Pool-riffle dynamics in mountain streams: implications for maintenance, formation and equilibrium by Shawn M. Chartrand BA, Environmental Geology, Case Western Reserve University, 1995 MS, Geological Sciences, Case Western Reserve University, 1997 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF Doctor of Philosophy in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Geography) The University of British Columbia (Vancouver) July 2017 c Shawn M. Chartrand, 2017 Abstract It is common for mountain riverbeds to exhibit a repetitive pattern of topographic lows and highs known respectively as pools and riffles. Pool-riffle structures are ecologically important because salmon rely on them for birth, growth and regeneration, and they are physically im- portant because pool-riffles are observed across diverse landscape settings. A common phys- ical characteristic of pool-riffles is that pool spacing is proportional to channel width, for lon- gitudinal bed slopes that vary by two-orders of magnitude. Furthermore, field, numerical and laboratory based studies observe that pools are colocated with points of channel narrow- ing, and riffles with points of widening. What is not known, however, is how downstream changes of channel width give rise to, and maintain pool-riffles. The goal of my thesis is to address this knowledge gap, and to specifically build physical understanding for the observed spatial correlation between channel width and pool-riffle architecture. I use field work, lab- oratory experiments and theory to address this goal. In Chapter 2 I apply non-parametric statistics and self-organizing maps to understand the spatial and temporal character of rif- fle bed surface texture spanning 11 different sediment mobilizing floods, and conclude that frequent texture adjustment is part of the maintenance process for pool-riffles which exhibit topographic stationarity. I build from this finding in Chapters 3, 4 and 5 with laboratory exper- iments designed to investigate how pool-riffles form and evolve along variable width channel reaches. In Chapter 4 I conclude that pool-riffle formation is physically driven by two com- peting timescales which reflect the tendency to build riverbed topography through sediment deposition, vs. the tendency to destroy topography through net particle entrainment. I cap- ture these timescales in a mathematical model I develop using theory with physical scaling. In Chapter 5 I show that the (dis)equilibrium state of pool-riffle evolution is quantitatively de- scribed by a competition between two rates which reflect the temporal adjustment of riverbed topography and riverbed surface texture. I conclude that equilibrium, or comparability be- tween the rates of topographic and sediment texture adjustment, is most likely to occur when overall sediment mobility and grain size sorting are relatively high. ii Lay Summary Mountain streams commonly display a riverbed shape that has a repetitive pattern of topo- graphic lows and highs known respectively as pools and riffles. Visually, pools appear as relatively deep portions of a river, with slow water velocities, and riffles appear as compara- tively shallow portions, with more rapid water velocities. Pool-riffles are ecologically impor- tant because salmon rely on them for birth, growth and regeneration, and they are physically important because pool-riffles are observed across diverse landscape settings. Despite their importance, the scientific community lacks a clear explanation for pool-riffle formation. This research shows that pool-riffles develop in response to how channel width and water velocity change moving in the downstream direction, reflecting a tendency to either build or destory riverbed topography. We demonstrate our finding with a mathematical model motivated by experimental observations, and built using a combination of theory and physical scaling. iii Preface This thesis is original work completed by Shawn Chartrand. Guidance was given by the super- visory committee, and laboratory assistance was provided by Rick Ketler, Carles Ferrer-Boix, and Ryan Buchanan. This thesis includes one manuscript, and three complementary Chapters that will be submit- ted for publication as two or more manuscripts. The published manuscript is presented in Chapter 2. Chapter 3, Chapter 4 and Chapter 5 are the complementary Chapters. A version of the work in Chapter 2 is published in Water Resources Research Chartrand et al. (2015). The co-authors are Marwan Hassan and Valentina Radic.