THEN: With the completion of the Hoover Dam in 1935 came the birth of water-sports playground Lake Mead, the largest man-made lake in the U.S. About 25 miles east of Las Vegas, this magnet for boaters, water-skiers, fishermen, swimmers, and scuba divers spills into two states: Nevada and Arizona. These lake revelers were photographed in 1940, as the Great Depression was ending (the economy had grown by ten percent in 1939) and FDR won reelection to an unprecedented third term.

Photograph: Transcontinental and Western Air, Inc./National Geographic Image Collection

NOW: As for water-sports gear, "We've come a long way," says Rick Doyle, whose shot captures modern-day Lake Mead action via a rear-view boat mirror. "From skiing on two wooden planks—where you can get beaten to a pulp—we've moved on to hydrofoil cruising, where you can ski all day in any conditions and never feel a bump." The Lake Mead National Recreation Area, which also includes Lake Mohave, was established in 1936. In 1940, it received 655,910 visitors; in 2003, the number was 7,915,581. Given below-normal runoff from the Colorado River, Lake Mead is now at its lowest level in 40 years.

Photograph by Rick Doyle/Corbis