[oldasrocks]

I would for a LARGE dam above DC

[Ouchthatonehurt]

Potomac River Reservoir Studies Army Corps;
https://babel.hathitrust.org/cgi/pt?id=uiug.30112060272249&view=1up&seq=9

[Covenantor]

If your thought is to drown the DC Swamp, you build the dam acoss the Potomac below the mouth of the Anacostia River which flows in to the Potomac.

That would pretty much take care of the downtown Federal zone which is on. reclaimed swamp anyway. I've Haines Point, that narrow penisula at south, flooded on from both sides. Would take care of SE on the east bank, been needing a Biblical flood there since I can remember. Marion Barry country.