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<br />o <br />!-:'l <br />C:,) . <br /> <br />l ;, <br /> <br />C) <br />CO <br /> <br />11 <br /> <br />Since the completion of Navajo Dam in 1962, flows below Navajo Reservoir have been <br />largely controlled and stabilized. Spring peak flows have been significantly reduced in <br />magnitude, base flows have been increased and stabilized. and late winter flows have <br />increased markedly to provide storage space in the reservoir for the spring runoff. In <br />addition to moderating natural flows, hypolimnetic releases from Navajo Reservoir have <br />decreased mean annual water temperature and diminished temperature fluctuations of the <br />San Juan River downstream to near the confluence of the Animas River. <br /> <br />As a component of the reasonable and prudent alternative issued in the October 1991 <br />Biological Opinion to the Bureau of Reclamation for the proposed Animas-La Plata project, <br />the operation of Navajo Dam has been revised to mimic the natural hydrograph of the <br />San Juan River. . <br /> <br />C. VEGETATION <br /> <br />The vegetation of the San Juan Basin is typical of the Great Basin biotic province and is <br />comprised of four general types of plant habitats: riparian wetland, desertscrub, desert <br />grassland, and conifer woodland. . <br /> <br />The riparian corridors are relatively narrow, confined to the river banks and disjunct <br />throughout the course of the San Juan River as it traverses the analysis area. Tributaries <br />to the San Juan River support varying degrees of intact riparian communities. The <br />riparian plant community is dominated by Rio Grande cottonwood (Pooulus fremontii). <br />and narrowleaf cottonwood (E. angustifolia); peachleaf willow (Salix amyadaloides) and <br />scrub willow (S. exigua); Russian olive (Elaeaanus anaustifolia); saltcedar (Tamarix sp.); <br />and common reed (Phraamites astralis). <br /> <br />Xeric lowlands and upland sites in the region are occupied by the Great Basin <br />desertscrub community. The following taxa are indicative of this community: shad scale <br />(Atriplex confertifolia) and broadscale (6. obovata); big sagebrush (Artemisia tridentata), <br />Bigelow sagebrush (6.biaelovii), sand sagebrush (6. filifolia), black sagebrush (6. <br />arbuscula ssp. nova), Parry sagebrush (6..QEIIYi), and bud sagebrush (6. spinescens); <br />fivehook bassia (Bassia hvssooifolial; three-leaf snakeweed (Gutierrezia microceohala) <br />and broom snakeweed (Q.. sarothrae); winterfat (Eurotia lanata); tanglebrush (Forestiera <br />neomexicana); tumbleweed (Salsola iberica and S. kalil; greasewood (Sarcobatus <br />vermiculatus); rabbitbrush (Chrysothamnus nauseosus); galleta grass (Hilaria iamesii); <br />whipple cholla (Oountia whipplei); pediocactus (Pediocactus spp. sclerocactus <br />(Sclerocactus spp.); and gray horsebrush (Tetradvmia canescens). <br /> <br />The Great Basin desert grasslands and Plains grasslands communities intergrade and <br />form transitional areas in northwestern and central New Mexico, southern Utah, northern <br />Arizona, and south central and western Colorado (Brown 1982). Because these two <br />communities overlap, they will be referred to herein as the grasslands community. Much <br />