´ I am responsible for devel- oping the field sampling program, implementation, and analysis of all field data presented in Chapter 2, except use of self-organizing maps (SOMs), which was completed by Valentina Radic.´ I completed a majority of the writing presented in Chapter 2. Marwan Hassan and Valentina Radic´ provided editorial review of the manuscript prior to publication. Chartrand, S. M., M. A. Hassan, and V. Radic´ (2015), Pool-riffle sedimentation and surface tex- ture trends in a gravel bed stream, Water Resources Research, 51, 9127-9140. iv Table of Contents Abstract . ii Lay Summary . iii Preface . iv Table of Contents . v List of Tables . ix List of Figures . x List of Symbols and Acronyms . xii Acknowledgments . xxiii Dedication . xxvi 1 Introduction and motivation . 1 1.1 Overview . 2 1.2 Motivation . 3 1.2.1 River size at the local scale . 3 1.2.2 Evidence and knowledge gaps of the physical connection between chan- nel width variation and pool-riffle architecture . 4 1.3 Making sense of coupling between channel width and bed architecture . 6 2 Pool-riffle sedimentation and surface texture trends in a gravel bed stream . 10 2.1 Summary . 10 2.2 Introduction . 10 2.3 Study site . 12 2.4 Data collection and analysis . 15 2.4.1 Riffle texture . 16 2.4.2 V* and pool cross-section surveys . 23 v 2.4.3 Bedload transport measurements and modeling . 24 2.4.4 Bed surface sampling . 27 2.5 Results . 27 2.5.1 Riffle texture adjustment . 27 2.5.2 V* and cross-section surveys . 32 2.5.3 Sediment transport and bed surface sampling . 33 2.6 Discussion . 34 2.6.1 Riffle texture dynamics and spatial organization . 34 2.6.2 Pool-riffle sedimentation coupling . 37 2.7 Concluding remarks and next steps . 38 2.8 Details of SOM methods . 39 3 Experimental setup and measurements . 41 3.1 Introduction . 41 3.2 Laboratory experiment and methods . 42 3.2.1 Setup and construction . 42 3.2.2 Experimental design . 43 3.2.3 Experimental procedure . 47 3.2.4 Experimental measurements and processing . 49 3.2.5 Bed surface grain size distributions . 53 3.2.6 Manual water and bed surface profiles . 54 3.2.7 Flow depth, flow area and average streamwise velocity . 55 4 Morphodynamics of a width-variable gravel-bed stream: new insights on pool- riffle formation . 56 4.1 Summary . 56 4.2 Introduction . 57 4.3 Results . 59 4.3.1 Identifying general response regimes with sediment flux, mean bed to- pography and bed sediment texture . 59 4.3.2 Topographic response: channel-wide and longitudinal profile develop- ment . 62 4.3.3 Effects of initial conditions on topographic responses . 70 4.3.4 Summary of main results . 71 4.4 Physically linking channel width changes to topographic response . 71 4.4.1 Downstream changes in flow speed and mobility . 71 4.4.2 Downstream changes in channel width and bed slope . 73 4.4.3 Theory for the local channel profile . 76 4.5 Discussion . 79 4.5.1 Predicting local channel slope along variable-width channels . 80 vi 4.5.2 Maintenance of bed topography along variable-width channels: support for an emerging view . 84 4.5.3 Development of pool-riffles along variable width channels . 85 4.5.4 General implications of unique profiles for sediment transport theory . 86 4.6 Conclusions and next steps . 87 5 Morphodynamic evolution of a width-variable gravel-bed stream: a battle be- tween local topography and grain size texture . 89 5.1 Summary . 89 5.2 Introduction . 90 5.3 Morphodynamic evolution metrics at the scale of a channel width . 92 5.3.1 Problem set-up . 92 5.3.2 Mass conservation . 95 5.3.3 Nondimensional Exner and Hirano equations . 97 5.3.4 Dimensionless channel response number: Ne . 99 5.3.5 Calculations of d2, Ub, Up and Ne . 100 5.4 Results . 104 5.4.1 Topographic and sediment texture response numbers: Nt and Np . 105 5.4.2 Sediment texture d2 .............................. 105 5.4.3 Channel response number: Ne . 107 5.4.4 Results summary . 107 5.5 Discussion . 108 5.5.1 Local contributions to equilibrium conditions during pool-riffle and rough- ened channel development and maintenance . 108 5.5.2 Local and channel response numbers: a new view of fluvial equilibrium 109 5.6 Conclusions . 110 6 Concluding remarks . 112 6.1 Summary . 112 6.2 Future directions . 113 Bibliography . 118 A Numerical channel evolution model description ..