COLONIAL AQUEDUCT 
Calle Rufino Tamayo, Centro 
When the city was founded in the 1520s, a rough canal was built to bring water to the settlement from a mountain spring to the north. In the mid-18th century this canal was reconstructed into a graceful stone aqueduct that served as  the main water provider to the city until 1940. The remains of the aqueduct can be seen by walking north on García Vigil (the street name changes to R. Tamayo); the aqueduct runs through a charming neighborhood of the city, appropriately called Los Arquitos, and through the barrio of Xochimilco. Another section lines the road to San Felipe del Agua, the source of the water.
From page 16 of Oaxaca Tips