Impacts of a dam on the hydrologic regime of the Matawin river (Québec, Canada)
Few studies have characterized the effect of dams on the hydrologic regime of rivers in Quebec. This is rather strange given the large number of hydroelectric dams that have been constructed in the province. To shed some light on the environmental impact of these dams, this paper aims at describing and quantifying the effect of the Matawin River dam on the hydrologic regime of the river on an annual, seasonal and daily basis. The Matawin River is located north of the St-Lawrence River and is a tributary of the St-Maurice River. The Matawin dam was built in 1929 by Shawinigan Water and Power Co. mainly to supply the Gabelle hydroelectric dam on the Saint-Maurice River. The dam is 26 m high and the storage capacity of the reservoir is 348,000,000 m3 when full. The catchment area of the dam is 4070 km2. To assess the effect of the dam, we used various statistical methods to compare discharge time series over 60 years as measured at two gauging stations on the river. One of the stations is located upstream whereas the other one is located downstream from the Matawin dam. The upstream drainage basin covers an area of 1390 km2. No major tributaries are found between the two gauging stations, thus allowing us to ascertain the effect of the Matawin dam on the natural hydrologic regime of the river at different time scales. On the annual scale, no difference in the mean annual discharge is observed upstream and downstream from the dam. The specific discharges upstream and downstream from the river's dam are respectively 17.2 and 17.1 l/s/km2. This is to be expected because the reservoir is used neither for irrigation nor for derivation. However, analysis of the interannual variability of mean annual discharges, using the Hanning low pass filter, reveals that wet and dry periods are far more persistent downstream than upstream from the dam. This persistence can be seen to occur within the two dry periods of 1930-1960 and of 1980-1990 and during the wet period from 1965 to 1980. This persistence does not affect the stationarity of the discharge time series downstream from the dam as no significant changes are detected from Mann-Kendall and Pettitt statistical tests. On the monthly and seasonal scale, the comparison of the time of occurrence of maximum and minimum discharges shows a strong inversion within the hydrologic regime. Upstream from the dam, the maximum and minimum discharges are measured respectively during the spring and the winter. Downstream from the dam, the regime is inverted, with the maximum and minimum discharges being measured in winter and spring respectively. This inversion is closely associated with the production of hydroelectricity during the cold winter season when large amounts of water are released from the reservoir. Furthermore, is worth noticing that the monthly and seasonal coefficients of maximum discharge are lowered downstream from the dam whereas those for the minimum discharge remain similar. On a daily basis, the comparison of dates of occurrence for the lowest annual discharge downstream and upstream from the dam shows these are found at different times of the year. Upstream from the dam, most of the minimum daily discharges are measured in August and September whereas downstream from the dam, they largely occur during April. On the other hand, the maximum daily discharges are recorded almost exclusively in April and May upstream from the dam but can occur throughout the year downstream from it, with a marginally larger number in January. These results are relevant for the assessment of the environmental impacts of dams on rivers in the province of Quebec. For example, the inversion of maximum and minimum discharges is likely to have an important impact on the winter habitat characteristics by increasing the area of suitable habitat, but also by increasing the likelihood of sediment being transported during periods where usually only sporadic transport events occur. In the future, it would be crucial to understand the exact effect of the inversion on the morphological and biological components of the river dynamics.