| 1. |  | Area below siphon, Big Thompson Canyon | Sylvan Dale (Colo.); Big Thompson River Flood, Larimer County, Colo., July 31, 1976; Big Thompson River (Colo.)--Aerial photographs | Big Thompson Flood; aerial view of area below where the siphon was located. Sylvan Dale area. The Big Thompson Canyon flood on July 31, 1976 occurred when a thunderstorm stalled over the canyon dropping... |