Orchard Beach is the only public beach in the Bronx, and almost none of it is natural. The 1.1-mile crescent of sand, the 115 acres of beach and parkland, even the ground the bathhouse sits on — all of it was assembled in the 1930s on the orders of one man, Parks Commissioner Robert Moses, out of materials that did not belong to this shoreline.

A lagoon, not a beach. Before Moses, the site was not a beach at all. Between Rodman's Neck on the mainland and the small offshore land of Hunter Island and Twin Islands lay a marshy tidal lagoon. The native shoreline of the western Long Island Sound is gray, pebbly, and unglamorous, the opposite of a swimming beach. To make Orchard Beach, the lagoon was filled and the islands were tied to the mainland, converting a tidal inlet into a continuous arc of dry land. The 'islands' at the north end are still there in name and in their wooded character, but you can now walk to them on solid ground because the gap between them and the Bronx was deliberately closed.

The sand came from somewhere else. Moses disliked the Sound's native sand so thoroughly that he refused to build a beach out of it. Instead he had fine, pale sand dredged from the Rockaway Peninsula in Queens and from Sandy Hook in New Jersey and carried north by barge — on the order of four million cubic yards of it — and laid down to make the swimming crescent. This is the literal definition of an artificial beach: the sand you stand on was mined from two other coastlines and shipped in. It is also why the beach needs periodic replenishment; imported sand on an engineered shore does not stay put on its own.

Built with relief money, in the architecture of the era. The project was Depression-era public works, funded and staffed through the federal Works Progress Administration. Its centerpiece was not a boardwalk but a piece of monumental civic architecture: the bathhouse and promenade pavilion designed by Aymar Embury II, Moses's go-to architect, completed in 1936. Two long limestone colonnades radiate from a raised central terrace, dressed in blue tile and terrazzo, fronting a promenade paved in hexagonal blocks. The grand colonnade is often likened to the Palais de Chaillot in Paris. New York City landmarked the bathhouse, promenade, and beach in 2006. A built beach got a building worthy of the claim.

The Bronx Riviera. The dedication on July 25, 1936 drew more than 18,000 people, and the calm Sound water and bright new sand earned the place its enduring nickname: the Bronx Riviera. But the more important thing the beach acquired was a community. Divided into thirteen numbered sections, Orchard Beach became social territory, and from the 1950s on its central sections became the gathering place of the borough's Puerto Rican and Dominican families. The open deck behind Sections 4 and 5 turned into a home for salsa and Afro-Caribbean music, a tradition the National Park Service has formally documented and that continues as Salsa Sundays. An engineered beach became one of the most authentic cultural spaces in the Bronx.

The catch is the water. Everything human about the beach can be maintained; the water cannot be commanded. Orchard Beach faces the western Long Island Sound, downstream of a dense city's combined sewers, and after heavy rain bacteria levels can rise far enough that New York City advises against swimming and wading. In drier stretches the same beach has earned an 'A' grade for water quality. That is the honest shape of the place: a beach that people built, on water that people did not, requiring maintenance on one side and a forecast check on the other. Robert Moses could barge in the sand and hire Aymar Embury II to design the colonnade, but he could not make the Sound clean. Ninety years on, that remains the one part of Orchard Beach that is still entirely natural.