DISSERTATION MODELING POOL SEDIMENT DYNAMICS IN A MOUNTAIN RIVER Submitted by Sara L. Rathburn Department of Earth Resources In partial fulfillment of the requirements for the Degree of Doctor of Philosophy Colorado State University Fort Collins, Colorado Fall 2001 COLORADO STATE UNIVERSITY August 31, 2001 WE HEREBY RECOMMEND THAT THE DISSERTATION PREPARED UNDER OUR SUPERVISION BY SARA L. RATHBURN ENTITLED MODELING POOL SEDIMENT DYNAMICS IN AMOUNTAIN RIVER BE ACCEPTED AS FULFILLING IN PART REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY. Committee on Graduate Work Adviser rJ:£L-If- Department Head £ 11 COLORADO STATE UNIVERSITY August 31, 2001 WE HEREBY RECOMMEND THAT THE DISSERTATION PREPARED UNDER OUR SUPERVISION BY SARA L. RATHBURN ENTITLED MODELING POOL SEDIMENT DYNAMICS IN AMOUNTAIN RIVER BE ACCEPTED AS FULFILLING IN PART REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY. Department Head 11 ABSTRACT OF DISSERTATION MODELING POOL SEDIMENT DYNAMICS IN A MOUNTAIN RIVER An increasingly important source of sediment into river systems is sediment that accumulates within reservoirs and is subsequently released into the downstream ecosystem. In Colorado alone, five large-scale sediment releases from reservoirs within the last decade have resulted in a host of environmental hazards, particularly the loss of aquatic biota and their habitat. The most recent example occurred in September 1996 when approximately 7,000 m3 of clay- to gravel-sized sediment were released from Halligan Dam into the North Fork Cache la Poudre River in northern Colorado. The sediment caused extensive aggradation of the original cobble-boulder bed, primarily in pools, and complete fish mortality for 12 km downstream from the dam. Because of the thriving, pre-release wild trout fishery downstream from Halligan Reservoir, flushing of sediment from pools to recreate overwinter fish habitat was of prime concern. The purpose of this investigation was to evaluate the applicability of various hydraulic and sediment transport models as predictors of pool recovery along the steep- gradient, bedrock-controlled North Fork River. Two modeling scenarios representing a low and high flushing discharge were modeled using one- and semi-two dimensional sediment transport models, HEC-6 and GSTARS2.0, respectively. The models were calibrated against quantitative measurements of pool bed elevation obtained during field iii surveys. HEC-6 results indicate that long-tenn, robust simulations yield the closest agreement between predicted and measured pool bed elevation change. Greater than 50 percent of the actual scour and deposition within three pools was modeled using HEC-6. Modeling accuracy using GSTARS2.0 was considerably more variable, and no pool-wide trends were obtained. A two-dimensional, finite element hydraulic model, RMA2, improved delineation of flow hydraulics in areas of flow separation and recirculation within a compound pool. RMA2 results of depth-averaged velocity magnitude and vectors broadly agree with timed photographs of surface flow patterns, and correspond with velocity measurements for low-velocity areas such as eddy pools. Patterns of boundary shear stress and a particle stability index accurately predict gross areas of scour and deposition, but fail to represent the simultaneous aggradation and degradation measured in pools. Estimates of bedload transport capacity from the two-dimensional modeling results are one order of magnitude greater than measured transport rates, and indicate that supply-limited conditions existed along the North Fork following a clear-water flushing release. Further correlations between observed and modeled sedimentation patterns are hindered by the disparity in resolution between the field data and modeled results; field-based cross sectional information is quickly outstripped by the finite element model RMA2. Finally, a conceptual model of pool sediment dynamics was developed for water resource specialists as an alternative to the time-intensive effort and expertise required of the numerical modeling. Predictable sites of channel aggradation and degradation resulting from a sediment pulse are identified on a reach-scale hierarchy_ Processes of sediment delivery, storage, and transfer into and out of eddies that influence fish occur on iv the width scale, however. Sedimentation within laterally confined pools is dependent on pool geometry, distance downstream from the dam (a surrogate of sediment supply), and the duration and magnitude of flows following the release. At low flows, sediment deposition is restricted to small areas of recirculating flow. As discharge increases, migration of the separation point and development of a strong shear zone limits the transfer of sediment between the eddy and the main