View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Carolina Digital Repository Impact of Geomorphic Structures on Hyporheic Exchange, Temperature, and Ecological Processes in Streams Erich T. Hester A dissertation submitted to the faculty of the University of North Carolina at Chapel Hill in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Curriculum in Ecology. Chapel Hill 2008 Approved by: Martin W. Doyle Lawrence E. Band Emily S. Bernhardt Michael F. Piehler Seth R. Reice Abstract Erich T. Hester: Impact of Geomorphic Structures on Hyporheic Exchange, Temperature, and Ecological Processes in Streams (Under the direction of Martin W. Doyle) Water exchange between streams and groundwater (hyporheic exchange) facilitates exchange of heat, nutrients, toxics, and biota. In-stream geomorphic structures (IGSs) such as log dams and steps are common in natural streams and stream restoration projects, and can significantly enhance hyporheic exchange. Hyporheic exchange is known to moderate temperatures in streams, a function important to a variety of stream organisms. Nevertheless, the connection between IGS form, hydrogeologic setting, hyporheic exchange, and hyporheic thermal impacts are poorly known. In this dissertation, I used hydraulic modeling and field experiments to quantify how basic characteristics of IGSs and their hydrogeologic setting impact induced hyporheic water and heat exchange and stream temperature. Model results indicate that structure size, background groundwater discharge, and sediment hydraulic conductivity are the most important factors controlling induced hyporheic exchange. Nonlinear relationships between many such driving factors and hyporheic exchange are important for understanding IGS functioning. Weir-induced hyporheic heat advection noticeably affected shallow sediment temperatures during the field experiments, and also caused slight cooling of the surface stream, an effect that increased with weir height. Nevertheless, such advection was far less important to the stream heat budget than atmospheric heat exchange, indicating streambed hydraulic conductivity was the overriding factor controlling hyporheic influence on surface water temperatures. Lotic organisms are ii adapted to the thermal regime typically experienced in their native ranges, and are therefore sensitive to thermal shifts from human activities. However, a basic survey of ecological sensitivity to temperature change in lotic systems is currently lacking. In this dissertation, I generated a quantitative synthesis of ecological sensitivity to temperature from the peer- reviewed scientific literature which I compared to a broad array of human thermal impacts to streams and rivers. Results indicate that on average, lotic organisms are more sensitive to warming than to cooling, and fish are more sensitive than invertebrates. Human thermal impacts entail warming more often than cooling and are of similar magnitude to that required to induce a 50% reduction in organism level functioning, indicating significant potential to impair ecological functioning. My results highlight the need for thermal mitigation in lotic systems at local, regional, and global scales. iii Acknowledgements This dissertation was funded by a United States Environmental Protection Agency Science to Achieve Results (STAR) graduate fellowship (FP-91667601), a William R. Kenan fellowship from the University of North Carolina, and a National Science Foundation grant (CAREER-BCS-0441504). I am grateful to Myles Killar, Meredith Harvill, Frank M. Smith, and Jason Johnson for valuable field assistance, data analysis, and interesting conversation between ibutton downloads. I wish to thank Geoffrey C. Poole for lengthy discussions of stream and river hydraulic and thermal dynamics, professional development, and life in general. Michael N. Gooseff, Tamao Kasahara, Mary I. O’Connor, Jay P. Zarnetske, John Bruno, Jeffrey J. Clark, and John Stofleth provided valuable advice and manuscript review. I am indebted to Karen Henry, Cottie Pasternak, Denise Kenney, Barbara Taylor, Nell Phillips, and Bob Peet, who provided important assistance and advice at many critical administrative junctures. I greatly appreciate the efforts of committee members Larry Band, Emily Bernhardt, Seth Reice, and Mike Piehler for their assistance and guidance during my PhD program. I will be forever grateful to my advisor Martin Doyle for his wisdom, patience, and flexibility over four years. And finally I wish to recognize the patient behind-the-scenes efforts of Linsey Marr, without which my sanity, let alone this dissertation, would be truly impossible. iv Table of Contents List of Tables ........................................................................................................................... ix List of Figures............................................................................................................................x List of Abbreviations and Symbols......................................................................................... xii 1 Introduction........................................................................................................................1 1.1 Research Rationale.....................................................................................................1 1.2 Research Questions....................................................................................................2 1.3 Research Approach ....................................................................................................3 1.4 Document Organization.............................................................................................4 2 In-Stream Geomorphic Structures as Drivers of Hyporheic Exchange.............................7 2.1 Introduction................................................................................................................7 2.1.1 Hyporheic Exchange and Lotic Ecosystem Management .................................7 2.1.2 Mechanisms of Hyporheic Exchange ................................................................8 2.1.3 Existing Approaches for Quantifying Hyporheic Exchange ...........................11 2.1.4 Motivation and Approach for Study ................................................................12 2.2 Methods....................................................................................................................13 2.2.1 Modeling..........................................................................................................13 2.2.1.1 Conceptual Model of Hypothetical Stream System.....................................13 2.2.1.2 Numerical Modeling Approach ...................................................................14 2.2.1.3 Sensitivity Analysis.....................................................................................18 2.2.2 Field Experiment..............................................................................................18 v 2.3 Results......................................................................................................................20 2.3.1 Field Experiment..............................................................................................20 2.3.2 Modeling Sensitivity Analysis.........................................................................20 2.3.2.1 Effect of Structure Size and Type................................................................20 2.3.2.2 Effect of Hydrologic and Geologic Setting .................................................22 2.4 Discussion................................................................................................................24 2.4.1 Model Confidence and Spatial Scale ...............................................................24 2.4.2 Magnitude of Structure-Induced Hyporheic Exchange ...................................27 2.4.3 Effect of Structure Size and Type....................................................................29 2.4.4 Effect of Hydrologic and Geologic Setting .....................................................34 2.4.5 Primary Factors Controlling Hyporheic Exchange..........................................39 2.4.6 Implications for Natural Streams.....................................................................40 2.4.7 Implications for Stream Restoration and Watershed Planning........................42 2.4.7.1 Stream Restoration.......................................................................................42 2.4.7.2 Watershed Planning .....................................................................................45 2.5 Summary and Conclusions ......................................................................................46 3 The influence of in-stream geomorphic structures on stream temperature via induced hyporheic exchange ........................................................................................................65 3.1 Introduction..............................................................................................................65 3.2 Methods....................................................................................................................67 3.2.1 Field Experiments ............................................................................................67 3.2.2 Calculations......................................................................................................69 3.2.2.1 Vertical Hydraulic