Studies were conducted on wind performance within a mainly hydroelectric system. Because windpower provides only intermittent electricity, the studies aimed at answering the two following questions: First, does windpower development require the installation of additional backup capacity, thereby imposing a hidden cost somewhere in the electricity system? Second, does windpower increase the environmental impacts of the hydro-facilities providing the backup when wind is down? To answer these questions, wind/hydro combinations were simulated using hourly real data (1990-1996) of patterns of electricity demand, wind speeds and hydraulic flows in Quebec. The simulations show that windpower requires backup capacity to compensate for wind fluctuations. In large networks, it may not be necessary to build additional capacity, because reliability could be maintained by other tools, such as buying ancillary services from other generators. However, this also represents a hidden cost of windpower. When hydropower is the option used to compensate for wind fluctuations, a relatively large development of windpower could have significant effects on river flows. It could reduce the minimum flow during dry season and increase the intensity of short-term flow fluctuations. These conclusions do not mean that windpower development is not justified, but simply that its assessment should include the impact of backup generation.