Monticello Dam is a dam in Napa County, California, United States constructed between 1953 and 1957. It is a medium concrete-arch dam with a structural height of 304 ft (93 m) and a crest length of 1,023 ft (312 m). A glory hole is used when a dam is at full capacity and water needs to be drained from the resevoir.3 t( J$ k7 ]" X$ x( h4 ?