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25 through single exposures to microgram quantities. The PAHs act at both the site of application and at organs distant to the site of absorption; their effects have been demonstrated in nearly every tissue and species tested, regardless of the route of administration (Lee and Grant 1981). The evidence implicating PAHs as inducers of cancerous and precancerous lesions is overwhelming, and this class of substances is probably a major contributor to the recent increase in cancer rates reported for industrialized nations (Cooke and Dennis 1984). Ongoing Service analyses of PAH contamination of aquatic biota of the San Juan River and hepato-histological examinations of fish in the river have raised concerns regarding the exposure of these organisms to contaminants introduced into the basin through the intensive development of energy resources in the area. Analyses of bile samples taken from fish in the San Juan River indicate that these organisms are being exposed to high levels of three PAH compounds, and strongly suggest that the aquatic environment of the river is heavily impacted by'PAHs. The primary PAH compounds of concern in the San Juan River are naphthalene, phenanthrene, and benzo(a)pyrene. The lower molecular weight unsubstituted PAH compounds, such as naphthalene and phenanthrene, have significant acute toxicity to some organisms (Krahn et al. 1984, 1986: Zitko 1975). Higher molecular weight PAHs, including benzo(a)pyrene, have been found to cause mutations and cancer in aquatic and terrestrial organisms (Eisler 1987). Background concentrations of these compounds are <10,000 nanograms per gram (ng/g) for naphthalene, <3,000 ng/g for phenanthrene, and <100 ng/g for benzo(a)pyrene (National Marine Fish. Servo 1989). Flannelmouth suckers (Catostomus latiDinnis) (n=36) analyzed from the San Juan River had mean concentrations of 97,110 ng/g wet weight naphthalene, 15,767 ng/g phenanthrene, and 256 ng/g benzo(a)pyrene in bile samples. From these data, it is apparent the flannelmouth suckers in the San Juan River are exposed to and accumulating PAH compounds in bile. Utilizing the flannelmouth suckers as indicators of similar exposures experienced by the Colorado squawfish and razorback sucker, these levels are of significant concern. Physical Habitat The impacts to physical habitat involve the loss of the quantity and quality of water in critical habitat and the change in flow regime. The quantity and timing of flows influence how various habitats are formed and maintained. Water depletions reduce the ability of the river to create and maintain these habitats; degradation of water quality lessens the ability of endangered species to survive in these habitats. Osmundson and Kaeding (1991) reported observations on the Colorado River (15-mile reach) during the drought years of 1988 -1990, that backwaters were filling in with silt and sand because spring flows were not sufficient to flush out the fine sediment. Also they reported that tamarisk colonized sand and cobble bars, stabilizing the river banks. On the San Juan River, lack of flooding since Navajo Dam was completed and introduction of exotic riparian vegetation (tamarisk and Russian olive) has armored the channel banks