The state built a bypass around Sheridan and Willamina that opened in 1957 to carry Oregon Route 18 on the south end of the city. The old alignment through the center of town became Oregon Route 18 Business. Sheridan had its largest flooding in history on December 22, 1964. The South Yamhill overflowed its banks and isolated the town for two days. The Jesuits sold their training school in 1974 to the Delphian Foundation, who then opened The Delphian School at the site in 1976.