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,. Case Review 249 problem in the 1320-km Rhine River that drains a catchment basin of 185,000 km2 in the European countries of Austria, France, Liechtenstein, Luxemburg, and the Netherlands (van Dijk et al., 1995). This river basin is the most densely populated (54 million inhabitants) and industrialized ("'10% of the world's chemical industry) in western Europe. Although pollution of the Rhine River remains a major problem, water quality has improved markedly through ecological rehabilitation efforts that have al- lowed biological communities to partially recover (van Dijk et al., 1995). Contaminants affect animal communities for a considerable distance downstream from point sources. For example, the number and density of benthic macroinvertebrates immediately downstream of a single uranium- vanadium mill on the Animas River (a tributary of the Colorado River in the southwestern United States) was decimated by the discharge of heavy metals (TsivogIov et al., 1959). Although some of the more tolerant species were found about 3 km downstream of the discharge, the species composition and abundance did not recover for about 35 km from the point source of the contamination. f Floodplain The most significant adverse impact of anthropogenic alterations of a river- floodplain ecosystem is undoubtedly the loss of connectivity with the floodplain that alters the productivity of large river systems through dis- ruptions in nutrient cycling and food webs (Junk et al., 1989; Ward and Stanford, 1989; Bayley, 1991; Power et al., 1995a, 1995b; Sparks, 1995; Welcomme, 1995). Fragmentation ofriverine habitat by high- and low-head dams in regulated rivers results in the loss of heterotrophic production (Ryder and Pesendorfer, 1989) that is vital to nutrient and energy transfer in the river continuum (Ward and Stanford, 1983, 1995). Construction of levees, dikes, or berms to control overbank streamflows in regulated rivers further reduces the connectivity of rivers with productive floodplains. The frequency and duration of flooding is reduced in regulated streams, affect- ing the structure and integrity of native fish . and invertebrate communi- ties that form a food web (Junk. et al., 1989; Bayley, 1991; Sparks, 1995; Welcomme, 1995). Plants and animals with narrow, specific tolerance lim~ its to various biological, chemical, and physical factors (Shelford's Law of Tolerance described by Odum and Odum, 1959) may decrease or disappear. Although floodplain habitats contribute to the productivity of a river- floodplain ecosystem, they can also become sinks for various contami- nants. Low-velocity habitats such as those in floodplains become deposi- tion areas for large amounts of contaminants in sediments (Graf, 1985). In addition to causing direct mortality of aquatic animals, sublethal effects of contaminants can be significant to various life stages of animals (Table 9.6). ,'. . , 'Y; ~, ., ~ ;1; : --~ Case Review: Impacts of Water Development on Endangered Fishes in the Upper Colorado River Basin The Colorado River and its tributaries flow through 2317 km of arid land (Carlson and MUth' 1989) and are separated into two distinct basins (an upper and a lower basin) by the 66-m Glen Canyon Dam that forms Lake Powell (see Figure 1.5 of Wohl, Chapter 1, this volume). This river flows