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~o (:.) ( '.:J . " .. CJ --.l 10 III. AFFECTED ENVIRONMENT Based on the perceived range of environmental impacts resulting from the proposed implementation of the preferred alternative, this EA incorporates as its analysis area the immediate environs of the San Juan River and its permanent and. ephemeral tributaries from Navajo Dam downstream to Lake Powell. Baseline information concerning the delineated affected environment is provided in the following discussions. The No Action Alternative sets the environmental baseline (Le., the affected environment) for comparison of the effects of the proposed action. The environmental effects (changes from present baseline conditions) reflect the identified major issues and other key elements of the environment. A. CLIMATE, AIR QUALITY, GEOLOGY, AND SOILS The San Juan Basin is typical basin and range topography with deep canyons, dry washes, upland mesas, and hogback ridges with igneous 6lkes, Soils are derived from sandstones, clays. and barren shales containing little organic matter. This area is classified as the Navajo section of the Colorado Plateau physiographic province (Fenneman 1931). Elevation within the basin varies from 1460 to 2100 meters (4790-6890 feet) . The climate and vegetation of the San Juan Basin is characteristic of the Great Basin, a cold-temperature desertland. The Great Basin has cold, harsh winters, low precipitation scattered throughout the year, with great extremes in both daily and seasonal temperatures (Brown 1982). Mean annual precipitation at Aztec, New Mexico is 242 mm or 9.53 inches. Winter 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. 19B?). 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 hydro graph 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.