The lower Potomac's troubles probably aren't over. I'm near the headwaters. The river and all tributaries are very high, although actual flood damage seems pretty minor here.
The problem is that all parts of the system from here to the Bay got dumped in last night, and as of this morning the lower Potomac was flowing backwards because the Bay, already high, was rising again with the tide. A cousin of mine on the Bay shore (Pasadena, MD) was stuck on the second floor with furniture floating on the first as of this AM.
What happens in this kind of situation is the whole drainage basin becomes one big traffic backup, with the maximum flood surge taking 2-3 days to reach the DC area.
So there's probably more to come.