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. . Wintering bald eagles were first observed to congregate in Grand Canyon in the early 1980's and the winter population there increased dramatically after 1985 (Brown et aI., 1989, Brown and Stevens 1991, Brown and Stevens 1992). The wintering bald eagle population has been monitored since 1988, and it occurs throughout the upper half of the Grand Canyon (in Marble Canyon) and on both Lakes Powell and Mead. Density of the Grand Canyon bald eagles during the winter peak (in late February and early March) ranged from 13 to 24 birds between Glen Canyon Dam and the LCR confluence from 1993 to 1995 (Sogge et aI., 1995). A concentration of wintering bald eagles occurs in late February at the mouth of Nankoweap Creek, where bald eagles forage on spawning rainbow trout (Brown et aI., 1989, Brown 1993). Bald eagle density there ranged from 6 in 1987 to 26 in 1990, and 18 bald eagles occurred at Nankoweap Creek in 1995 (Sogge et aI., 1995). Eagle density was correlated with trout density in the lower 0.5 km of Nankoweap Creek, and trout density was correlated with tributary stream water temperature (Leibfried and Montgomery 1993). Territorial behavior, but no breeding activity, has been detected in Grand Canyon. Life Requisites At Nankoweap Creek in Grand Canyon, wintering bald eagles preferentially capture rainbow trout in the shallow creek, rather than in the mainstream where foraging success is low (Brown 1993, Sogge et aI., 1995). Bald eagles at Nankoweap Creek prefer roosting and feeding areas that are relatively free of vegetation. The eagle population there consists of all age classes, with considerable piracy and other interactions between individuals (Brown and Leibfried 1990, Brown and Stevens 1991). The ease and relative safety of foraging in Nankoweap Creek affords wintering bald eagles at Nankoweap Creek the opportunity to accumulate energy reserves needed for their long, northward migration flights and initiation of nesting. Bald eagle distribution in Glen and Grand canyons appears to be negatively related to human disturbance (Brown and Stevens 1997). Although bald eagles are widely known as opportunistic foragers, they are rare in the Glen Canyon and uppermost Grand Canyon reaches. This is surprising given that those reaches contain the most abundant aquatic food base and trout populations (Stevens et al. 1997c). Those reaches support the highest intensity of recreation and other human uses, and Brown and Stevens (1991) reported that bald eagles in Grand Canyon are extremely sensitive to human disturbance, often abandoning their foraging sites when human came within 0.5 km. For these reasons, Brown and Stevens (1997) concluded that human disturbance is responsible for the general rarity of bald eagles in the upper reaches. Impacts of BHBFs and Conclusions Wintering and migrant bald eagles have largely left the Grand Canyon region by late March (Stevens et al. 1997b). The few remaining eagles in April forage opportunistically and may continue to catch trout in the mainstream. The rainbow trout 1998 GCD Beach/Habitat Building Flow 17 Biological Assessment