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3.30 in the region, it is very important to wildlife. Aspen (Populus tremuloides) is dominant at higher elevations, but is replaced by boxelder (Acer negundo) and narrowleaf cottonwood (Populus angustifolia) at lower elevations (Table 3.12). The Shrubland Riparian Formation occurs from 4,000 to 6,000 feet elevation, also occurring on less than 1 percent of the available land. This formation is more extensive than the Mountain Formation, and serves as a major habitat for wildlife. Salt cedar (Tamarix pentandra) covers much of the riparian zone (Table 3.12). Mature cottonwoods are important for the endangered bald eagle (Haliaeetus leucocephalus) as diurnal perches or hunting sites, nocturnal roost sites, and as nest trees (Northwest Colorado Wildlife Consortium, 1982b). Cottonwood and willow regeneration characteristics are presented below because the regeneration of these trees has become a subject of concern in recent years (Crouch, 1982). Cottonwoods and willows disperse their seeds from May through July, pri- marily by means of wind. The seeds must land on a favorable germination site within a very few days or germination success is reduced. Such sites must be moist, bare soils in full sunlight. Seedlings rapidly develop an extensive root system and may grow from 4 to 6 feet in height the first year. Seedlings are highly tolerant of flooding and can withstand up to 30 days of inundation. Stands of mature cottonwoods begin to decline after 30 to 40 years because seedlings cannot survive in the shade of mature stands. Changes in streamflows are now being investigated as a possible cause in decreased regeneration of cottonwood and willows. The bald eagle and peregrine falcon (Falco pere4rinus anatum) depend on riparian zones during certain times of the year. Bald eagles use riparian areas in the summer for nest trees, and in the winter for diurnal perch sites and nocturnal roost sites. Peregrine falcons use riparian areas mainly for hunting. Information concerning the occurrence of both species in northwestern Colorado and their use of riparian habitat is presented in Table 3.13. Waterfowl, nongame birds, and furbearers are other wildlife groups in the study region. Waterfowl are important winter food items for bald eagles (Fisher et al., 1981). Nongame birds occur throughout the study region, and potentially are important prey items for peregrine falcons. Peregrines often hunt over riparian areas (Northwest Colorado Wildlife Consortium, 1982b) due to the high number and diversity of nongame birds. Muskrat (Ondatra zibethica), mink