flow. The sediment release from Halligan induced persistent, long-term storage of fine sediment because of an elevated channel bed and loss of channel capacity_ Recognition of the hazards associated with a large influx of sediment into a riverine ecosystem is critical for a greater understanding of the effects of sediment releases, and future management of sediment within reservoirs. Sara L. Rathburn Department of Earth Resources Colorado State University Fort Collins, CO 80523 Fall 2001 v ACKNOWLEDGEMENTS Many people have contributed in a myriad of ways to the completion of this document. It was in no way singly undertaken, and I offer a modification to an existing saying; 'It takes a village to complete a Ph.D.' First, I want to thank my advisor Dr. Ellen Wohl for agreeing to take on a part-time, highly non-traditional student. These past four years have been stimulating, challenging, and exceedingly enjoyable. I have relished our hours in the field, in the office, and elsewhere, discussing my project, geomorphology in general, and mountain rivers in particular. Ellen's clarity and insight always kept me focused and ensured thoroughness and perseverance in spite of my struggles with the model-of-the-day. I look forward to more scientific interactions with her. Thank you to my other committee members Drs. Deborah Anthony, Pierre Julien, and Stanley Schumm. Deb offered substantive input and encouragement, provided key insight into sediment transport processes and evaluation of the modeling results, and allowed me to sit in on an excellent class on fluvial and eolian transport. Pierre taught two semesters of courses on erosion and sedimentation and river mechanics, and his courses, along with Deb's, were the best classes I took during my graduate tenure. I refer to those notes and the texts more than any other. Stan kindly offered to fill in as a third department committee member, as a favor, after I lost one committee member to a job change. Thank you to Dr. Robert Ward for substituting during my orals, and for his overall interest in my research. Dr. Brian Cluer provided encouragement and initial assistance during early phases of my research. Initial phases of this research were supported by funding from the Colorado Water Conservation Board, U.S. Bureau of Reclamation, and Trout Unlimited. Major support was provided by National Science Foundation Grant CMS-9727061 to Dr. Ellen Wohl. Additionally, I received a Career Development Grant from the American Association of University Women, the American Water Resources Association Rich Herbert Scholarship, a Rocky Mountain Association of Geologist Foundation Grant, and the Ned vi and Barbara Dils Scholarship. All of this monetary support was crucial to the completion of my degree. Heather Knight, John Stokes, and various interns working at The Nature Conservancy's Phantom Canyon Preserve have been enthusiastic and helpful, providing logistical support throughout the duration of this research. North Poudre Irrigation Co. provided access to Halligan Dam and to upstream reaches of the study site. I appreciate the opportunity to explore sediment dynamics in such a lovely, fascinating setting. Numerous graduate students assisted me throughout various phases of my research. I thank Dan Cenderelli, Janet Curran, Jasper Hardison, and Ron ZeIt, Doug Thompson, and John O'Brien for assisting in the data collection prior to my beginning school. Dan Cenderelli' s effort was critical to the overall high quality of data collected along the North Fork, and I appreciate discussions with him and field companionship. Greg Springer provided Adobe Illustrator guidance, and was open to an exchange of ideas about eddy system hydraulics. Greg Stewart offered important, patient insight into a fickle two-dimensional model, and served as a main-stay in my support system. I have missed our daily discussions tremendously. Additional individuals to whom I am grateful include Jason Alexander and Allison Thorton, summer assistants through the RED program sponsored by NSF. Karla Schmidt, work study, helped with a poster presentation. Francisco Simoes, U.S. Bureau of Reclamation, graciously reviewed my GSTARS2.0 data files, and provided insight into model particulars that would have been missed. Lyle Zevenbergen, Ayres Associates, kindly offered his time and expertise with RMA2 and SMS when I felt really stuck. To my parents, Buz and Katie Rathburn, I am tremendously grateful for their emotional support, help with childcare, and understanding the multifaceted roles in my life. Thank you to my mother, who in a mother-daughter phone conversation encouraged me to pursue a Ph.D. in the first place, and offered immense support in all ways. To my father, who worked on a doctorate for seven years while maintaining