WSP04505 - Laserfiche WebLink

Home Browse Search

<5 c:s (~ t ."" W tiel 11 precipitation is dominant (more than 50 percent of the total precipitation falls during winter months), although the Chihuahuan desert monsoon provides warm weather moisture during summer months (Cully et al. 1987). B. WATER Flows in the San Juan River are dependent upon a number of factors. Climatic conditions; water use demands for municipal, domestic, industrial, and agricultural activities; and release of flows from Navajo Dam in response to the first two factors all influence the flow regime of the river. These effects are attenuated as distance from the dam increases; the river reflects more short term fluctuations from unregulated tributaries intersected en route to Lake Powell. Prior to the construction of Navajo Dam, the hydrograph of the San Juan River was characterized by large spring peaks and low base flow. Typically, spring runoff began in March, peaked in mid-May to early June and ended by the first week of July. During the remainder of the year, flow was characteristically low, punctuated by large, short duration peaks caused by summer and fall storm events. Since the completion of Navajo Dam in 1962, flows below Navajo Reservoir have been largely controlled and stabilized. Spring peak flows have been significantly reduced in magnitude, base flows have been increased and stabilized, and late winter flows have increased markedly to provide storage space in the reservoir for the spring runoff. In addition to moderating natural flows, hypolimnetic releases from Navajo Reservoir have decreased mean annual water temperature and diminished temperature fluctuations of the San juan River downstream to near the confluence of the Animas River. As a component of the reasonable and prudent alternative issued in the October 1991 Biological Opinion to the Bureau of Reclamation for the proposed Animas-La Plata project, the operation of Navajo Dam has been revised to mimic the natural hydrograph of the San Juan River. c. VEGETATION The vegetation of the San Juan Basin is typical of the Great Basin biotic province and is comprised of four general types of plant habitats: riparian wetland, desertscrub, desert grassland, and conifer woodland. The riparian corridors are relatively narrow, confined to the river banks and disjunct throughout the course of the San Juan River as it traverses the analysis area. Tributaries to the San Juan River support varying degrees of intact riparian communities. The riparian plant community is dominated by Rio Grande cottonwood (Populus fremontii), and narrowleaf cottonwood (.E. anaustifolia); peachleaf willow (Salix amvadaloides) and scrub willow (.s.. exiaua); Russian olive (Elaeaanus anaustifolia); saltcedar (Tamarix sp.); and common reed (Phraamites astralis). Xeric lowlands and upland sites in the region are occupied by the Great Basin desertscrub J