The engineering is remarkable for what it consumed. When farmers in the Merced River valley outgrew the old Exchequer Dam's 281,000 acre-foot capacity by the 1950s, the Merced Irrigation District proposed something audacious: rather than demolish the existing dam, they would build a much larger one just downstream and incorporate the old structure as an upstream toe, a buttress buried within the new embankment. New Exchequer is a concrete-faced, rock-fill dam -- its wall constructed in vertical zones of compacted, alternating layers of coarse and fine material, then armored with reinforced concrete on the upstream face. At the time of completion, it was the largest dam of its kind in the world. The reservoir it impounds, Lake McClure, holds more than one million acre-feet of water, enough to serve the irrigation district's sprawling farmland and generate hydroelectric power besides.

Pioneering engineering carries pioneering risks. New Exchequer was among the first high concrete-faced rock-fill dams ever built, and the untested design leaked -- sometimes dramatically, with seepage reaching 100 cubic feet per second. For nearly two decades the Merced Irrigation District managed the problem before the California Division of Safety of Dams ordered formal repairs in 1985. The leaks were sealed, but they had already taught the dam-building world a lesson: the concrete face of a rock-fill dam must be designed with far greater attention to joints and waterproofing than early engineers had assumed. New Exchequer became a case study, its problems informing the design of subsequent dams worldwide. The structure also proved its worth as a flood barrier, halting the devastating New Year's Day Flood of 1997 -- a storm system that overwhelmed rivers across California but met its match at Lake McClure.

California's water story is one of extremes, and Lake McClure embodies both ends. The reservoir has never overflowed since the dam's completion, a fact opponents cite when the irrigation district proposes raising the spillway gates to add 70,000 acre-feet of storage. That proposal has drawn fierce resistance because it would partially flood a stretch of the Merced River designated Wild and Scenic -- one of the few remaining undammed sections of a river that flows from Yosemite's high country to the San Joaquin Valley floor. At the other extreme, three years of persistent drought drove the reservoir to its lowest recorded level in February 2015: just 63,489 acre-feet, less than seven percent of capacity. The exposed lakebed revealed contours of the landscape that existed before the dam -- a valley, a river channel, and perhaps a glimpse of the old Exchequer arch, still standing in the dark beneath all that missing water.

Today Lake McClure serves double duty as infrastructure and recreation. Named for Wilbur F. McClure, California's State Engineer during the original dam's construction, the reservoir stretches through the Sierra Nevada foothills with 515 campsites, four boat ramps, and two marinas along its shores. Below the dam, an ogee-type gated spillway two miles to the north handles high-water releases, governed by flood-control rules tied to downstream flows at Stevinson. The dam stands 1,220 feet long and 1,000 feet wide at the base -- a mass of more than five million cubic yards of fill holding back a river born in Yosemite's glacial valleys. From the air, Lake McClure appears as a sinuous blue shape cut into golden-brown foothills, its arms reaching into side canyons, its surface reflecting the same Sierra light that once illuminated the town now resting beneath it.