Laserfiche WebLink
<br />o <br /><::5 <br />,-, <br />::1 <br />~ <br />f-:' <br /> <br />11 <br /> <br />peaks caused by summer and fall storm events. <br /> <br />Since the completion of Navajo Dam in 1962, flows below Navajo Reservoir have been largely <br />controlled and stabilized. Spring peak flows have been significantly reduced in magnitude, <br />base flows have been increased and stabilized, and late winter flows have increased markedly <br />to provide storage space in the reservoir for the spring runoff. In addition to moderating <br />natural flows, hypolimnetic releases from Navajo Reservoir have decreased mean annual water <br />temperature and diminished temperature fluctuations of the San Juan River downstream to <br />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, the <br />operation of Navajo Dam has been revised to mimic the natural hydrograph of the San Juan <br />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 throughout <br />the course of the San Juan River as it traverses the analysis area. Tributaries to the San Juan <br />River support varying degrees of intact riparian communities. The riparian plant community <br />is dominated by Rio Grande cottonwood (Pooulus fremontii), and narrowleaf cottonwood (E. <br />anaustifolia); peachleaf willow (Salix amvadaloides) and scrub willow (S. exiaua); Russian olive <br />(Elaeaanus an9ustifolia); saltcedar (Tamarix sp.); and common reed (Phraamites astralis).. <br /> <br />Xeric lowlands and upland sites in the region are occupied by the Great Basin desertscrub <br />community. The following taxa are indicative of this community: shadscale (Atriolex <br />confertifolia) and broad scale (A. obovata); big sagebrush (Artemisia tridentata), Bigelow <br />sagebrush (8.biaelovii), sand sagebrush (A. filifolia), black sagebrush (A. arbuscula ssp. nova), <br />Parry sagebrush (A. 12<!illli), and bud sagebrush (A. soinescens); fivehook bassi a (Bassi a <br />hvssooifolial; three-leaf snakeweed (Gutierrezia microceohala) and broom snakeweed (G. <br />sarothrae); winterfat (Eurotia lanata); tanglebrush (Forestiera neomexicana); tumbleweed <br />(Sa Isola iberica and .s. kali); greasewood (Sarcobatus vermiculatusl; rabbitbrush <br />(Chrvsothamnus nauseosus); galleta grass (Hilaria iamesii); whipple cholla (Oountia whioolei); <br />pediocactus (Pediocactus spp. scierocactus (Scierocactus spp.); and gray horse brush <br />(Tetradvmia canescens). <br /> <br />The Great Basin desert grasslands and Plains grasslands communities intergrade and form <br />transitional areas in northwestern and central New Mexico, southern Utah, northern Arizona, <br />and southcentral and western Colorado (Brown 1982). Because these two communities <br />overlap, they will be referred to herein as the grasslands community. Much of this general <br />community has been converted to irrigated cropland. Extensive over-grazing and fire <br />suppression have led to shrub invasion of grassland habitats. The following species are <br />