Michel Lapointe

Effects of large floods on sediment transport and reach morphology in the cobble-bed Sainte Marguerite River

Abstract Sediment transport rates were estimated for two flood events on the cobble-bed Sainte Marguerite River in the Saguenay region, Canada. Morphologic methods were used to derive one set of estimates, and a combination of the Meyer-Peter and Muller equation with a dimensionless sediment transport ratio (after Dietrich et al. [Nature 340 (1989) 215]) was used to derive another set of estimates. Both sets of estimates give consistent results for the first event (which had a decade-scale return period), and for the second event (which was the largest flood on record and had a century-scale return period). The transport occurring during the second event was an order of magnitude greater than that occurring during the first event: despite this disparity in the transport intensity of the two events, the channel morphology remained qualitatively similar. The observed degree of channel stability is attributed to a change of channel pattern and the initiation of bed degradation following channel rectification in the 1960s.

Response of the Ha! Ha! River to the flood of July 1996 in the Saguenay region of Quebec: Large‐scale avulsion in a glaciated valley

Modifications to valley form due to extreme flooding in the lower 34 km of the Ha! Ha! River were analyzed. Heavy regional precipitation in July 1996 triggered extreme runoff and the catastrophic drainage of Ha! Ha! Lake, producing discharges of 1100 m3/s, 8 times the 100-year flood. Dominant valley modifications, revealed by comparing preflood and postflood topographies derived photogrammetrically, were related to two large-scale avulsions; in particular, a deep retrogressive incision which bypassed the 30-m-high Perron Falls, exporting 6 × 106 m3 of glacial stratified drift from a 2-km section of valley and producing massive sedimentation in the reach downstream. Reconstructed maximum flow power values support the existence of a 300 W/m2 threshold for major scouring of the alluvial valley bottom. The evidence highlights the potential for massive scour and fill and reorganization of the long profile, with potentially catastrophic effects on infrastructure, during extreme floods in glaciated valley settings.

Visual characterization technique for gravel-cobble river bed surface sediments; validation and environmental applications Contribution to the programme of CIRSA (Centre Interuniversitaire de Recherche sur le Saumon Atlantique)

This paper presents an evaluation of the feasibility and the reliability of a visual characterization technique for gravel–cobble river bed surface substrate. Based on principal axis regressions, using phi scale (ϕ), comparisons of visual estimation and grid sampling techniques show that useful predictive relations (R2 = 0·78–0·88) exist between visual estimates of the surface d16, d50 and d84 and estimates obtained for the same percentiles with the grid sampling technique. Comparisons of visual estimation and the surface-bulk sampling technique also indicate a predictive relation (R2 = 0·70) between the d50 of the two methods. Trained operators can visually estimate gravel–cobble bed surface d16 to uncertainties of 41 per cent, d50 to 15 per cent and d84 to 11 per cent (for example, there is a 5·5 mm error on a d84 size of 50 mm). Furthermore, evidence shows that if operators are properly trained, a calibration relation for each percentile can be applied independently of operators. This visual characterization allows effective detailed mapping of spatial patterns in substrate size distribution along extensive reaches of gravel-bed rivers. The technique can be very useful in creating terrain models for various geomorphological, hydrological and biological applications such as the determination of entrainment thresholds, hydraulic roughness and substrate suitability for benthic insects or salmonid habitat. Copyright

Spatial distribution of suspended pollen in the Mississippi River as an example of pollen transport in alluvial channels

Abstract A comparison of pollen concentrations on a velocity profile within a cross-section of the Mississippi River at St. Francisville, Louisiana is used to examine processes of pollen suspension and transport. The profile consists of five verticals sampled at six different depths paired with current velocity measurements, as well as surface samples. Our results show no significant correlation between pollen load and velocity. Application of sediment mechanics to pollen grain transport demonstrates that such relationships should not be expected in a river. Due to the negligible terminal fall velocities of pollen grains, one should expect a nearly uniform distribution of pollen load with river depth. Differential deposition of pollen grains due to spatial heterogeneity in a fluvial environment is also an unlikely phenomenon. In the Mississippi we find that other mechanisms, such as pollen rain and resuspension of grains from the bed, are more likely to control the spatial distribution of pollen.

Implications of low-pass filtering on power spectra and autocorrelation functions of turbulent velocity signals

Filtering either through the electronics of an instrument or through digital procedure is performed routinely on geophysical data. When velocity fluctuations are measured in turbulent flows using electromagnetic current meters (ECMs), a builtin lowpass Butterworth filter of order n usually attenuates fluctuations at high frequencies. However, the effects of this filter may not be acknowledged in turbulence studies, thus impeding comparisons between data collected with different ECMs. This paper explores the implications of the filters on the characteristics of velocity signals, mainly on variance, power spectra, and correlation analyses. Variance losses resulting from filtering can be important but will vary with the order n of the Butterworth filter, decreasing as n increases. Knowing the filter response, it is possible to reconstruct the original signal spectrum to evaluate the effect of filtering on variance and to allow comparisons between data collected with different instruments. The autocorrelation function also is affected by filtering which increases the value of the coefficients in the first lags, resulting in an overestimation of the integral length scale of coherent structures. These important effects add to those related to size and shape differences in ECM sensors and must be taken into account in comparative studies.

