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The Mighty Kensico

Spanning the northern end of the Bronx River valley, the 307 foot-high Kensico Dam is a spectacular sight for walkers and bicyclists finishing a trip up the Bronx River Parkway Reservation. Behind it, the original sources of the Bronx River now lie beneath the waters of the Kensico Reservoir.

Together with the Ashokan Dam in the Catskills, the Kensico can share the title of the “last of the handmade dams.” Built to a classic design and covered with a quarried granite facing, it represents one of the final dams of its generation. The Kensico’s one million cubic feet of masonry is enough to build an Egyptian pyramid.

With its conventional straight wall, the Kensico is what engineers term a “gravity dam,” because it derives its stability from its sheer weight. The granite blocks covering its face are more than decorative; they lend essential weight to a structure holding back the 30 billion gallons of water in the Kensico Reservoir.

Six years after the Kensico was completed in 1915, the Swiss engineer Fred Noetzli proposed a design for an “arch support” dam, which by bending inward against the weight of the water would transfer some of the stress onto the anchorages at either side of the dam. This meant that less material would be needed to construct such a dam, making it cheaper to build, and with a greater margin of safety.

While engineers tested and refined Noetzli’s arched dam design in the mid-1920s, the development of poured concrete technology also proceeded apace, with the result that the next generation of high dams in the 1930s would be arched, concrete dams, leaving the once-celebrated Kensico looking like the last of its kind. Curiously, though, while the new arched support dams were hailed as a triumphant innovation, the humble beaver had been harnessing its principles for thousands of years: nearly every beaver dam is built with a slight inward arch.

Stephen Paul DeVillo

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The Bronx River Alliance is a coordinated voice for the river that works in partnership to protect, improve and restore the Bronx River corridor so that it can be a healthy ecological, recreational, resource for the communities through which it flows.

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