................... 133 PLATES...........................................................................................Rear Pocket Plate 1—Artillery Reach............................................................Rear Pocket Plate 2—Hole in the Ground Reach...........................................Rear Pocket vi LIST OF TABLES Table Page 1 Landslide, Landslide Dam, and Related Geomorphic Feature Characterization .............................................................................................. 49 2 Age Data Collected for Both Study Reaches ................................................. 54 3 Shear Stress of Measured Boulders ................................................................ 58 4 Velocity of Boulder Deposition...................................................................... 59 5 Boulder Measurements of East Spring Earthflow and Greeley Bar ............... 62 6 Slope of Terrace Surfaces and River Channel ................................................ 67 vii LIST OF FIGURES Figure Page 1 Location map .................................................................................................... 3 2 Larger scale map of the study area ................................................................... 5 3 Hand sample and thin section sample locations ............................................. 21 4 Terrace