Which one of the following events significantly occurred in 1957?

The Delta Protection Act was a significant event in 1957 as it marked a crucial step in the long-term protection and management of California's Delta region. The act aimed to ensure the preservation of the Delta's unique ecology and support its economic resources. It came in response to increasing concerns about water quality, habitat loss, and the socio-economic challenges posed by urbanization and agricultural demands in the area. By establishing protective measures, this legislation aimed to balance environmental sustainability with agricultural and urban water needs, highlighting the importance of managing California's water resources comprehensively.

The other options represent important developments in California's water management history, but they did not occur in 1957. The California Water Plan was published earlier in the 20th century, and while flooding in the Sacramento Valley has been a persistent issue, specific major flooding events in that area did not occur in 1957. The Feather River Project is a significant infrastructure effort but was completed at a different time. Therefore, the Delta Protection Act’s enactment in 1957 stands out as a pivotal moment in the context of California’s water management policies.