The substantial size of some hydroelectric projects and the extensive total surface area convered by reservoirs globally require that research determining the impacts of these developments be done at ever-increasing spatial and temporal scales. As a consequence of this research, new views are emerging about the spatial extent and longevity of the environmental and social impacts of such developments. New findings challenge the notion of hydroelectric development as a benign alternative to other forms of power generation. This review examines the intertwined environmental and social effects of methylmercury bioaccumulation I th efood web, emission of greenhouse gases from reservoirs, downstream effects of altered flows, and impacts on biodiversity, each of which operates at its own unique spatial and temporal scales. Methylmercury bioaccumulation occurs at the smallest spatial and temporal scales of the four impacts reviewed, whereas downstream effects usally occur at the largest scales. Greenhouse gas emissions, the newest surprise connected with large-scale hydroelectric development, are relatively short term but eventually may have important global-scale consequences. Limitation of biodiversity by hudroelectric development usually occurs at intermediate spatial and temporal scales. Knowledge developed from working at expanded spatial and temporal scales should be an important part of furure decision making for large-scale hydroelectric development.

The most often heard claims in support of large scale hydroelectric development are: (1) hydropower generation is 'clean', (2) water flowing freely to the ocean is 'wasted', and (3) local residents (usually aboriginals) will benefit from the development. These three claims are critically examined using case histories from Canada and elsewhere in the world. The critique is based mainly on journal articles and books, material that is readily available to the public, and reveals that the three claims cannot be supported by fact. Nevertheless, large scale hydroelectric development continues on a worldwide basis. The public needs to be well informed about the environmental and social consequences of large scale hydroelectic development in order to narrow the gap between its wishes for environmental protection and what is really occurring.

The development of waterways, for hydropower and other industrial uses, has substantially altered many of the freshwater habitats of the planet and this has had considerable impact upon aquatic organisms. Industrial changes in aquatic ecosystems, including hydropower development, can restrict or delay fish migration, increase predation, affect water quantity and quality, and subject fish to direct damage and stress. This review will focus on the consequences for fish welfare and the progress towards developing the means to pass and protect fish at hydropower dams, at water withdrawal facilities, and in other engineered aquatic environments. It primarily concerns the large mainstem hydropower dams in the Columbia-Snake River Basin in the northwestern United States. Some methods for improving fish passage and protection at hydropower damsinvolve modifications and additions to engineered structures and occasionally use sensory stimuli such as light, sound, turbulence, or electric fields to influence fish distributions. Measures to improve fish survival, like spilling water at a dam to provide non-turbine passage, can cause other problems for fish, for example higher dissolved gas concentrations downstream. Reducing losses of fish in industrial environments is desirable in both the industrialized world, where many fish-related problems currently exist, and in the developing world, here lessons already learned may make future development more cost-effective and benign.

This paper presents the results of an ex post survey of recreational anglers for the lower Kennebec River, post-Edwards Dam removal. To the best of our knowledge, this study represents one of the first ex post analyses of fisheries restoration from dam removal. We find significant benefits have accrued to anglers using the restored fishery. Specifically, anglers are spending more to visit the fishery, a direct indication of the increased value anglers place on the improved fishery. Anglers are also willing to pay for increased angling opportunities on the river. These findings have policy implications for other privately owned dams that are currently undergoing relicensing and ⁄ or dam removal considerations. Our findings may also hold implications for fisheries that have deteriorated due to historic dam construction.