We consider electricity generation industries where thermal operators imperfectly compete with hydro operators that manage a (scarce) water stock stored in reservoirs over a natural cycle. We explore how the exercise of intertemporal market power affects social welfare and environmental quality. We show that, as compared to the outcome of spot markets, long-term contracting either exacerbates or alleviates price distortions, depending upon the consumption pattern over the water cycle. Moreover, it induces a second-order environmental effect that, in the presence of a thermal competitive fringe, is critically related to the thermal market shares in the different periods of the cycle. We conclude by providing policy insights.