Claudine Boyer

Bed morphology and sedimentology at the confluence of unequal depth channels

Abstract Spatial and temporal variations in the bed geometry and bed material size of a sand-bed river confluence with unequal-depth channels were monitored during a sequence of floods. Additionally, two other sand-bed confluences were surveyed to test the replicability of these observations. The confluences studied here have only one avalanche face, which corresponds to the front of a tributary mouth bar in the shallower channel, and do not exhibit a marked scour zone. Three distinct morpho-sedimentological zones are present: (1) an area around the upstream corner of the junction where the sediments are generally finer than the mean, (2) a maximum depth zone with coarser than average particles and (3) a bar at the downstream junction corner where grain size is finer than the mean and decreases slightly downstream. Changes in relative discharge between the two channels had little effect on the grain size of the downstream junction corner bar, but exerted a strong influence on the position of the maximum depth zone and the front of the tributary mouth bar. The downstream junction corner bar showed little evidence of the separation zone which is commonly observed at the confluences of laboratory channels. The contrasting depths of the approach channels at the sites studied here may be partly responsible for the absence of the separation zone.

Habitat Choice by Atlantic Salmon Parr in Relation to Turbulence at a Reach Scale

Abstract The variables commonly used to describe the physical habitat of Atlantic salmon Salmo salar parr are average velocity, water depth, and substrate. A variety of micro- and mesohabitat models have been developed using these variables to assess habitat quality. However, Atlantic salmon parr live in highly turbulent streams and rivers in which intense fluctuations of water velocity occur. Laboratory experiments have shown that turbulence affects the behavior and energetics of fish. Nevertheless, habitat use in relation to the strong temporal variability of velocity in natural environments has rarely been studied. In this study, Atlantic salmon parr habitat was examined in relation to turbulence in the Patapedia River, Quebec. Rather than taking the usual approach of surveying a large population at one point in time, we used an intensive radiotelemetry tracking survey that focused on the habitat use of a few individual fish over an extended period. We analyzed habitat use in relation to several dynami...

Recent evolution of the Yamachiche delta (Québec) : Natural processes and anthropogenic impacts

Le lac Saint-Pierre, un elargissement du fleuve Saint-Laurent, pourrait connaitre une baisse importante de niveau d’eau dans les cinquante prochaines annees, consequence du rechauffement climatique global. Sept rivieres debouchent dans ce lac fluvial peu profond qui risquerait d’enregistrer une sedimentation importante. Nous avons etudie le delta de l’un de ces tributaires, la Yamachiche, pour evaluer les processus de sedimentation et les reponses de la riviere aux changements environnementaux passes. L’evolution du delta de la Yamachiche a ete reconstituee par des photographies aeriennes, des cartes anciennes, des analyses sedimentologiques et stratigraphiques de six coupes et de quatorze forages et la datation par luminescence optique, 210Pb et 14C. Le delta est caracterise par deux types de depots. La base est constituee de depots sableux de lit mineur du chenal, mis en place dans des conditions de sedimentation et d’erosion frequentes. Ils sont surmontes par des depots limoneux de plaine d’inondation deltaique, comprenant des lits sableux locaux. Ces depots presentent une variabilite laterale et longitudinale expliquee par les variations de niveau d’eau du lac et de la riviere a une echelle annuelle et decennale, par les migrations du chenal et par l’action erosive des vagues du lac Saint-Pierre. Le delta s’est forme depuis 150 ans, comme le montre la carte de 1859 ou le delta n’etait que tres peu developpe. La luminescence optique attribue des âges de 140 et de 280 ans aux depots sableux a la base. Les datations revelent des taux d’accumulation verticale moyens compris entre 0,5 et 1,5 cm/an. Cette accumulation importante serait liee aux perturbations anthropiques affectant les sources sedimentaires du bassin versant depuis le debut de la colonisation il y a 200 ans. L’âge recent du delta serait egalement explique par la stabilisation tardive du lac Saint-Pierre a son niveau actuel, qui est intervenue entre 1000 et 150 ans. Recent evolution of the Yamachiche delta (Quebec) : Natural processes and anthropogenic impacts. Lake St. Pierre, being an enlargement of the St. Lawrence River, could experience a significant base level drop in the next fifty years as a consequence of global climate warming. Seven tributaries flow into this shallow lake, and base level changes could lead to increased sedimentation rates. We have studied the delta of the Yamachiche River, a tributary of Lake St. Pierre. The objective of the study is to document the active geomorphic processes and the response of the river to past environmental changes. We have reconstructed the evolution of the Yamachiche delta from aerial photos, historical maps, sedimentological and stratigraphic analysis of six sections and fourteen boreholes, and by sediment dating using optically stimulated luminescence, Pb and 14C. The subsurface deposits are divided into two facies : sandy low-water channel deposits at the base, and silty delta plain deposits at the top. Sedimentary sequences show changes along a longitudinal gradient driven by the level of the lake and the river dynamics on annual and decadal scales, and a high lateral variability driven by migration of the channel and by high-energy waves from Lake St. Pierre, which erode the downstream eastern part of the delta plain. Rapid progradation of the delta has occurred in the last 150 years, as demonstrated by the poor development of the delta on the 1859 map. Optically stimulated luminescence dating provides ages of 140 and 280 years at the base of the depositional sequence. These results are consistent with the chronological sequence deduced from the other dating methods.This framework gives mean accumulation rates between 0.5 and 1.5 cm/year. These high rates of sedimentation are likely linked to human activities in the watershed, which affected sedimentary sources since colonization of the St. Lawrence Lowlands 200 years ago. The young age of the delta could also be explained by the late stabilization of Lake St. Pierre at its present level, between 1000 and 150 years ago.

