Silverwood Lake, a large reservoir created by the construction of a dam on the northern flank of the San Bernardinos in 1973. From Silverwood, the water passes through the mountains via the San Bernardino Tunnel, and drops down to the Devil Canyon Power Plant in the San Bernardino Valley, using the enormous hydraulic head afforded by the mountains to generate up to 276 MW of power. The streams of the San Bernardino Mountains are also prone to flash floods, a danger that has prompted the construction of numerous flood control dams throughout the range. The largest of these is Seven Oaks Dam – the sixth highest dam in the United States – on the Santa Ana River. In 1969, the U.S. Army Corps of Engineers deemed the Santa Ana the greatest flood threat in the United States west of the Mississippi River because of its course through heavily developed areas. Completed in 1999, the dam is designed to completely contain a 350-year flood. Many other dams, including Mojave Forks Dam on the Mojave River and various retention basins and check dams on smaller drainages, provide more localized flood and sediment control. Ecology and wildlife The San Bernardino Mountains, along with the nearby San Gabriel and San Jacinto ranges, is considered a sky island – a high mountain region whose plants and animals vary dramatically from those in the surrounding semi-arid lands. The San Bernardinos in particular comprise the largest forested region in Southern California, and support some 1,600 species of plants. Foothill regions are primarily composed of chaparral and evergreen oak woodland communities, with a transition to forests of deciduous oak, yellow pine, Jeffrey pine, incense cedar and several fir species at elevations above . Deeper within the mountains, perennial streams fed by springs and lakes nourish stands of alders, willows and cottonwoods. About 440 species of wildlife inhabit the mountains, including many endangered species such as the San Bernardino flying squirrel, California Spotted Owl, Mountain yellow-legged frog, Southern rubber boa, and Andrew's marbled butterfly. The mountains once had an abundant population of California grizzly, but hunting eliminated their populations by 1906. Black bears roam the highlands today, but they are not native to the region: they were imported from the Sierra Nevada by the California Department of Fish and Game in the 1930s, in part to attract tourists to the mountains. See also List of mountain ranges of California