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FIGURE 13 3 Thermal regimes of the (a) unregulated Middle Fork and (b) regulated South Fork of the Flathead River, Montana, during 1977. The shaded area approximates the daily temperature range. (Reproduced from Ward and Stanford 1979.)

appeared to be responsible for substantial changes in the benthic fauna. Sediment loads normally limit the diversity and abundance of benthic algae in the Colorado River, but in the tailwaters below Glen Canyon Dam, transparency exceeds 7 m, and the filamentous green alga Cladophora glomerata is abundant. Although Cladophora enters the food web primarily as detritus, it provides epiphytic diatoms and habitat for invertebrates, including the amphipod Gammarus lacustris, and thus helps to sustain a blue-ribbon fishery for nonnative trout (Shannon et al. 1994, Blinn and Poff 2005).

The benthic invertebrate community immediately below dams often shows a reduction in species richness. In the Gunnison River, Colorado, the greatest number of species of Trichop-tera (Hauer et al. 1989) and Plecoptera (Stanford and Ward 1989) are found in unregulated reaches, and the lowest species richness occurs in the tailwaters below deep-release dams. Often the faunal changes are fairly predictable from knowledge of habitat requirements. From a survey of the effects of dams on mayflies, Brittain and Saltveit (1989) report that low flows usually cause a shift from lotic to lentic species such as Cloeon, Paraleptophlebia, and Siphlonurus, whereas high flows and fast currents favor other species including Baetis, Rhithrogena, and Epeorus.

In addition to the specific and local-scale effects of individual dams on the impounded river section and downstream reach (which may extend for tens or even hundreds of km), dams sever the upstream-downstream connectivity that is a core characteristic of river ecosystems. The consequences of this loss of connectivity is readily apparent for the many freshwater species from shrimp to river dolphins to migratory fishes that use different habitats at different stages of their life cycles and so require unrestricted movement both upstream and downstream. Dams on the Yangtse River in China have contributed to declines of sturgeons and paddlefish, blocking migrations of the pota-

modromous (migrating within river systems) Acipenser sinensis in the Yangtse River, fragmenting populations of the endemic Yangtse sturgeon (A. dabryanus) and contributing to the decline of the paddlefish Psephurus gladius (Dudgeon 2000). Many aquatic species that dwell in the streams of tropical islands migrate between rivers and coastal zones (March et al. 2003). Although some shrimp, fish, and snail species have impressive abilities to overcome obstacles such as waterfalls, large dams without spillways totally block migrations, causing the extirpation of species that may play important roles in headwaters (Pringle 1996). Experimental exclusion of freshwater shrimps from sections of headwater streams of Puerto Rico has shown effects on sediments, algal and insect assemblages, and on litter processing (Pringle et al. 1999, March et al. 2003), suggesting that the elimination of shrimp may have significant consequences.

The influence of dams on migrating salmon of the Pacific Northwest of North America may be the best known yet still controversial example of biological impacts on migratory species. The total number of individuals of all species of salmon returning from the sea to spawn in the Columbia River prior to development is conservatively estimated to be 6.2 million, of which spring- and summer-run Chinook were most abundant. By the close of the 20th century, numbers of naturally produced salmon had declined to about one eighth of their predevelopment abundance (NRC 1996). Some 19 major hydroelectric facilities were constructed between the 1930s and 1970s, impeding and in some cases completely blocking upstream migrations of returning salmon. Juveniles that once passed swiftly downstream with the river's current now must expend more energy in swimming through slack-water impoundments, risk damage from passing over spillways or through turbines, and run a gauntlet of predatory fish and birds that congregate below dams. Despite the well-documented declines of salmon runs

(Figure 13.4) and the obvious negative effects of dams, other factors including deteriorating habitat in nursery streams, commercial and recreational harvest, and varying ocean conditions all affect salmon survival (Ruckelshaus et al. 2002). Because it is uncertain how much salmon would recover by the removal of just one of these threats, and hydropower provides 90% of the energy needs of the Pacific Northwest, the breaching of Columbia and Snake River dams to restore salmon has failed to gain widespread support. In other instances where dams block access to extensive salmon spawning area and generate only modest hydropower, such as on the Elwha River in Washington State, advocates for dam breaching have met with greater success.

Floodplains are a natural feature of large lowland rivers, a fact that is easy to forget in North America and Europe due to construction of dams, dikes, and levees to control flooding and permit agricultural use and human settlement in former floodplains. Natural floodplains are highly imperiled ecosystems, with over 90% under cultivation in Europe and North America (Tockner and Stanford 2002).

The ecological benefits of flooding are evident from studies of the Missouri River subsequent to catastrophic floods of the mid-1990s. A partially

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