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The objective of this study is to use two hydrological indices (coefficients of variation and immoderation) to analyze the impacts of dam management methods on seasonal daily flow rate change downstream of three dams: Manouane (diversion-type management method), Ouareau (natural-type management method) and Matawin (inversion-type management method). The results show that this change is far greater downstream of the Matawin dam (characterized by an inversion-type management method) than downstream of the two other dams. Moreover, downstream of the Matawin dam, this daily flow rate change increases significantly over time, while decreasing downstream of the two other dams and in natural rivers. Lastly, this change is better correlated with climate downstream of the Ouareau dam than downstream of the two other dams. It is positively correlated with winter and spring temperatures as well as summer and fall rain. Contrary commonly accepted hypothesis, this study shows that the impacts of dams generally result in an increase of the seasonal flow rate change in Quebec.

The goal of this study was to analyze the impact of agriculture on the spatial and temporal variability of flow rate change indices from 1930 to 2008. The two indices used are the coefficient of immoderation (CI) and the coefficient of variation (CV). Values of these two indices are higher for the L’Assomption River agricultural watershed than for the Matawin River forested watershed due to higher runoff in the former than in the latter. The difference in these values between the two watersheds is greater for winter, but it is lower for summer, when the difference in runoff between the two watersheds is strongly attenuated by the presence of crops. Regarding the temporal variability, a difference between the two watersheds is observed in the fall. For the agricultural watershed, mean values of neither index show a break in slope, while a break is observed for the forested watershed. In both watersheds, both indices are positively correlated with maximum temperature and total rainfall in winter, but only to this latter climate variable in the fall. In springtime, the two indices are negatively correlated with minimum temperature in the forested watershed, but only CV is correlated, positively, with this same climate variable in the agricultural watershed.

This study presents a comparison of the spatio-temporal variability of characteristics (magnitude, duration and timing) of annual minimum daily extreme flows (AMEF) as a function of land use and the mode of management of dams. Streamflow measured at stations not affected by dams at Joliette, along the L’Assomption River (agricultural watershed, 1340 km2), and at Saint-Michel-des-Saints, on the Matawin River (forested watershed, 1390 km2) on one hand, and downstream from the Rawdon dam (regulated natural-type management mode), on the Ouareau River (1260 km2), which is the main tributary of the L’Assomption River, and from the Matawin dam (inverted-type management mode), on the Matawin River (4070 km2), on the other hand, were compared over the period from 1930 to 2010. As far as the spatial variability of natural rivers is concerned, the magnitude and duration of AMEF are higher in the forested watershed than in the agricultural watershed. In regulated rivers, AMEF magnitude is higher downstream from the dam characterized by a natural-type management mode than downstream from the dam characterized by inversion-type management. However, downstream from the latter, AMEF occur much more frequently and very early in the year. As for temporal variability, the Lombard method did not reveal any influence of land use differences on the stationarity of series of AMEF characteristics. In contrast, differences in dam management mode result in occurrences of AMEF downstream from the inversion-type dam progressively earlier in the year. The duration and timing of AMEF are not correlated with the same climate variables, be it in natural rivers or downstream from dams.

We analyze the relationship between four climate variables (maximum and minimum extreme temperatures, rainfall and snowfall) measured in winter (December to March) at 17 stations from 1950 to 2000 in the three hydroclimate regions of southern Quebec, and six seasonal climate indices using canonical correlation analysis (CCA) and the copula method. This analysis yielded these major results: (1) extreme temperatures are not correlated with the amount of winter rain or snow in southern Quebec; (2) winter seasonal climate indices show better correlations with climate variables than do fall climate indices; (3) winter extreme temperatures are best correlated (positive correlation) with the Atlantic Multidecadal Oscillation (AMO) in the eastern region, but show a negative correlation with the Arctic Oscillation (AO) in the southwestern region; (4) the total amount of winter snow is best correlated (negative correlation) with the Pacific Decadal Oscillation (PDO) in the three hydroclimate regions; (5) the total amount of winter rain is best (negatively) correlated with PDO in the eastern region, but shows a positive correlation with AO in the southeast region. Finally, the copula method revealed very little change in the dependence between climate indices and climate variables in the three hydroclimate regions.

Although climate models predict that the impacts of climate change on the temporal variability of water levels in the St. Lawrence River will be seasonally-dependent, such a seasonal effect on the current variability of extreme water levels has never been analyzed. To address this, we analyzed the temporal variability of three hydrological variables (monthly daily maximums and minimums, as well as their ratio) of water levels in the St. Lawrence River measured at the Sorel station since 1912, as they relate to climate indices. As for stationarity, the shifts in the mean values of maximum and minimum water levels revealed by the Lombard method took place prior to 1970 for spring water levels, but after that year, for winter water levels. Changes in the winter stationarity are thought to mainly relate to the decreasing snowfall observed in the St. Lawrence River watershed after 1970. In contrast, for spring, these changes are likely primarily related to human activity (digging of the St. Lawrence Seaway and construction of dams). Two shifts in the mean values of fall minimum extreme water levels were highlighted. The first of these shifts, which occurred in the first half of the 1960s decade, can also be linked to human activity (digging of the St. Lawrence Seaway and construction of dams), whereas the second shift, observed after the 1970s for the months of November and December, can be linked to decreasing amounts of snow in winter. AMO (Atlantic Multidecadal Oscillation) is the climate index that is most frequently correlated negatively with the hydrologic variables, mainly in winter and spring.

The interannual variability of streamflow affects the composition and species richness of vegetation in low-flow channels and alluvial plains. Although climate conditions in 2003 and 2004 were nearly identical, large differences in streamflow were observed downstream from the Matawin dam. These differences resulted in numerous days without flow (no water release) during the growing period (May to August) in 2003, leading to man-made hydrological drought. While this drought had no effect on abiotic variables (grain-size distribution and nutrient concentrations in sediments), a significant decrease in the number of terrestrial species was observed in 2004 (year without drought) relative to 2003 (drought year) on three sand bars studied. This decrease is interpreted to result from prolonged submergence of the sites in 2004. Principal component analysis highlighted the effect of individual sites (first principal component) and of the interannual variability of streamflow (second component) on the number of species. The study suggests that, from a flow management standpoint, it is advisable to release enough water downstream from the dam during the growing season to prevent low-flow channel colonization by invasive terrestrial species.

The goal of this study is to demonstrate the usefulness of the Lombard method for analyzing dam-induced hydrologic impacts. The method was used to accurately detect the effects of the construction of a diversion dam in 2003 on annual and seasonal maximum and minimum daily flows in the Manouane River, Quebec, Canada, measured from 1980 to 2014. The Lombard method yields results that are nearly identical to results obtained using the monitoring (Kruskal-Wallis test) and long-term trend (Mann-Kendall test) methods. The Lombard method revealed a shift in mean values of annual and seasonal minimum daily flows in 2003, the year the dam was built. This shift is sharp for all four seasons. The dam induced a significant decrease in minimum daily flows in all four seasons. As far as maximum daily flows are concerned, unlike the monitoring method, the Lombard method detected a significant decrease only in the mean values of annual and spring maximum daily flows. This decrease occurred two years prior to the construction of the diversion dam. Instead, this decrease is interpreted to be the result of a significant decrease in spring precipitation after 1997. These hydrological changes are different from those induced by other types of dams in Quebec.