A comparative study of downstream flows and ecologic impacts of hydropower dams

The primary emphasis of this study is on the alteration of river flow regimes by closure of hydropower dams. The purpose of this research was to investigate the changes in flow regimes caused by hydropower dams and to explore the possibility of integrating the available data and statistical methods to develop a comprehensive methodology for evaluation and quantification of flow impacts caused by hydropower dams.; This study addressed two main questions. The first question was concerned with the verification of the general theory that the extent of hydropower dam related flow regimes changes depend upon the characteristics of rivers and dams. An analysis method called IHA (Indicators of Hydrologic Alteration; Richter et al. 1997) and other statistical tools were applied to the observed data for a sample of 17 dams and stream-sites located on 17 different rivers. A linear-regression model was also used to define selected hydrologic parameters as a function of the spatial and temporal variations in dam and river characteristics. Among the river and dam characteristics, dam height proved to be the best predictor of the changes in river flow regimes.; The second research question concerned changes of channel elevations caused by changed flow regimes. Available information from USGS streamflow gauging stations was used to assess channel-bed elevation changes downstream for seven stream-sites. Analysis for these seven gauging sites downstream of dams showed statistically significant negative trends (decreasing stage with time) at four sites. Thus, for these locations, channel-bed lowering was indicated.; Overall, this research has shown that flow regulation by hydropower dams was associated with reduction in magnitude and duration of the annual flood pulse, increase in magnitude and duration of annual discharge minima, reduction in frequency of annual low-flow pulses, earlier timing of low-flow pulses, and general increase in frequency of flow reversals with a reduction in the rate of change in river flows. For river channel elevations affected by dams, a statistically significant negative trend (i.e. presenting degradation of river bed) was indicated. Also, the study demonstrated that it is possible to integrate the available data and statistical methods in order to develop a comprehensive methodology for evaluation and quantification of flow impacts caused by hydropower dams. The results of this study can be used as reference or decision-making tools for the building or breaching dams in the future.