Impacts des barrages sur les débits annuels minimums en fonction des régimes hydrologiques artificialisés au Québec (Canada)

Les débits annuels minimums des rivières déterminent le volume d’habitat minimum disponible pour assurer la survie des espèces aquatiques en période d’étiage. Dans cette étude, nous comparons les impacts de barrages sur les caractéristiques (période d’occurrence, magnitude, amplitude de variation et asymétrie) de ces débits dans trois régimes hydrologiques artificialisés d’une part, et les débits annuels minimums mesurés en aval des barrages aux normes de débits réservés pour protéger les habitats du poisson au Québec, d’autre part. Nous avons analysé 72 stations appartenant aux régimes artificialisés d’Inversion (26 stations), d’Homogénéisation (18 stations) et de Type Naturel (28 stations). Toutes ces stations appartiennent au bassin versant du fleuve Saint-Laurent. La présente analyse est fondée sur la comparaison des débits mesurés en rivières naturelles (75 stations) à ceux mesurés en aval des barrages au moyen des méthodes de proportionnalité et graphique. Il ressort de ces comparaisons les principaux résultats suivants.En régime artificialisé d’Inversion caractérisé par les débits mensuels maximums en hiver et les débits mensuels minimums au printemps, les impacts des barrages se traduisent par une hausse significative de fréquence des débits annuels minimums au printemps au moment de la fonte des neiges mais une baisse en été, une diminution significative de la magnitude des débits pour les bassins versants de taille < 10 000 km2, une hausse de la variabilité inter-annuelle et une forte asymétrie de la distribution. En régime artificialisé de Type Naturel caractérisé par des débits mensuels maximums au printemps et des débits mensuels minimums en hiver ou en été, on observe une hausse de la fréquence des débits annuels minimums pendant la première moitié de la période froide (de novembre à janvier), une diminution significative de la magnitude pour certaines rivières de taille < 6000 km2. En régime d’Homogénéisation caractérisé par des débits mensuels quasi constats toute l’année, les barrages provoquent une hausse de la fréquence des débits annuels minimums (printemps et automne) mais une baisse en été. Mais contrairement aux deux régimes précédents, l’impact des barrages se manifeste surtout par une hausse de la magnitude des débits annuels minimums pour quelques rivières.Pour les trois régimes artificialisés et durant les quatre saisons, les débits réservés sont systématiquement supérieurs aux débits annuels minimums lâchés en aval des barrages. L’écart entre les deux types de débits est surtout observé au printemps et en été pour les bassins versants > 10 000 km2.
Annual minimum discharges represent a crucial hydrologic parameter for the health of aquatic ecosystems. They determine the volume of available habitat for aquatic species and influence the concentration of pollutant within the fluvial system during low flows. They are also of importance for instream infrastructures and for the regulation of fluvial transport. For these reasons, the minimum discharges constitute the main hydrologic parameters for which clear regulation have been defined in several countries. In the province of Québec, albeit the large amount of dams on several important fluvial systems, there seems to exist a lack of studies examining their effects on the annual minimum discharges. This paper is aiming at highlighting the effects of dams (1) by examining their effect on the characteristics of annual minimum discharges for artificialised flow regimes in Québec, and (2) by comparing those discharges with recommended instream flows to protect fish habitats.Firstly, the effect of dams on annual minimum discharges is examined for the three types of artificialised flow regimes found in Québec. From the analysis of seasonal and monthly discharges, ASSANI et al. (2004) documented the three types of artificialised hydrologic regime downstream from dams: the inversion, the homogenization, and the natural type flow regimes. The inversion flow regime presents high monthly discharge values in winter and low monthly discharge values during spring. This type of regime occurs solely on the north shore of the St-Lawrence River and pertains to rivers with large reservoirs feeding in hydropower stations. The homogenization flow regime presents small annual fluctuations of the monthly discharge. The maximum monthly discharges are recorded during spring where- as the minimum monthly discharges frequently occur during fall. This type of regime is often associated with reservoirs created on large streams for which the storage of spring water is less important. This regime is observed mainly on the north shore of the St-Lawrence river. In the natural type flow regime, the maximum monthly discharges take place during spring snowmelt while minimum monthly discharges occur either during summer or winter. The annual natural flow characteristics are thus conserved albeit the existence of the dam. This regime pertains to dams with small reservoirs and it is found on both side of the St-Lawrence River.Secondly, annual mimimum discharges are compared with minimum instream flows recommended by BELZILE et al. (1997). These ones defined the minimum instream flows based on the different species of fish and their life cycle. Downstream from dams, the instream flows (Qr) can be estimated using the following relation:Qr = ek.Sawhere S represents the drainage area upstream from the dam; a and k are respectively regional and seasonal parameters. These parameters are associated to the ecohydrological region, to the season as well as to the critical phases of life cycle for the fish species found within the ecohydrological regions.From the Historical Stream Flow Summary of Environmental Canada, the distribution of discharge from 107 stations were selected and analysed. From those, 72 were located on rivers with dams and 75 on rivers with no regulation. On regulated rivers, 26, 18 and 28 were identified as belonging to the inversed, homogeneous and natural type regimes, respectively. All stations were located in the St-Lawrence drainage area. To highlight the effect of dams, we performed a comparison between the annual minimum discharges for stations on artificialised rivers to those from stations belonging to rivers with no regulation. The comparison is performed according to the size of the drainage basins (proportionality method) and uses a set of parametric and non-parametric statistical tests depending on the type of data. The proportionality method was chosen because of the non-availability of the discharges for the pre-dam periods. According to RICHTER et al. (1996), river flows can be described using several parameters relating to the daily discharges: the magnitude, the frequency, the duration, the timing and the rate of change (amplitude of the variability). The daily discharges required to compute these parameters were not available. The date of occurrence of annual minimum discharges, their magnitude, the interannual variability of the magnitude and the skewness of the distribution could however be obtained from the Historical Stream Flow Summary of Environmental Canada.The analysis of annual minimum discharges for the three types of artificialised flow regimes highlights several key elements associated with the effect of dams. For the inversion flow regime, the presence of dams increases and decreases significantly the occurrence of annual minimum discharges during spring and summer, respectively. For drainage area smaller than 10 000 km2, the magnitude of the annual minimum discharge is decreased significantly. Finally, the between-year variability is increased and the distribution presents a strong skewness. For the natural type flow regime, an increase in annual minimum discharges during the period between November and January can be observed as well as a significant decrease in magnitude for the small fluvial systems (drainage area < 6000 km2). For the homogenization flow regime, the frequency of annual minimum discharge is increased during spring and fall while decreased during summer time. However, in contrast with the two previous artificialised flow regimes, there is an increase in magnitude for the minimum annual discharges.Finally, for the three types of artificialised flow regime and for the four seasons, the minimum annual discharges released downstream from the dams appear to be systematically smaller than the instream flows recommended by BELZILE et al. (1997). The main differences are observed during spring and summer for drainage basins > 10 000 km2.