UBC Theses and Dissertations
Pool-riffle dynamics in mountain streams : implications for maintenance, formation and equilibrium Chartrand, Shawn M.
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 important because pool-riffles are observed across diverse landscape settings. A common physical characteristic of pool-riffles is that pool spacing is proportional to channel width, for longitudinal 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 narrowing, 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, laboratory 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 riffle 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 experiments 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 competing timescales which reflect the tendency to build riverbed topography through sediment deposition, vs. the tendency to destroy topography through net particle entrainment. I capture 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 described by a competition between two rates which reflect the temporal adjustment of riverbed topography and riverbed surface texture. I conclude that equilibrium, or comparability between the rates of topographic and sediment texture adjustment, is most likely to occur when overall sediment mobility and grain size sorting are relatively high.
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