The Christmas Flood was a disastrous event that took place on Christmas Day, 2082, in Appalachia. David Thorpe, the leader of the Cutthroats, destroyed Summersville Dam with a stolen mini nuke. His intent was to attack the Responders, who had captured his lover Rosalynn Jeffries. It devastated the city of Charleston and caused the death of over 1,000 Appalachian survivors, including Jeffries. While the Responders lost almost everything, they didn't give up, and they tried to rebuild in Morgantown.
The flood left Charleston in a sorry state. Many buildings were destroyed, some completely dislodged from their original positions. The formerly prospering city was turned into a ghost town.
The Christmas Flood is mentioned, and its aftermath only appear in Fallout 76.