Évolution récente du delta de la Yamachiche (Québec) : processus naturels et impacts anthropiques

Le lac Saint-Pierre, un elargissement du fleuve Saint-Laurent, pourrait connaitre une baisse importante de niveau d’eau dans les cinquante prochaines annees, consequence du rechauffement climatique global. Sept rivieres debouchent dans ce lac fluvial peu profond qui risquerait d’enregistrer une sedimentation importante. Nous avons etudie le delta de l’un de ces tributaires, la Yamachiche, pour evaluer les processus de sedimentation et les reponses de la riviere aux changements environnementaux passes. L’evolution du delta de la Yamachiche a ete reconstituee par des photographies aeriennes, des cartes anciennes, des analyses sedimentologiques et stratigraphiques de six coupes et de quatorze forages et la datation par luminescence optique, 210Pb et 14C. Le delta est caracterise par deux types de depots. La base est constituee de depots sableux de lit mineur du chenal, mis en place dans des conditions de sedimentation et d’erosion frequentes. Ils sont surmontes par des depots limoneux de plaine d’inondation deltaique, comprenant des lits sableux locaux. Ces depots presentent une variabilite laterale et longitudinale expliquee par les variations de niveau d’eau du lac et de la riviere a une echelle annuelle et decennale, par les migrations du chenal et par l’action erosive des vagues du lac Saint-Pierre. Le delta s’est forme depuis 150 ans, comme le montre la carte de 1859 ou le delta n’etait que tres peu developpe. La luminescence optique attribue des âges de 140 et de 280 ans aux depots sableux a la base. Les datations revelent des taux d’accumulation verticale moyens compris entre 0,5 et 1,5 cm/an. Cette accumulation importante serait liee aux perturbations anthropiques affectant les sources sedimentaires du bassin versant depuis le debut de la colonisation il y a 200 ans. L’âge recent du delta serait egalement explique par la stabilisation tardive du lac Saint-Pierre a son niveau actuel, qui est intervenue entre 1000 et 150 ans. Recent evolution of the Yamachiche delta (Quebec) : Natural processes and anthropogenic impacts. Lake St. Pierre, being an enlargement of the St. Lawrence River, could experience a significant base level drop in the next fifty years as a consequence of global climate warming. Seven tributaries flow into this shallow lake, and base level changes could lead to increased sedimentation rates. We have studied the delta of the Yamachiche River, a tributary of Lake St. Pierre. The objective of the study is to document the active geomorphic processes and the response of the river to past environmental changes. We have reconstructed the evolution of the Yamachiche delta from aerial photos, historical maps, sedimentological and stratigraphic analysis of six sections and fourteen boreholes, and by sediment dating using optically stimulated luminescence, Pb and 14C. The subsurface deposits are divided into two facies : sandy low-water channel deposits at the base, and silty delta plain deposits at the top. Sedimentary sequences show changes along a longitudinal gradient driven by the level of the lake and the river dynamics on annual and decadal scales, and a high lateral variability driven by migration of the channel and by high-energy waves from Lake St. Pierre, which erode the downstream eastern part of the delta plain. Rapid progradation of the delta has occurred in the last 150 years, as demonstrated by the poor development of the delta on the 1859 map. Optically stimulated luminescence dating provides ages of 140 and 280 years at the base of the depositional sequence. These results are consistent with the chronological sequence deduced from the other dating methods.This framework gives mean accumulation rates between 0.5 and 1.5 cm/year. These high rates of sedimentation are likely linked to human activities in the watershed, which affected sedimentary sources since colonization of the St. Lawrence Lowlands 200 years ago. The young age of the delta could also be explained by the late stabilization of Lake St. Pierre at its present level, between 1000 and 150 years ago.