Lyric discussion by Saruman 

Cover art for Uncle Frank lyrics by Drive-By Truckers

"Got back home from overseas." The only reason a illiterate Alabama farm boy like Uncle Frank would be overseas is to fight in a war - probably WW1.

The dam in question is likely Wheeler Dam (completed 1936) or perhaps Guntersville Dam (completed 1939) both of which were constructed by the TVA during the Depression and flooded a significant amount of Alabama farmland.