More than 75,000 dams exist in the continental United States to provide water storage, flood control, and hydropower generation (Graf, 1999). Many of these were built during the early twentieth century and are due for relicensing consideration now and in the near future. The cost of repairing aging dams, together with growing understanding of the ecologic effects of river regulation (Williams and Wolman, 1984; Dynesius and Nilsson, 1994; Graf, 1999, 2003; Yang et al., 2007), in some places have prompted dam removal, facilitating restoration of riparian habitat to a more natural state. In the Pacific Northwest region of the U.S., river-restoration efforts are commonly targeted to improve habitat quality for native salmonid fish species, many runs of which have declined precipitiously from their historical conditions (owing, in part, to overfishing and habitat loss and degradation) and are now endangered (e.g., Nehlsen, 1997; Larsen et al., 2004; Pess et al., 2008). Removal of dams that block the upstream migration of anadromous fish is considered an important step toward any potential recovery of Pacific Northwest salmon and steelhead populations.
