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. . mouths exceeded fecal coliform densities greater than 10,000 FC/100 ml. The pathogen, Shigella sonnei, was identified from victims of gasterenteritis during low water in mid-summer, 1972 and 1979, periods with maximum river temperature and maximum sediment discharge from tributaries. The Grand Canyon reach is also rarely a source of Giardia infection, presumably vectored by beaver, coyote or cattle grazing in the headwaters of tributaries (Stevens, personal observation). Multiple Pollution Effects: The Yampa River As an example of pollution problems in an undammed system, the Yampa River has been subjected to numerous sources of pollution. Headwaters quality is generally good. Three headwater tributaries studied bl Britton (1983) had a mean water temperature between 11.10 to 17.5 C, specific conductance of unpolluted sites ranged from 142 to 808 umho, mean pH ranged from 8.0 to 8.4, and mean dissolved oxygen ranged from 8.0 to 8.6 mg/L. Wentz and Steele (1980) found significant natural sources of water pollution in the Yampa River, including pyritic materials associated with coal deposits and natural gas leakage. They attributed a 14% increase in specific conductance in the Yampa River between 1951 and 1980 to increased agriculture and municipal uses of that basin's surface waters. Anthropogenic sources of pollution included higher than acceptable concentrations of dissolved iron and manganese (Colorado Department of Health 1977). Dissolved and trace element and nutrient concentrations were highest in March, when increased suspended sediment loads were greater and pH was lower. Thermal pollution from power plant cooling has been alluded to above. In addition, a significant oil spill occurred on the middle Yampa River in mid-summer 1989, with as yet unknown results on native fish and wildlife. Sedimentation The Colorado River and its tributaries are renown for their tremendous sediment loads and transport capacity. Sediment transport has measured in many tributaries, particularly because it pertains to the longevity of the system's many impoundments. Suspended sediment load is generally correlated with drainage basin area. Ringen (1984) concluded that suspended sediment loads varied significantly between years and drainages of the upper Green River drainage above Flaming Gorge Reservoir, with the majority of sediments transported during spring runoff and, to a lesser extent, summer rainstorm floods. The White River alone transports more than 82,000 mt/yr of sediment into the Green River (Boyle et al. 1984). The mechanisms and effects of sediment transport on mainstream channel geometry have been examined in the Grand Canyon reach. . . . . . . . . 29 .