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. plateau's highest point with an elevation of 10,347 feet. The southeast portion of the plateau, known as Log Hill Mesa, slopes northeast from Horsefly Peak in an area about 1,000 feet above DaIlas Creek and terminates about 200 feet above the Uncompahgre River. McKenzie Butte, formed by igneous intrusive rock, rises about 500 feet above the general slope of the mesa adjacent to the river vaIley. On the east, the basin is bounded by Cimarron Ridge, an extension of the San Juan Mountains, and the Black Canyon Uplift. The San Juan Mountains, with peaks rising above 14,000 feet, form the southern boundary of the river basin. . . . The main inflow into Ridgway Reservoir is the Uncompahgre River, which generaIly flows in a northern direction and has many intermittent tributary streams, Headwaters of the Uncompahgre River are in the San Juan Mountains, approximately 25 miles south of the existing reservoir. The Uncompahgre River flows north for about 70 miles to its confluence with the Gunnison River west of Delta. Along its course, the river is joined by numerous tributaries, including Red Mountain and Dallas Creeks from the southwest and Alkali and Cow Creeks from the southeast. The drainage area comprises 264 square miles of mostly intermountain area. . . In their upper reaches in the San Juan Mountains, the Uncompahgre River and its tributaries are entrenched in deep and narrow gorges but the streams in their lower reaches flow through relatively broad irrigated valleys. The vaIley bottom of the Uncompahgre River varies in width from a few hundred feet north of Ridgway up to 15 miles in the vicinity of Montrose and Delta. Typical land forms in the nonmountainous areas include mesas, aIluvial fans, stream terraces, flood plains, and low roIling hiIls. Elevations in the river basin range from more than 14,000 feet in the high mountains to about 4,900 feet at Delta. The vegetation in the basin varies primarily by differences in eltivation, from desert shrubs and grasses (at 4,900 to 5,500 feet) to alpine plants (over 12,000 feet). . . 4.3 Geology. Igneous rock formations are predominant in the surrounding mountains, while layers of sandstone, shale, and mudstone characterize the predominant. sedimentary formations in the vaIley. Mineral deposits including gold, silver, lead, copper, and zinc are present in the San Juan Mountains. Ridgway Dam is located at a constricted point in the valley eroded by the Uncompahgre River from the Morrison, Burrow Canyon, and Dakota Formations. Sandstone of the Dakota Formation produces a rim ofrock about 30 feet high near both sides of the valley. Highly alkaline calcareous deposits of Mancos Shale are also drained to the reservoir via Alkali Creek. . . A major east-west fault is located about 3,000 feet at its nearest point from the southern boundary of Ridgway Reservoir. Two smaller faults are along the eastern side, although the west side of the reservoir exhibits more geologic activity than the east. The closest significant earthquake occurred on September 13, 1994. The . 10 .