Hydraulic and morpho-sedimentary differences between pools and riffles have always intrigued fluvial geomorphologists. Surprisingly, earlier explanations of pool-riffle morpho-sedimentology dynamic focus too exclusively on hydraulic patterns and neglect analysis of sediment transport. Understanding the mechanisms governing pool-riffle morpho-sedimentary dynamics, such as the sediment transport patterns represent a big challenge in fluvial geomorphology, considering the stochastic nature of bedload transport in gravel-bed rivers. The main objective of this thesis is to understand the two-dimensional relationship between hydraulic patterns, sediment transport patterns and morpho-sedimentary changes in pool-riffle sequences and to use these insights to understand how pool-riffle sequences maintain their form over time. / This thesis is organized into four chapters. These chapters analyze hydraulic, sediment transport and morpho-sedimentary measurements collected on a riffle-pool-riffle sequence of the Des Coeurs river during seven controlled experimental floods. Chapter 2 tests the velocity reversal hypothesis (Keller, 1971), with results demonstrating that no velocity reversal was observed in the field, for peak flow up to 123% of the bankfull discharge. Chapter 3 demonstrates the existence of a bedload transport reversal: below 60% of bankfull discharge, sediment transport on the riffle exceeds transport in the pool; as discharge increased, more sediment was exported from the pool than imported, thereby maintaining the pool. Chapter 4 demonstrates that the bedload transport reversal is caused by the spatial heterogeneity of the sediment transport patterns (sediment supply, particle displacements and sediment sorting), which underlies the importance of developing two-dimensional bedload transport formulae. Chapter 5 tests the two-dimensional applicability of well-known bedload transport formulae developed in a one-dimensional environment. None of the formulae could be applied with accuracy in a two-dimensional environment. However, site-specific calibration considerably improves bedload transport rate and grain-size distribution prediction. / This thesis improves the understanding of the morpho-sedimentary dynamics of pool-riffle sequence; it argues that a purely hydraulic view of pool-riffle sequence is insufficient to understand their stability. It explains pool-riffle maintenance as a result of a bedload transport reversal caused by the spatial heterogeneity of the sediment transport patterns, and raises the importance of acquiring two-dimensional sediment transport data to improve two-dimensional bedload transport formulae.

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The segmenting of gravel-bed rivers into 'sedimentary links', characterized by single-sediment sources and downstream fining of alluvial sediments, is a relatively new technique which has had limited application. The sedimentary link concept has been primarily applied to alpine river environments where link formations are supplied by coarse sediment from active point sources. The purpose of this study is to apply the sedimentary link concept on the Ste Marguerite River in the Saguenay region of the Canadian Shield, where valley-segment deposits of coarse sediment from ancient glacial processes are dominant in forming links. / Specifically, this study examines link-scale trends in surface grain size and channel slope. This study also uses the sedimentary link concept within an ecological context to explain the spatial organization and quality of Atlantic salmon spawning and rearing habitat. (Abstract shortened by UMI.)

Changes in channel morphology in response to two flood events were measured within three reaches on the Sainte Marguerite River, Quebec. The first event was the spring freshet---peaking in mid-May, 1996---while the second event---peaking on July 20, 1996---was the largest flood on record for the region. The resultant channel adjustments can be classified as either bedform evolution---in which a clear, systematic pattern of adjustment is evident---or as bedform change, in which local hydraulic and sedimen tologic conditions produce a seemingly random pattern of channel mobilization. Where bedform evolution has occurred, it is consistent with the existing paradigm for meander development. Sediment transport calculations based on the 'inverse' or 'morphologic' method were strongly correlated to reach average mobility ratios. The average rate of transport in response to the larger flood approached those reported by others for a braided system (Goff and Ashmore, 1994). Potential spawning zones within the three reaches were all subjected to significant net scour and/or fill following the second event; several potential spawning zones were significantly affected by the first event The presence of bank protection upstream of a potential spawning zone seems to be a determinant 10 the severity of the impact by promoting erosion of these zones.

Forest harvesting is blamed for a perceived increase in the flashiness and turbidity of the Cascapedia River's water. This has raised questions over the source of the sediment (harvest parcels, roads, or stream banks) and its potential impact on the sedimentology of the river. The objectives of this research are twofold. The first is to ascertain if harvesting operations are associated to a widening of low-order tributaries, creating a source of sediment. The second is to determine if variations in the sedimentology along four segments of the Cascapedia can be associated to harvesting operation intensity. Firstly, analysis of stream width in low-order tributaries shows that, once the variations associated with basin area and D50 are removed and within the range of harvesting in our dataset, there appears to be a 25% increase in width associated with the harvesting activities of the last five years, as well as with road density, both in a 60 m stream buffer for a number of the sampled streams. Secondly, the models relating harvesting intensity and changes in sedimentology are sensitive to a few sites or contrary to theory. Future studies should determine the underlying hydrological processes responsible for stream enlargement and the process of sediment deposition.

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