Since 1935, Hoover Dam and Lake Mead have provided flood control, irrigation, drinking water, and power to desert communities, transforming the Southwest into productive farmland and thriving communities. The dam was originally built to protect farmland in southern California from flooding by the Colorado River, and the Bureau of Reclamation planned the project and designed the dam. Engineering geologists played an important role by surveying the Colorado River for potential dam sites, conducting subsurface investigations, and mapping foundation conditions during construction. Hoover Dam has long been recognized nationally and internationally as one of the world's greatest engineering and construction achievements. Built of 3.23 million cubic yards of concrete, the dam is 726 feet high, 660 feet thick at the base, and 1,244 feet long at the crest. It was the highest dam in the world from 1935 to 1967 and the largest hydroelectric plant in the world from 1936 to 1949. Lake Mead continues to be the largest man-made reservoir in North America. Built during the Great Depression, Hoover Dam was completed in less than 5 years by more than 5,000 workers from virtually every state in the nation, and it became a symbol of national pride. Water and power from Hoover Dam and Lake Mead have provided vital benefits to the Southwest and the nation. Water from Lake Mead irrigates farmland in southern California and southwestern Arizona, where fruit and vegetables are consumed across the country year round. Lake Mead supplies municipal water to Las Vegas, Phoenix, and 33 communities in the Los Angeles area. Hoover Dam generates 4 billion kilowatt hours of electricity a year, enough for 1.3 million people; this power is renewable and does not produce air pollution or toxic water, and income from electricity sales pays all operating costs of the dam. Lake Mead became the country's first national recreation area in 1964 and is the fifth most visited park in the National Park system. Nine million people a year come to Lake Mead for boating, fishing, swimming, camping, hiking, and picnicking. The lake covers 247 square miles and has 700 miles of shoreline, while the recreation area covers 1.5 million acres and protects desert wildlife including bighorn sheep, coyotes, jack rabbits, and desert tortoises. Over the years, environmental impacts from construction of Hoover Dam have become apparent, and the federal government and the states of Nevada, Arizona, and California have begun a 50-year program to protect fish and wildlife and restore habitats along the river from Lake Mead to the U.S.-Mexico border. Two endangered native fish species in the Lower Colorado River Basin are being raised for release into Lake Mead and other lower Colorado River areas, and four national wildlife refuges and one national wildlife area provide habitat for fish and waterfowl on the lower Colorado River. For 70 years, Hoover Dam and Lake Mead have supplied the benefits that made the desert Southwest productive and livable, and major cities in Nevada, Arizona, and southern California could not exist as they do today without them.