Built by Franciscan missionaries and mission Indian labor around 1745 to carry water over Piedras Creek to Mission San Francisco de la Espada, the Espada Aqueduct is the oldest Spanish aqueduct still in use in the United States. It forms part of the broader Espada acequia system, a roughly 15-mile gravity-flow network of ditches, dams, and this aqueduct that once irrigated approximately 3,500 acres of mission farmland, supplying water that also powered a mill and provided drinking water to the mission community. Rather than a monument built to commemorate an event, the aqueduct was purely functional infrastructure, engineered using Spanish colonial techniques rooted in Roman and Moorish irrigation traditions adapted to South Texas terrain. Its everyday "use" has always been literal and unbroken: the acequia madre it carries still delivers water toward the mission and nearby fields, and neighboring property owners continue to draw on the system today, making it one of the few pieces of Spanish colonial infrastructure in the country still performing its original function nearly three centuries after construction. In recognition of this engineering and historical significance, the aqueduct has been designated both a National Historic Civil Engineering Landmark and a National Historic Landmark, and it received a federal preservation grant in 2023 to fund structural repairs. Administered by the National Park Service as part of San Antonio Missions National Historical Park, the aqueduct today also serves a commemorative and educational role, with an adjacent trail and interpretive signage drawing visitors who come to see a still-working example of 18th-century Spanish colonial water engineering.