Comparison of impacts of dams on the annual maximum flow characteristics in three regulated hydrologic regimes in Québec (Canada)

Despite the presence of numerous dams in Québec, no study has yet been devoted to their impacts on flood levels. To compensate for this deficiency, we have compared the impacts of dams on the five characteristics (magnitude and its interannual variability, timing and its interannual variability, and asymmetry) of the maximum annual flows between natural rivers and regulated rivers by means of several statistical approaches (analysis of variance, chi‐square test, nonparametric tests, etc.). In the course of this study, we analysed 88 stations on pristine rivers and 60 stations on regulated rivers. The latter group was subdivided into three regulated hydrologic regimes, i.e. inversed flow regimes (25 stations), homogenization flow regimes (15 stations) and natural‐type flow regimes (20 stations). The following observations emerge from this study. (1) In inversed and homogenization flow regimes, generally associated with reservoirs, all the flow characteristics are modified. These modifications notably entrain a decrease in magnitude, a significant reduction in the frequency of the maximum annual spring flows when the snow is melting and an increase in skewness of the distribution and interannual variability of the magnitude and dates of occurrence of the annual maximum flows. We also observed the disappearance of most flows with a recurrence of over 10 years. All these changes particularly affect watersheds larger than 10 000 km². (2) In natural‐type flow regimes, often associated with run‐of‐river dams, very few changes were observed compared with pristine rivers. These changes primarily affected watersheds smaller than 1000 km². Copyright © 2006 John Wiley & Sons, Ltd.