Spatial patterns in periphyton biomass after low-magnitude flow spates: geomorphic factors affecting patchiness across gravel–cobble riffles

Abstract The patchy spatial distribution of benthic algae (periphyton) on gravel-bed rivers might be caused by physical disturbances during small frequent flow spates. During such spates, the gravel–cobble river bed is stable, but flows are often strong enough to transport large quantities of sand by a hopping motion called saltation. We tested the hypothesis that a spate-related refuge habitat exists in a transition zone (TZ) between the edge of the varial zone and the thalweg of the river channel where high hydraulic stress and saltating sand reduce biomass. We documented physical disturbance and periphyton biomass across 15 riffles after 3 summer spate periods in an oligomesotrophic river in Quebec. Periphyton perturbation thresholds were identified for near-bed water velocity during prespate growth (0.25 m/s) and for sand transport (64–180 g m−1event−1) and flow shear stress (15 Pa) during spates. Generalized linear models were used to examine cross-riffle trends in these 3 disturbance variables and in postspate periphyton biomass. The highest biomass occurred in the TZ. Periphyton increased away from the thalweg as sand transport rates decreased. Biomass continued to increase toward the edge of this zone unless disturbed by high rates of sand transport that were associated with a small, near-shore secondary peak in sand transport rate. Of the 3 disturbance variables, sand transport patterns controlled the spatial distribution of periphyton biomass after small spates with an average recurrence interval of 7 d. No cross-riffle refuge was found after a higher-magnitude spate (3× mean annual discharge) when disturbance thresholds were typically exceeded across the entire riffle. The intensity and distribution of physical disturbance, particularly sand abrasion, over the streambed dictated size and arrangement of periphyton refuge zones. These zones are crucial to promote stream system resilience to landuse change.

Human modification of a large meandering Amazonian river: genesis, ecological and economic consequences of the Masisea cutoff on the central Ucayali, Peru.

Abstract Evidence is mounting regarding the significant extent and scope of long-term human modification of “pristine nature” in the neotropics. In Amazonia, recent studies point to the landscape imprint of human activity that has transformed the forests, savannas, soils, and waterways of the basin. In this report, we describe a massive meander cutoff in the Peruvian Amazon along the Ucayali River—the fifth-longest river in the Amazon basin—that was triggered by small-scale human actions and resulted in significant ecological and economic consequences for the region. The modern case of the Masisea cutoff—near the Amazonian port city of Pucallpa, Peru (285 000 inhabitants)—indicates that humans using simple tools can play a major role in transforming large meandering rivers and their floodplains.

Climate change and resilience of tributary thermal refugia for salmonids in eastern Canadian rivers

Abstract River water temperature regimes are expected to change along with climate over the next decades. This work focuses on three important salmon rivers of eastern Canada, two of which warm up most summers to temperatures higher than the Atlantic salmon lethal limit (>28°C). Water temperature was monitored at 53 sites on the three basins during 2–18 summers, with about half of these sites either known or potential thermal refugia for salmon. Site-specific statistical models predicting water temperature, based on 10 different climate scenarios, were developed in order to assess how many of these sites will remain cool enough to serve as refugia in the future (2046–2065). The results indicate that, while 19 of the 23 identified refugia will persist, important increases in the occurrence and duration of temperature events in excess of 24°C and 28°C, respectively, in the mainstems of the rivers, will lead to higher demands for thermal refugia in the salmonid populations. Editor Z.W. Kundzewicz; Associate editor T. Okruszko

Temporal Development of Scour Holes around Submerged Stream Deflectors

Deflector structures used in many fish habitat rehabilitation schemes are frequently overtopped, yet few studies have examined the scour patterns created around submerged models. Furthermore, laboratory studies typically test smooth-surfaced structures, whereas those installed in natural rivers are generally made of logs or boulders. This study uses rough-surfaced paired deflectors to investigate the temporal evolution of scour for three overtopping ratios in identical approach flow conditions in a flume. Results show that maintaining identical discharge and raising the deflector height, which reduces the overtopping ratio (i.e., flow depth divided by structure height), generates increased depth and volume of scour next to the structures. The location of maximum depth and the rate of scouring with time is similar for the two highest deflectors (overtopping ratios of 1.22 and 1.83), but different for the lowest deflector model (overtopping ratio of 3.67). To improve the success rate of river restoration pr...

NSERC's HydroNet: A National Research Network to Promote Sustainable Hydropower and Healthy Aquatic Ecosystems

Abstract NSERCs HydroNet is a collaborative national five-year research program initiated in 2010 involving academic, government, and industry partners. The overarching goal of HydroNet is to improve the understanding of the effects of hydropower operations on aquatic ecosystems, and to provide scientifically defensible and transparent tools to improve the decision-making process associated with hydropower operations. Multiple projects are imbedded under three themes: 1) Ecosystemic analysis of productive capacity offish habitats (PCFH) in rivers, 2) Mesoscale modelling of the productive capacity offish habitats in lakes and reservoirs, and 3) Predicting the entrainment risk of fish in hydropower reservoirs relative to power generation operations by combining behavioral ecology and hydraulic engineering. The knowledge generated by HydroNet is essential to balance the competing demands for limited water resources and to ensure that hydropower is sustainable, maintains healthy aquatic ecosystems and a vibr...