The Potomac is always running, although it's apparently quite low right now. If you're a solid class V boater, then you've got Great Falls to run. Otherwise you can put in just below the falls and mill around in Mather Gorge. There are always boaters in both the Great Falls NP parking lot as well as a parking lot across from the Anglers Inn off of Clara Barton Parkway.
You're best bet is to post onwww.monocacycanoe.org
and click on "Paddle Prattle" in the left column. Someone should be willing to meet up with you. I no longer live near DC or I'd probably be able to meet up with you myself. Good luck.