Board of Water Supply. Catskill Aqueduct. New York City Mayor George B. McClellan and Chief Engineer of the Board of Water Supply, J. Waldo Smith standing inside an experimental section of cut-and-cover arch at 79th Street and the Hudson River. In order for water to flow from Ashokan Reservoir to New York City using only gravity, the Catskill Aqueduct's tunnels had to be sloped in a particular way for the entire length. Therefore the cut-and-cover method--digging a trench and building a concrete tunnel within it, then covering the tunnel completely with earth--could only be used where the natural geography of the land was naturally sloped. November 22, 1906.