Impacts of dams on monthly flow characteristics. The influence of watershed size and seasons

We have compared the monthly flow characteristics between natural rivers (76 stations) and reservoir-regulated rivers (25 stations) based on watershed size, using regression analysis on the monthly flows observed in Québec. The flow characteristics, defined according to the "natural flows regime" paradigm, are as follows: the periods of occurrence of the monthly maximum and minimum flows and their inter-annual variability, the magnitude and its inter-annual variability, the frequency, and the coefficients of skewness and kurtosis. This study revealed that dams alter all monthly flow characteristics but the extent of these modifications is variable. Two factors influence this extent: the watershed size and the season. The behaviour of large watersheds (>10,000 km²) seems to be different from that of small and medium-sized watersheds. Large watersheds are characterized by a greater inter-annual variability of the timing of maximum monthly discharges but a lower inter-annual variability of the timing of the minimum monthly flows than smaller basins. Concerning the monthly winter flows, large watersheds also exhibit an increase in the magnitude and a smaller variability in between years of discharges. Concerning the season, the hydrological changes were mainly observed in winter and spring. However, in both seasons, the extent of these hydrological changes varies from month to month. These results show that the dam-induced hydrological changes render the method of estimating minimum flow requirements in winter and spring inapplicable to reservoir dams in Québec to protect Québec fish habitats effectively. In winter, the method would underestimate the flows released downstream from the dams, while overestimating them in the spring. Moreover, the method cannot be applicable to all watersheds in certain months, because the hydrological changes during those months only affect either small watersheds or large watersheds.