The Summersville Dam is just east of the Charleston Capitol Building. After raider Rosalynn Jeffries was captured by the Responders, her partner, David Thorpe, thinking she was dead, used a mini nuke to avenge her by destroying the dam. This caused Summersville Lake to catastrophically flood Charleston, killing Rosalynn who was being held there, in what would be known as the Christmas Flood.
At some point after the Christmas Flood and prior to 2102, a group of raiders built watchtowers and a crude bridge across the gap. The bridge has not been maintained in some time and has a broken section in the middle that must be jumped across; failing the jump means a long drop to certain death.
The Summersville Dam appears only in Fallout 76.