FIGURE 3-1 Channelization of the Willamette River since the 1800s has reduced channel complexity, riparian trees, and off-channel habitat. SOURCE: Reprinted, with permission, from Sedell and Froggatt (1984). © 1984 by Science Publishers.

twentieth century—an extremely short time period compared to the many thousands of years over which riparian plant communities have adapted to shifting climatic regimes, runoff patterns, and adjustments in channel morphology. There are currently 75,000 dams on the streams and rivers of the United States (Meyer, 1996; Graf, 1999), and large dams1 worldwide are being completed at an estimated rate of 160 to 320 per year (World Commission on Dams, 2000). Dams have been constructed for hydropower generation, irrigation, flood control, domestic and industrial water use, recreational use, improved navigation, or some combination of these uses. Although detailed methods for the design of dams (e.g., Bureau of Reclamation, 1977) have been available for many years, such methods have provided little or no context for understanding the potential impacts such structures might have on other portions of a river and its riparian system.


A large dam is 15 meters or more high (from the foundation). A dam 5–15 meters deep with a reservoir volume over 3 million cubic meters is also classified as a large dam. Using this definition, there are more than 45,000 large dams worldwide. (World Commission on Dams

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